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Questions and Answers
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 What is the difference between an Interface and an Abstract class?
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| Abstract class |
Interface |
| An abstract class can have instance methods that implement a default behavior. |
An Interface can only declare constants and instance methods, but cannot implement default behavior. |
| An abstract class may have the usual flavors of class members (private, protected, etc.) |
All the methods and properties defined in Interface are by default public and abstract. |
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Interfaces are recommended to be used when something in design will change frequently |
| We use abstract class and Interface for the base class in our application. |
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How can we take decision about when we have to use Interface and when Abstract Class?
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A. When should we use an abstract class, when an interface, when both? Interfaces and abstract classes seem superficially to provide almost the same capability. How do we decide which to use?
Basicly abstact class is a abstract view of any realword entity and interface is more abstract one. When we thinking about the entity there are two things one is intention and one is implemntation. Intention means I know about the entity and also may have idea about its state as well as behaviour but don’t know about how its looks or works or may know partially. Implementation means actual state and behaviour of entity.
Enough theory lets take an example. I think it would be better. :)
I am trying to make a Content Management System where content is a genralize form of article, reviews, blogs etc.
CONTENT
ARTICLE
BLOGS
REVIEW
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So content is our base class now how we make a decision whether content class should be Abstract class, Interface or normal class.
First normal class vs other type (abstract and interface). If content is not a core entity of my application means as per the business logic if content is nothing in my application only Article, Blogs, Review are the core part of business logic then content class should not be a normal class because I’ll never make instance of that class. So if you will never make instance of base class then Abstract class and Interface are the more appropriate choice.
Second between Interface and Abstract Class.
CONTENT
Publish ()
ARTICLE
BLOGS
REVIEW
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As you can see content having behavior named "Publish". If according to my business logic Publish having some default behavior which apply to all I’ll prefer content class as an Abstract class. If there is no default behavior for the "Publish" and every drive class makes their own implementation then there is no need to implement "Publish" behavior in the base case I’ll prefer Interface.
These are the in general idea of taking decision between abstract class, interface and normal class. But there is one catch. As we all know there is one constant in software that is "CHANGE". If I made content class as Interface then it is difficult to make changes in base class because if I add new method or property in content interface then I have to implement new method in every drive class. These problems will over come if you are using abstract class for content class and new method is not an abstract type. So we can replace interface with abstract class except multiple inheritance.
CAN-DO and IS-A relationship is also define the deference between Interface and abstract class. As we already discuss Interface can be use for multiple inheritance for example we have another interface named "ICopy" which having behavior copy and every drive class have to implements its own implementation of Copy. If "Article" class drive from abstract class Content as well as ICopy then article "CAN-DO" copy also.
IS-A is for "generalization" and "specialization" means content is a generalize form of Article, Blogs, Review and Article, Blogs, Review are a specialize form of Content.
So, abstract class defines core identity. If we are thinking in term of speed then abstract is fast then interface because interface requires extra in-direction.
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So as per this example Abstract class having upper-hand in compare to interface. Using interface having only advantage of multiple inheritance. If you don’t understand the things then don’t worry because it’s my mistake because I am not able to describe the topic.
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An interface allows somebody to start from scratch to implement your interface or implement your interface in some other code whose original or primary purpose was quite different from your interface. To them, your interface is only incidental, something that have to add on to the their code to be able to use your package.
An abstract class, in contrast, provides more structure. It usually defines some default implementations and provides some tools useful for a full implementation. The catch is, code using it must use your class as the base. That may be highly inconvenient if the other programmers wanting to use your package have already developed their own class hierarchy independently. In Java, a class can inherit from only one base class.
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Describe synchronization in respect to multithreading.
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A: With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources. Without synchonization, it is possible for one thread to modify a shared variable while another thread is in the process of using or updating same shared variable. This usually leads to significant errors.
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Explain different way of using thread?
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A: The thread could be implemented by using runnable interface or by inheriting from the Thread class. The former is more advantageous, 'cause when you are going for multiple inheritance..the only interface can help.
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What are pass by reference and passby value?
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A: Pass By Reference means the passing the address itself rather than passing the value. Passby Value means passing a copy of the value to be passed.
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What is HashMap and Map?
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A: Map is Interface and Hashmap is class that implements that.
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Difference between HashMap and HashTable?
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A: The HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls. (HashMap allows null values as key and value whereas Hashtable doesnt allow). HashMap does not guarantee that the order of the map will remain constant over time. HashMap is non synchronized and Hashtable is synchronized.
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What is the purpose of garbage collection in Java, and when is it used?
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A: Garbage collection is one of the most important feature of Java. Garbage collection is also called automatic memory management as JVM automatically removes the unused variables/objects (value is null) from the memory. User program cann't directly free the object from memory, instead it is the job of the garbage collector to automatically free the objects that are no longer referenced by a program. Every class inherits finalize() method from java.lang.Object, the finalize() method is called by garbage collector when it determines no more references to the object exists. In Java, it is good idea to explicitly assign null into a variable when no more in use. I Java on calling System.gc() and Runtime.gc(), JVM tries to recycle the unused objects, but there is no guarantee when all the objects will garbage collected. The purpose of garbage collection is to identify and discard objects that are no longer needed by a program so that their resources can be reclaimed and reused. A Java object is subject to garbage collection when it becomes unreachable to the program in which it is used.
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Difference between Vector and ArrayList?
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A: Vector is synchronized whereas arraylist is not.
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Difference between Swing and Awt?
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A: AWT are heavy-weight componenets. Swings are light-weight components. Hence swing works faster than AWT.
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What is the difference between a constructor and a method?
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A: A constructor is a member function of a class that is used to create objects of that class. It has the same name as the class itself, has no return type, and is invoked using the new operator. A method is an ordinary member function of a class. It has its own name, a return type (which may be void), and is invoked using the dot operator.
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What is an Iterators?
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A: Some of the collection classes provide traversal of their contents via a java.util.Iterator interface. This interface allows you to walk a collection of objects, operating on each object in turn. Remember when using Iterators that they contain a snapshot of the collection at the time the Iterator was obtained; generally it is not advisable to modify the collection itself while traversing an Iterator.
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State the significance of public, private, protected, default modifiers both singly and in combination and state the effect of package relationships on declared items qualified by these modifiers.
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A: public : Public class is visible in other packages, field is visible everywhere (class must be public too) private : Private variables or methods may be used only by an instance of the same class that declares the variable or method, A private feature may only be accessed by the class that owns the feature. protected : Is available to all classes in the same package and also available to all subclasses of the class that owns the protected feature.This access is provided even to subclasses that reside in a different package from the class that owns the protected feature. default :What you get by default ie, without any access modifier (ie, public private or protected).It means that it is visible to all within a particular package.
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What is an abstract class?
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A: Abstract class must be extended/subclassed (to be useful). It serves as a template. A class that is abstract may not be instantiated (ie, you may not call its constructor), abstract class may contain static data. Any class with an abstract method is automatically abstract itself, and must be declared as such. A class may be declared abstract even if it has no abstract methods. This prevents it from being instantiated.
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What is static in java?
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A: Static means one per class, not one for each object no matter how many instance of a class might exist. This means that you can use them without creating an instance of a class.Static methods are implicitly final, because overriding is done based on the type of the object, and static methods are attached to a class, not an object. A static method in a superclass can be shadowed by another static method in a subclass, as long as the original method was not declared final. However, you can't override a static method with a nonstatic method. In other words, you can't change a static method into an instance method in a subclass.
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What is final?
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A: A final class can't be extended ie., final class may not be subclassed. A final method can't be overridden when its class is inherited. You can't change value of a final variable (is a constant).
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What if the main method is declared as private?
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A: The program compiles properly but at runtime it will give "Main method not public." message.
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What if the static modifier is removed from the signature of the main method?
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A: Program compiles. But at runtime throws an error "NoSuchMethodError".
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What if I write static public void instead of public static void?
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A: Program compiles and runs properly.
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What if I do not provide the String array as the argument to the method?
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A: Program compiles but throws a runtime error "NoSuchMethodError".
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What is the first argument of the String array in main method?
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A: The String array is empty. It does not have any element. This is unlike C/C++ where the first element by default is the program name.
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If I do not provide any arguments on the command line, then the String array of Main method will be empty of null?
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A: It is empty. But not null.
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How can one prove that the array is not null but empty?
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A: Print args.length. It will print 0. That means it is empty. But if it would have been null then it would have thrown a NullPointerException on attempting to print args.length.
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What environment variables do I need to set on my machine in order to be able to run Java programs?
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A: CLASSPATH and PATH are the two variables.
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Can an application have multiple classes having main method?
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A: Yes it is possible. While starting the application we mention the class name to be run. The JVM will look for the Main method only in the class whose name you have mentioned. Hence there is not conflict amongst the multiple classes having main method.
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Can I have multiple main methods in the same class?
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A: No the program fails to compile. The compiler says that the main method is already defined in the class.
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Do I need to import java.lang package any time? Why ?
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A: No. It is by default loaded internally by the JVM.
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Can I import same package/class twice? Will the JVM load the package twice at runtime?
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A: One can import the same package or same class multiple times. Neither compiler nor JVM complains abt it. And the JVM will internally load the class only once no matter how many times you import the same class.
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What are Checked and UnChecked Exception?
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A: A checked exception is some subclass of Exception (or Exception itself), excluding class RuntimeException and its subclasses. Making an exception checked forces client programmers to deal with the possibility that the exception will be thrown. eg, IOException thrown by java.io.FileInputStream's read() method· Unchecked exceptions are RuntimeException and any of its subclasses. Class Error and its subclasses also are unchecked. With an unchecked exception, however, the compiler doesn't force client programmers either to catch the exception or declare it in a throws clause. In fact, client programmers may not even know that the exception could be thrown. eg, StringIndexOutOfBoundsException thrown by String's charAt() method· Checked exceptions must be caught at compile time. Runtime exceptions do not need to be. Errors often cannot be.
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What is Overriding?
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A: When a class defines a method using the same name, return type, and arguments as a method in its superclass, the method in the class overrides the method in the superclass. When the method is invoked for an object of the class, it is the new definition of the method that is called, and not the method definition from superclass. Methods may be overridden to be more public, not more private.
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What are different types of inner classes?
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A: Nested top-level classes, Member classes, Local classes, Anonymous classes
Nested top-level classes- If you declare a class within a class and specify the static modifier, the compiler treats the class just like any other top-level class. Any class outside the declaring class accesses the nested class with the declaring class name acting similarly to a package. eg, outer.inner. Top-level inner classes implicitly have access only to static variables.There can also be inner interfaces. All of these are of the nested top-level variety.
Member classes - Member inner classes are just like other member methods and member variables and access to the member class is restricted, just like methods and variables. This means a public member class acts similarly to a nested top-level class. The primary difference between member classes and nested top-level classes is that member classes have access to the specific instance of the enclosing class.
Local classes
s - Local classes are like local variables, specific to a block of code. Their visibility is only within the block of their declaration. In order for the class to be useful beyond the declaration block, it would need to implement a more publicly available interface.Because local classes are not members, the modifiers public, protected, private, and static are not usable.
Anonymous classes - Anonymous inner classes extend local inner classes one level further. As anonymous classes have no name, you cannot provide a constructor.
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Are the imports checked for validity at compile time? e.g. will the code containing an import such as java.lang.ABCD compile?
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A: Yes the imports are checked for the semantic validity at compile time. The code containing above line of import will not compile. It will throw an error saying,can not resolve symbol symbol : class ABCD location: package io import java.io.ABCD;
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Does importing a package imports the subpackages as well? e.g. Does importing com.MyTest.* also import com.MyTest.UnitTests.*?
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A: No you will have to import the subpackages explicitly. Importing com.MyTest.* will import classes in the package MyTest only. It will not import any class in any of it's subpackage.
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What is the difference between declaring a variable and defining a variable?
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A: In declaration we just mention the type of the variable and it's name. We do not initialize it. But defining means declaration + initialization. e.g String s; is just a declaration while String s = new String ("abcd"); Or String s = "abcd"; are both definitions.
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What is the default value of an object reference declared as an instance variable?
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A: null unless we define it explicitly.
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Can a top level class be private or protected?
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A: No. A top level class can not be private or protected. It can have either "public" or no modifier. If it does not have a modifier it is supposed to have a default access.If a top level class is declared as private the compiler will complain that the "modifier private is not allowed here". This means that a top level class can not be private. Same is the case with protected.
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What type of parameter passing does Java support?
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A: In Java the arguments are always passed by value .
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Primitive data types are passed by reference or pass by value?
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A: Primitive data types are passed by value.
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Objects are passed by value or by reference?
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A: Java only supports pass by value. With objects, the object reference itself is passed by value and so both the original reference and parameter copy both refer to the same object .
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What is serialization?
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A: Serialization is a mechanism by which you can save the state of an object by converting it to a byte stream.
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How do I serialize an object to a file?
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A: The class whose instances are to be serialized should implement an interface Serializable. Then you pass the instance to the ObjectOutputStream which is connected to a fileoutputstream. This will save the object to a file.
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Which methods of Serializable interface should I implement?
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A: The serializable interface is an empty interface, it does not contain any methods. So we do not implement any methods.
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How can I customize the seralization process? i.e. how can one have a control over the serialization process?
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A: Yes it is possible to have control over serialization process. The class should implement Externalizable interface. This interface contains two methods namely readExternal and writeExternal. You should implement these methods and write the logic for customizing the serialization process.
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What is the common usage of serialization?
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A: Whenever an object is to be sent over the network, objects need to be serialized. Moreover if the state of an object is to be saved, objects need to be serilazed.
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What is Externalizable interface?
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A: Externalizable is an interface which contains two methods readExternal and writeExternal. These methods give you a control over the serialization mechanism. Thus if your class implements this interface, you can customize the serialization process by implementing these methods.
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What happens to the object references included in the object?
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A: The serialization mechanism generates an object graph for serialization. Thus it determines whether the included object references are serializable or not. This is a recursive process. Thus when an object is serialized, all the included objects are also serialized alongwith the original obect.
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What one should take care of while serializing the object?
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A: One should make sure that all the included objects are also serializable. If any of the objects is not serializable then it throws a NotSerializableException.
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What happens to the static fields of a class during serialization?
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A: There are three exceptions in which serialization doesnot necessarily read and write to the stream. These are 1. Serialization ignores static fields, because they are not part of ay particular state state. 2. Base class fields are only hendled if the base class itself is serializable. 3. Transient fields.
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Does Java provide any construct to find out the size of an object?
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A: No there is not sizeof operator in Java. So there is not direct way to determine the size of an object directly in Java.
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Does importing a package imports the subpackages as well? e.g. Does importing com.MyTest.* also import com.MyTest.UnitTests.*?
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A: Read the system time just before the method is invoked and immediately after method returns. Take the time difference, which will give you the time taken by a method for execution. To put it in code... long start = System.currentTimeMillis (); method (); long end = System.currentTimeMillis (); System.out.println ("Time taken for execution is " + (end - start)); Remember that if the time taken for execution is too small, it might show that it is taking zero milliseconds for execution. Try it on a method which is big enough, in the sense the one which is doing considerable amout of processing.
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What are wrapper classes?
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A: Java provides specialized classes corresponding to each of the primitive data types. These are called wrapper classes. They are e.g. Integer, Character, Double etc.
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Why do we need wrapper classes?
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A: It is sometimes easier to deal with primitives as objects. Moreover most of the collection classes store objects and not primitive data types. And also the wrapper classes provide many utility methods also. Because of these resons we need wrapper classes. And since we create instances of these classes we can store them in any of the collection classes and pass them around as a collection. Also we can pass them around as method parameters where a method expects an object.
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What are checked exceptions?
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A: Checked exception are those which the Java compiler forces you to catch. e.g. IOException are checked Exceptions.
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What are runtime exceptions?
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A: Runtime exceptions are those exceptions that are thrown at runtime because of either wrong input data or because of wrong business logic etc. These are not checked by the compiler at compile time.
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What is the difference between error and an exception?
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A: An error is an irrecoverable condition occurring at runtime. Such as OutOfMemory error. These JVM errors and you can not repair them at runtime. While exceptions are conditions that occur because of bad input etc. e.g. FileNotFoundException will be thrown if the specified file does not exist. Or a NullPointerException will take place if you try using a null reference. In most of the cases it is possible to recover from an exception (probably by giving user a feedback for entering proper values etc.).
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How to create custom exceptions?
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A: Your class should extend class Exception, or some more specific type thereof.
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If I want an object of my class to be thrown as an exception object, what should I do?
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A: The class should extend from Exception class. Or you can extend your class from some more precise exception type also.
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If my class already extends from some other class what should I do if I want an instance of my class to be thrown as an exception object?
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A: One can not do anytihng in this scenarion. Because Java does not allow multiple inheritance and does not provide any exception interface as well.
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What happens to an unhandled exception?
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A: One can not do anytihng in this scenarion. Because Java does not allow multiple inheritance and does not provide any exception interface as well.
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How does an exception permeate through the code?
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A: An unhandled exception moves up the method stack in search of a matching When an exception is thrown from a code which is wrapped in a try block followed by one or more catch blocks, a search is made for matching catch block. If a matching type is found then that block will be invoked. If a matching type is not found then the exception moves up the method stack and reaches the caller method. Same procedure is repeated if the caller method is included in a try catch block. This process continues until a catch block handling the appropriate type of exception is found. If it does not find such a block then finally the program terminates.
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What are the different ways to handle exceptions?
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A: There are two ways to handle exceptions, 1. By wrapping the desired code in a try block followed by a catch block to catch the exceptions. and 2. List the desired exceptions in the throws clause of the method and let the caller of the method hadle those exceptions.
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What is the basic difference between the 2 approaches to exception handling...1> try catch block and 2> specifying the candidate exceptions in the throws clause? When should you use which approach?
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A: In the first approach as a programmer of the method, you urself are dealing with the exception. This is fine if you are in a best position to decide should be done in case of an exception. Whereas if it is not the responsibility of the method to deal with it's own exceptions, then do not use this approach. In this case use the second approach. In the second approach we are forcing the caller of the method to catch the exceptions, that the method is likely to throw. This is often the approach library creators use. They list the exception in the throws clause and we must catch them. You will find the same approach throughout the java libraries we use.
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Is it necessary that each try block must be followed by a catch block?
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A: It is not necessary that each try block must be followed by a catch block. It should be followed by either a catch block OR a finally block. And whatever exceptions are likely to be thrown should be declared in the throws clause of the method.
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If I write return at the end of the try block, will the finally block still execute?
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A: Yes even if you write return as the last statement in the try block and no exception occurs, the finally block will execute. The finally block will execute and then the control return.
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If I write System.exit (0); at the end of the try block, will the finally block still execute?
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A: No in this case the finally block will not execute because when you say System.exit (0); the control immediately goes out of the program, and thus finally never executes.
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How are Observer and Observable used?
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A: Objects that subclass the Observable class maintain a list of observers. When an Observable object is updated it invokes the update() method of each of its observers to notify the observers that it has changed state. The Observer interface is implemented by objects that observe Observable objects.
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What is synchronization and why is it important?
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A: With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources. Without synchronization, it is possible for one thread to modify a shared object while another thread is in the process of using or updating that object's value. This often leads to significant errors.
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How does Java handle integer overflows and underflows?
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A: It uses those low order bytes of the result that can fit into the size of the type allowed by the operation.
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Does garbage collection guarantee that a program will not run out of memory?
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A: Garbage collection does not guarantee that a program will not run out of memory. It is possible for programs to use up memory resources faster than they are garbage collected. It is also possible for programs to create objects that are not subject to garbage collection
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What is the difference between preemptive scheduling and time slicing?
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A: Under preemptive scheduling, the highest priority task executes until it enters the waiting or dead states or a higher priority task comes into existence. Under time slicing, a task executes for a predefined slice of time and then reenters the pool of ready tasks. The scheduler then determines which task should execute next, based on priority and other factors.
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When a thread is created and started, what is its initial state?
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A: A thread is in the ready state after it has been created and started.
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What is the purpose of finalization?
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A: The purpose of finalization is to give an unreachable object the opportunity to perform any cleanup processing before the object is garbage collected.
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What is the Locale class?
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A: The Locale class is used to tailor program output to the conventions of a particular geographic, political, or cultural region.
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What is the difference between a while statement and a do statement?
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A: A while statement checks at the beginning of a loop to see whether the next loop iteration should occur. A do statement checks at the end of a loop to see whether the next iteration of a loop should occur. The do statement will always execute the body of a loop at least once.
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What is the difference between static and non-static variables?
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A: A static variable is associated with the class as a whole rather than with specific instances of a class. Non-static variables take on unique values with each object instance.
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How are this() and super() used with constructors?
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A: Othis() is used to invoke a constructor of the same class. super() is used to invoke a superclass constructor.
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What are synchronized methods and synchronized statements?
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A: Synchronized methods are methods that are used to control access to an object. A thread only executes a synchronized method after it has acquired the lock for the method's object or class. Synchronized statements are similar to synchronized methods. A synchronized statement can only be executed after a thread has acquired the lock for the object or class referenced in the synchronized statement.
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What is daemon thread and which method is used to create the daemon thread?
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A: Daemon thread is a low priority thread which runs intermittently in the back ground doing the garbage collection operation for the java runtime system. setDaemon method is used to create a daemon thread.
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Can applets communicate with each other?
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A: At this point in time applets may communicate with other applets running in the same virtual machine. If the applets are of the same class, they can communicate via shared static variables. If the applets are of different classes, then each will need a reference to the same class with static variables. In any case the basic idea is to pass the information back and forth through a static variable. An applet can also get references to all other applets on the same page using the getApplets() method of java.applet.AppletContext. Once you\'ve got a reference to an applet, you can communicate with it by using its public members. It is conceivable to have applets in different virtual machines that talk to a server somewhere on the Internet and store any data that needs to be serialized there. Then, when another applet needs this data, it could connect to this same server. Implementing this is non-trivial.
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What are the steps in the JDBC connection?
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A: While making a JDBC connection we go through the following steps : Step 1 : Register the database driver by using : Class.forName(\" driver classs for that specific database\" ); Step 2 : Now create a database connection using : Connection con = DriverManager.getConnection(url,username,password); Step 3: Now Create a query using : Statement stmt = Connection.Statement(\"select * from TABLE NAME\"); Step 4 : Exceute the query : stmt.exceuteUpdate();
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How does a try statement determine which catch clause should be used to handle an exception?
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A: When an exception is thrown within the body of a try statement, the catch clauses of the try statement are examined in the order in which they appear. The first catch clause that is capable of handling the exceptionis executed. The remaining catch clauses are ignored.
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Can an unreachable object become reachable again?
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A: An unreachable object may become reachable again. This can happen when the object's finalize() method is invoked and the object performs an operation which causes it to become accessible to reachable objects.
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What method must be implemented by all threads?
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A: All tasks must implement the run() method, whether they are a subclass of Thread or implement the Runnable interface.
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What are synchronized methods and synchronized statements?
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A: Synchronized methods are methods that are used to control access to an object. A thread only executes a synchronized method after it has acquired the lock for the method's object or class. Synchronized statements are similar to synchronized methods. A synchronized statement can only be executed after a thread has acquired the lock for the object or class referenced in the synchronized statement.
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What is Externalizable?
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A: Externalizable is an Interface that extends Serializable Interface. And sends data into Streams in Compressed Format. It has two methods, writeExternal(ObjectOuput out) and readExternal(ObjectInput in)
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What modifiers are allowed for methods in an Interface?
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A: Only public and abstract modifiers are allowed for methods in interfaces.
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What are some alternatives to inheritance?
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A: Delegation is an alternative to inheritance. Delegation means that you include an instance of another class as an instance variable, and forward messages to the instance. It is often safer than inheritance because it forces you to think about each message you forward, because the instance is of a known class, rather than a new class, and because it doesn't force you to accept all the methods of the super class: you can provide only the methods that really make sense. On the other hand, it makes you write more code, and it is harder to re-use (because it is not a subclass).
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What does it mean that a method or field is "static"?
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A: Static variables and methods are instantiated only once per class. In other words they are class variables, not instance variables. If you change the value of a static variable in a particular object, the value of that variable changes for all instances of that class. Static methods can be referenced with the name of the class rather than the name of a particular object of the class (though that works too). That's how library methods like System.out.println() work out is a static field in the java.lang.System class.
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What is the difference between preemptive scheduling and time slicing?
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A: Under preemptive scheduling, the highest priority task executes until it enters the waiting or dead states or a higher priority task comes into existence. Under time slicing, a task executes for a predefined slice of time and then reenters the pool of ready tasks. The scheduler then determines which task should execute next, based on priority and other factors.
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What is the catch or declare rule for method declarations?
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A: If a checked exception may be thrown within the body of a method, the method must either catch the exception or declare it in its throws clause.
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Can a private method of a superclass be declared within a subclass?
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A: Sure. A private field or method or inner class belongs to its declared class and hides from its subclasses. There is no way for private stuff to have a runtime overloading or overriding.
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Why Java does not support multiple inheritence?
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A: Java DOES support multiple inheritance via interface implementation.
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What is the difference between final, finally and finalize?
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A: final - declare constant; finally - handles exception; finalize - helps in garbage collection.
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Where and how can you use a private constructor?
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A: Private constructor can be used if you do not want any other class to instanstiate the object , the instantiation is done from a static public method, this method is used when dealing with the factory method pattern when the designer wants only one controller (fatory method ) to create the object.
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In System.out.println(),what is System,out and println,pls explain.
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A: System is a predefined final class,out is a PrintStream object and println is a built-in overloaded method in the out object.
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What is meant by "Abstract Interface"?
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A: First, an interface is abstract. That means you cannot have any implementation in an interface. All the methods declared in an interface are abstract methods or signatures of the methods.
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Can you make an instance of an abstract class? For example - java.util.Calender is an abstract class with a method getInstance() which returns an instance of the Calender class.
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A: No! You cannot make an instance of an abstract class. An abstract class has to be sub-classed. If you have an abstract class and you want to use a method which has been implemented, you may need to subclass that abstract class, instantiate your subclass and then call that method.
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What is the output of x<y? a:b = p*q when x=1,y=2,p=3,q=4?
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A: When this kind of question has been asked, find the problems you think is necessary to ask before you give an answer. Ask if variables a and b have been declared or initialized. If the answer is yes. You can say that the syntax is wrong. If the statement is rewritten as: x |
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What is the difference between Swing and AWT components?
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A: AWT components are heavy-weight, whereas Swing components are lightweight. Heavy weight components depend on the local windowing toolkit. For example, java.awt.Button is a heavy weight component, when it is running on the Java platform for Unix platform, it maps to a real Motif button.
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Why Java does not support pointers?
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A: Because pointers are unsafe. Java uses reference types to hide pointers and programmers feel easier to deal with reference types without pointers. This is why Java and C# shine.
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What are Parsers?
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A: Parsers are xml components, a bridge between XML documents and applications that process that XML. The parser is responsible for handling xml syntax, checking the contents of the document against constraints established in a DTD or Schema.
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What is a platform?
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A: A platform is the hardware or software environment in which a program runs. Most platforms can be described as a combination of the operating system and hardware, like Windows 2000/XP, Linux, Solaris, and MacOS.
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What is the main difference between Java platform and other platforms?
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A: The Java platform differs from most other platforms in that it's a software-only platform that runs on top of other hardware-based platforms. The Java platform has two components:
1. The Java Virtual Machine (Java VM)
2. The Java Application Programming Interface (Java API)
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What is the Java Virtual Machine?
|
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A: The Java Virtual Machine is a software that can be ported onto various hardware-based platforms.
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What is the Java API?
|
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A: The Java API is a large collection of ready-made software components that provide many useful capabilities, such as graphical user interface (GUI) widgets.
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What is the package?
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A: The package is a Java namespace or part of Java libraries. The Java API is grouped into libraries of related classes and interfaces; these libraries are known as packages.
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What is native code?
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A: The native code is code that after you compile it, the compiled code runs on a specific hardware platform.
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Is Java code slower than native code?
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|
A: Not really. As a platform-independent environment, the Java platform can be a bit slower than native code. However, smart compilers, well-tuned interpreters, and just-in-time bytecode compilers can bring performance close to that of native code without threatening portability.
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What is the serialization?
|
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A: The serialization is a kind of mechanism that makes a class or a bean persistence by having its properties or fields and state information saved and restored to and from storage.
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How to make a class or a bean serializable?
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|
A: By implementing either the java.io.Serializable interface, or the java.io.Externalizable interface. As long as one class in a class's inheritance hierarchy implements Serializable or Externalizable, that class is serializable.
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How many methods in the Serializable interface?
|
|
A: There is no method in the Serializable interface. The Serializable interface acts as a marker, telling the object serialization tools that your class is serializable.
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How many methods in the Externalizable interface?
|
|
A: There are two methods in the Externalizable interface. You have to implement these two methods in order to make your class externalizable. These two methods are readExternal() and writeExternal().
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What is the difference between Serializalble and Externalizable interface?
|
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A: When you use Serializable interface, your class is serialized automatically by default. But you can override writeObject() and readObject() two methods to control more complex object serailization process. When you use Externalizable interface, you have a complete control over your class's serialization process.
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What is a transient variable?
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|
A: A transient variable is a variable that may not be serialized. If you don't want some field to be serialized, you can mark that field transient or static.
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Which containers use a border layout as their default layout?
|
|
A: The Window, Frame and Dialog classes use a border layout as their default layout.
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How are Observer and Observable used?
|
|
A: Objects that subclass the Observable class maintain a list of observers. When an Observable object is updated it invokes the update() method of each of its observers to notify the observers that it has changed state. The Observer interface is implemented by objects that observe Observable objects.
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What is synchronization and why is it important?
|
|
A: With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources. Without synchronization, it is possible for one thread to modify a shared object while another thread is in the process of using or updating that object's value. This often causes dirty data and leads to significant errors.
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What are synchronized methods and synchronized statements?
|
|
A: Synchronized methods are methods that are used to control access to an object. A thread only executes a synchronized method after it has acquired the lock for the method's object or class. Synchronized statements are similar to synchronized methods. A synchronized statement can only be executed after a thread has acquired the lock for the object or class referenced in the synchronized statement.
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What is thread?
|
|
A: A thread is an independent path of execution in a system.
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What is multithreading?
|
|
A: Multithreading means various threads that run in a system.
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How does multithreading take place on a computer with a single CPU?
|
|
A: The operating system's task scheduler allocates execution time to multiple tasks. By quickly switching between executing tasks, it creates the impression that tasks execute sequentially.
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What is the difference between preemptive scheduling and time slicing?
|
|
A: Under preemptive scheduling, the highest priority task executes until it enters the waiting or dead states or a higher priority task comes into existence. Under time slicing, a task executes for a predefined slice of time and then reenters the pool of ready tasks. The scheduler then determines which task should execute next, based on priority and other factors.
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How to create multithread in a program?
|
|
A: You have two ways to do so. First, making your class "extends" Thread class. Second, making your class "implements" Runnable interface. Put jobs in a run() method and call start() method to start the thread.
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Can Java object be locked down for exclusive use by a given thread?
|
|
A: Yes. You can lock an object by putting it in a "synchronized" block. The locked object is inaccessible to any thread other than the one that explicitly claimed it.
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Can each Java object keep track of all the threads that want to exclusively access to it?
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What state does a thread enter when it terminates its processing?
|
|
A: When a thread terminates its processing, it enters the dead state.
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What invokes a thread's run() method?
|
|
A: After a thread is started, via its start() method of the Thread class, the JVM invokes the thread's run() method when the thread is initially executed.
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What is the purpose of the wait(), notify(), and notifyAll() methods?
|
|
A: The wait(),notify(), and notifyAll() methods are used to provide an efficient way for threads to communicate each other.
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What are the high-level thread states?
|
|
A: The high-level thread states are ready, running, waiting, and dead.
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What are three ways in which a thread can enter the waiting state?
|
|
A: A thread can enter the waiting state by invoking its sleep() method, by blocking on I/O, by unsuccessfully attempting to acquire an object's lock, or by invoking an object's wait() method. It can also enter the waiting state by invoking its (deprecated) suspend() method.
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Can a lock be acquired on a class?
|
|
A: Yes, a lock can be acquired on a class. This lock is acquired on the class's Class object.
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What's new with the stop(), suspend() and resume() methods in JDK 1.2?
|
|
A: The stop(), suspend() and resume() methods have been deprecated in JDK 1.2.
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|
What is the difference between yielding and sleeping?
|
|
A: When a task invokes its yield() method, it returns to the ready state. When a task invokes its sleep() method, it returns to the waiting state.
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|
What is the preferred size of a component?
|
|
A: The preferred size of a component is the minimum component size that will allow the component to display normally.
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|
What method is used to specify a container's layout?
|
|
A: The setLayout() method is used to specify a container's layout.
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Which containers use a FlowLayout as their default layout?
|
|
A: The Panel and Applet classes use the FlowLayout as their default layout.
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|
What is the Collections API?
|
|
A: The Collections API is a set of classes and interfaces that support operations on collections of objects.
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What is the List interface?
|
|
A: The List interface provides support for ordered collections of objects.
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What is an Iterator interface?
|
|
A: The Iterator interface is used to step through the elements of a Collection.
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How does Java handle integer overflows and underflows?
|
|
A: It uses those low order bytes of the result that can fit into the size of the type allowed by the operation.
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|
How many bits are used to represent Unicode, ASCII, UTF-16, and UTF-8 characters?
|
|
A: Unicode requires 16 bits and ASCII require 7 bits. Although the ASCII character set uses only 7 bits, it is usually represented as 8 bits. UTF-8 represents characters using 8, 16, and 18 bit patterns. UTF-16 uses 16-bit and larger bit patterns.
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What is the Vector class?
|
|
A: The Vector class provides the capability to implement a growable array of objects What modifiers may be used with an inner class that is a member of an outer class? A (non-local) inner class may be declared as public, protected, private, static, final, or abstract.
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If a method is declared as protected, where may the method be accessed?
|
|
A: A protected method may only be accessed by classes or interfaces of the same package or by subclasses of the class in which it is declared.
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|
Is sizeof a keyword?
|
|
A: The sizeof operator is not a keyword.
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|
What are wrapped classes?
|
|
A: Wrapped classes are classes that allow primitive types to be accessed as objects.
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|
Does garbage collection guarantee that a program will not run out of memory?
|
|
A: No, it doesn't. It is possible for programs to use up memory resources faster than they are garbage collected. It is also possible for programs to create objects that are not subject to garbage collection.
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|
Name Component subclasses that support painting?
|
|
A: The Canvas, Frame, Panel, and Applet classes support painting.
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|
What is a native method?
|
|
A: A native method is a method that is implemented in a language other than Java.
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|
How can you write a loop indefinitely?
|
|
A: for(;;)--for loop; while(true)--always true, etc.
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Can an anonymous class be declared as implementing an interface and extending a class?
|
|
A: An anonymous class may implement an interface or extend a superclass, but may not be declared to do both.
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|
What is the purpose of finalization?
|
|
A: The purpose of finalization is to give an unreachable object the opportunity to perform any cleanup processing before the object is garbage collected.
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|
Which class is the superclass for every class?
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|
|
What is the difference between the Boolean & operator and the && operator?
|
|
A: If an expression involving the Boolean & operator is evaluated, both operands are evaluated. Then the & operator is applied to the operand. When an expression involving the && operator is evaluated, the first operand is evaluated. If the first operand returns a value of true then the second operand is evaluated. The && operator is then applied to the first and second operands. If the first operand evaluates to false, the evaluation of the second operand is skipped. Operator & has no chance to skip both sides evaluation and && operator does. If asked why, give details as above.
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What is the GregorianCalendar class?
|
|
A: The GregorianCalendar provides support for traditional Western calendars.
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|
What is the SimpleTimeZone class?
|
|
A: The SimpleTimeZone class provides support for a Gregorian calendar.
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|
Which Container method is used to cause a container to be laid out and redisplayed?
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|
What is the Properties class?
|
|
A: The properties class is a subclass of Hashtable that can be read from or written to a stream. It also provides the capability to specify a set of default values to be used.
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What is the purpose of the Runtime class?
|
|
A: The purpose of the Runtime class is to provide access to the Java runtime system.
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What is the purpose of the System class?
|
|
A: The purpose of the System class is to provide access to system resources.
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|
What is the purpose of the finally clause of a try-catch-finally statement?
|
|
A: The finally clause is used to provide the capability to execute code no matter whether or not an exception is thrown or caught.
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|
What is the Locale class?
|
|
A: The Locale class is used to tailor program output to the conventions of a particular geographic, political, or cultural region.
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|
What must a class do to implement an interface?
|
|
A: It must provide all of the methods in the interface and identify the interface in its implements clause.
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What is an abstract method?
|
|
A: An abstract method is a method whose implementation is deferred to a subclass. Or, a method that has no implementation (an interface of a method).
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What is a static method?
|
|
A: A static method is a method that belongs to the class rather than any object of the class and doesn't apply to an object or even require that any objects of the class have been instantiated.
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|
What is a protected method?
|
|
A: A protected method is a method that can be accessed by any method in its package and inherited by any subclass of its class.
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What is the difference between a static and a non-static inner class?
|
|
A: A non-static inner class may have object instances that are associated with instances of the class's outer class. A static inner class does not have any object instances.
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|
What is an object's lock and which object's have locks?
|
|
A: An object's lock is a mechanism that is used by multiple threads to obtain synchronized access to the object. A thread may execute a synchronized method of an object only after it has acquired the object's lock. All objects and classes have locks. A class's lock is acquired on the class's Class object.
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When can an object reference be cast to an interface reference?
|
|
A: An object reference be cast to an interface reference when the object implements the referenced interface.
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|
What is the difference between a Window and a Frame?
|
|
A: The Frame class extends Window to define a main application window that can have a menu bar.
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|
What do heavy weight components mean?
|
|
A: Heavy weight components like Abstract Window Toolkit (AWT), depend on the local windowing toolkit. For example, java.awt.Button is a heavy weight component, when it is running on the Java platform for Unix platform, it maps to a real Motif button. In this relationship, the Motif button is called the peer to the java.awt.Button. If you create two Buttons, two peers and hence two Motif Buttons are also created. The Java platform communicates with the Motif Buttons using the Java Native Interface. For each and every component added to the application, there is an additional overhead tied to the local windowing system, which is why these components are called heavy weight
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Which package has light weight components?
|
|
A: javax.Swing package. All components in Swing, except JApplet, JDialog, JFrame and JWindow are lightweight components.
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What are peerless components?
|
|
A: The peerless components are called light weight components.
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|
What is the difference between the Font and FontMetrics classes?
|
|
A: The FontMetrics class is used to define implementation-specific properties, such as ascent and descent, of a Font object.
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|
What happens when a thread cannot acquire a lock on an object?
|
|
A: If a thread attempts to execute a synchronized method or synchronized statement and is unable to acquire an object's lock, it enters the waiting state until the lock becomes available.
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|
What is the difference between the Reader/Writer class hierarchy and the InputStream/OutputStream class hierarchy?
|
|
A: The Reader/Writer class hierarchy is character-oriented, and the InputStream/OutputStream class hierarchy is byte-oriented.
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|
What classes of exceptions may be caught by a catch clause?
|
|
A: A catch clause can catch any exception that may be assigned to the Throwable type. This includes the Error and Exception types.
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|
What is the difference between throw and throws keywords?
|
|
A: The throw keyword denotes a statement that causes an exception to be initiated. It takes the Exception object to be thrown as argument. The exception will be caught by an immediately encompassing try-catch construction or propagated further up the calling hierarchy. The throws keyword is a modifier of a method that designates that exceptions may come out of the mehtod, either by virtue of the method throwing the exception itself or because it fails to catch such exceptions that a method it calls may throw.
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If a class is declared without any access modifiers, where may the class be accessed?
|
|
A: A class that is declared without any access modifiers is said to have package or friendly access. This means that the class can only be accessed by other classes and interfaces that are defined within the same package.
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|
What is the Map interface?
|
|
A: The Map interface replaces the JDK 1.1 Dictionary class and is used associate keys with values.
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|
|
Does a class inherit the constructors of its superclass?
|
|
A: A class does not inherit constructors from any of its superclasses.
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|
|
Name primitive Java types.
|
|
A: The primitive types are byte, char, short, int, long, float, double, and boolean.
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|
Which class should you use to obtain design information about an object?
|
|
A: The Class class is used to obtain information about an object's design.
|
|
|
How can a GUI component handle its own events?
|
|
A: A component can handle its own events by implementing the required event-listener interface and adding itself as its own event listener.
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|
|
How are the elements of a GridBagLayout organized?
|
|
A: The elements of a GridBagLayout are organized according to a grid. However, the elements are of different sizes and may occupy more than one row or column of the grid. In addition, the rows and columns may have different sizes.
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|
What advantage do Java's layout managers provide over traditional windowing systems?
|
|
A: Java uses layout managers to lay out components in a consistent manner across all windowing platforms. Since Java's layout managers aren't tied to absolute sizing and positioning, they are able to accommodate platform-specific differences among windowing systems.
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|
What are the problems faced by Java programmers who don't use layout managers?
|
|
A: Without layout managers, Java programmers are faced with determining how their GUI will be displayed across multiple windowing systems and finding a common sizing and positioning that will work within the constraints imposed by each windowing system.
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|
|
What is the difference between static and non-static variables?
|
|
A: A static variable is associated with the class as a whole rather than with specific instances of a class. Non-static variables take on unique values with each object instance.
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|
What is the difference between the paint() and repaint() methods?
|
|
A: The paint() method supports painting via a Graphics object. The repaint() method is used to cause paint() to be invoked by the AWT painting thread.
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|
|
What is the purpose of the File class?
|
|
A: The File class is used to create objects that provide access to the files and directories of a local file system.
|
|
|
What restrictions are placed on method overloading?
|
|
A: Two methods may not have the same name and argument list but different return types.
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|
|
What restrictions are placed on method overriding?
|
|
A: Overridden methods must have the same name, argument list, and return type. The overriding method may not limit the access of the method it overrides. The overriding method may not throw any exceptions that may not be thrown by the overridden method.
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|
|
What is casting?
|
|
A: There are two types of casting, casting between primitive numeric types and casting between object references. Casting between numeric types is used to convert larger values, such as double values, to smaller values, such as byte values. Casting between object references is used to refer to an object by a compatible class, interface, or array type reference.
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|
Name Container classes.
|
|
A: Window, Frame, Dialog, FileDialog, Panel, Applet, or ScrollPane
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|
What class allows you to read objects directly from a stream?
|
|
A: The ObjectInputStream class supports the reading of objects from input streams.
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|
|
How are this() and super() used with constructors?
|
|
A: this() is used to invoke a constructor of the same class. super() is used to invoke a superclass constructor.
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|
How is it possible for two String objects with identical values not to be equal under the == operator?
|
|
A: The == operator compares two objects to determine if they are the same object in memory. It is possible for two String objects to have the same value, but located indifferent areas of memory.
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|
|
What an I/O filter?
|
|
A: An I/O filter is an object that reads from one stream and writes to another, usually altering the data in some way as it is passed from one stream to another.
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|
|
What is the Set interface?
|
|
A: The Set interface provides methods for accessing the elements of a finite mathematical set. Sets do not allow duplicate elements.
|
|
|
What is the List interface?
|
|
A: The List interface provides support for ordered collections of objects.
|
|
|
What is the purpose of the enableEvents() method?
|
|
A: The enableEvents() method is used to enable an event for a particular object. Normally, an event is enabled when a listener is added to an object for a particular event. The enableEvents() method is used by objects that handle events by overriding their event-dispatch methods.
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|
|
What is the difference between the File and RandomAccessFile classes?
|
|
A: The File class encapsulates the files and directories of the local file system. The RandomAccessFile class provides the methods needed to directly access data contained in any part of a file.
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|
|
What interface must an object implement before it can be written to a stream as an object?
|
|
A: An object must implement the Serializable or Externalizable interface before it can be written to a stream as an object.
|
|
|
What is the ResourceBundle class?
|
|
A: The ResourceBundle class is used to store locale-specific resources that can be loaded by a program to tailor the program's appearance to the particular locale in which it is being run.
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|
|
What is the difference between a Scrollbar and a ScrollPane?
|
|
A: A Scrollbar is a Component, but not a Container. A ScrollPane is a Container. A ScrollPane handles its own events and performs its own scrolling.
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What is a Java package and how is it used?
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A: A Java package is a naming context for classes and interfaces. A package is used to create a separate name space for groups of classes and interfaces. Packages are also used to organize related classes and interfaces into a single API unit and to control accessibility to these classes and interfaces.
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What are the Object and Class classes used for?
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A: The Object class is the highest-level class in the Java class hierarchy. The Class class is used to represent the classes and interfaces that are loaded by a Java program.
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What is Serialization and deserialization?
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A: Serialization is the process of writing the state of an object to a byte stream. Deserialization is the process of restoring these objects.
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what is tunnelling?
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A: Tunnelling is a route to somewhere. For example, RMI tunnelling is a way to make RMI application get through firewall. In CS world, tunnelling means a way to transfer data.
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Does the code in finally block get executed if there is an exception and a return statement in a catch block?
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A: If an exception occurs and there is a return statement in catch block, the finally block is still executed. The finally block will not be executed when the System.exit(1) statement is executed earlier or the system shut down earlier or the memory is used up earlier before the thread goes to finally block.
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How you restrict a user to cut and paste from the html page?
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A: Using javaScript to lock keyboard keys. It is one of solutions.
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Is Java a super set of JavaScript?
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A: No. They are completely different. Some syntax may be similar.
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What is a Container in a GUI?
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A: A Container contains and arranges other components (including other containers) through the use of layout managers, which use specific layout policies to determine where components should go as a function of the size of the container.
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How the object oriented approach helps us keep complexity of software development under control?
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A: We can discuss such issue from the following aspects: o Objects allow procedures to be encapsulated with their data to reduce potential interference. o Inheritance allows well-tested procedures to be reused and enables changes to make once and have effect in all relevant places. o The well-defined separations of interface and implementation allows constraints to be imposed on inheriting classes while still allowing the flexibility of overriding and overloading.
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What is Polymorphism?
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A: The meaning of Polymorphism is something like one name many forms. Polymorphism enables one entity to be used as as general category for different types of actions. The specific action is determined by the exact nature of the situation. The concept of polymorphism can be explained as "one interface, multiple methods". Polymorphism allows methods to be written that needn't be concerned about the specifics of the objects they will be applied to. That is, the method can be specified at a higher level of abstraction and can be counted on to work even on objects of yet unconceived classes.
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Explain the different forms of Polymorphism?
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A: From a practical programming viewpoint, polymorphism exists in three distinct forms in Java:
a. Method overloading
b. Method overriding through inheritance
c .Method overriding through the Java interface
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What is design by contract?
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A: The design by contract specifies the obligations of a method to any other methods that may use its services and also theirs to it. For example, the preconditions specify what the method required to be true when the method is called. Hence making sure that preconditions are. Similarly, postconditions specify what must be true when the method is finished, thus the called method has the responsibility of satisfying the post conditions. In Java, the exception handling facilities support the use of design by contract, especially in the case of checked exceptions. The assert keyword can be used to make such contracts.
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What are use cases?
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A: A use case describes a situation that a program might encounter and what behavior the program should exhibit in that circumstance. It is part of the analysis of a program. The collection of use cases should, ideally, anticipate all the standard circumstances and many of the extraordinary circumstances possible so that the program will be robust.
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What is the difference between interface and abstract class?
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A: o interface contains methods that must be abstract; abstract class may contain concrete methods. o interface contains variables that must be static and final; abstract class may contain non-final and final variables. o members in an interface are public by default, abstract class may contain non-public members. o interface is used to "implements"; whereas abstract class is used to "extends". o interface can be used to achieve multiple inheritance; abstract class can be used as a single inheritance. o interface can "extends" another interface, abstract class can "extends" another class and "implements" multiple interfaces. o interface is absolutely abstract; abstract class can be invoked if a main() exists. o interface is more flexible than abstract class because one class can only "extends" one super class, but "implements" multiple interfaces. o If given a choice, use interface instead of abstract class.
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