Java Concepts

Woke up in the morning and a tea from my wife :)

thinking about relearning some java concepts. Here are few -

Q
What is the difference between an Interface and an Abstract class?
A
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 and all methods are implicitly abstract. An interface has all public members and no implementation. An abstract class is a class which may have the usual flavors of class members (private, protected, etc.), but has some abstract methods.
An Abstract class without any implementation just looks like an Interface; however there are lot of differences than similarities between an Abstract class and an Interface. Let's explain both concepts and compare their similarities and differences.

What is an Abstract Class?

An abstract class is a special kind of class that cannot be instantiated. So the question is why we need a class that cannot be instantiated? An abstract class is only to be sub-classed (inherited from). In other words, it only allows other classes to inherit from it but cannot be instantiated. The advantage is that it enforces certain hierarchies for all the subclasses. In simple words, it is a kind of contract that forces all the subclasses to carry on the same hierarchies or standards.

What is an Interface?

An interface is not a class. It is an entity that is defined by the word Interface. An interface has no implementation; it only has the signature or in other words, just the definition of the methods without the body. As one of the similarities to Abstract class, it is a contract that is used to define hierarchies for all subclasses or it defines specific set of methods and their arguments.


The main difference between them is that a class can implement more than one interface but can only inherit from one abstract class.

Q.
What is the purpose of garbage collection in Java, and when is it used?
A.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.

Garbage collection is one of the most important features of Java. 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. Garbage collection is also called automatic memory management as JVM automatically removes the unused variables/objects (value is null) from the memory. 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. In 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. Garbage collection is an automatic process and can't be forced. There is no guarantee that Garbage collection will start immediately upon request of System.gc().

Q.
Describe synchronization in respect to multithreading.
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.

Q. Daemon Thread and User Threads
A.
In java we have two type of Threads : Daemon Thread and User Threads. Generally all threads created by programmer are user thread (unless you specify it to be daemon or your parent thread is a daemon thread). User thread are generally meant to run our programm code. JVM doesn't terminates unless all the user thread terminate.

On the other hand we have Daemon threads. Typically these threads are service provider threads. They should not be used to run your program code but some system code. These thread run paralley to your code but survive on the mercy of the JVM. When JVM finds no user threads it stops and all daemon thread terminate instantly. Thus one should never rely on daemon code to perform any program code.

For better understanding consider a well known example of Daemon thread : Java garbage collector. Garbage collector runs as a daemon thread to recalim any unused memory. When all user threads terminates, JVM may stop and garbage collector also terminates instantly.

Q. What is the purpose of finalization?

The purpose of finalization is to give an unreachable object the opportunity to perform any cleanup, before the object gets garbage collected. For example, closing an opened database Connection.

A. The purpose of finalization is to give an unreachable object the opportunity to perform any cleanup, before the object gets garbage collected. For example, closing an opened database Connection.
The main function of finalization is to provide the opportunity to an object which is unattended or can say unreachable to complete the cleanup process task . This activity is done before the object is collected as considered as garbage . Eg Closing a database connection.

Q.
Explain different way of using thread?
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.

Q.
What are pass by reference and passby value?
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.

Q:
What is HashMap and Map?
A: Map is Interface and Hashmap is class that implements that.

Q. HashMap vs HashTable

Hashmap is not synchronised and allows null for key and values where as Hashtable is synchronised and does not allow null values. The elements may not be in an order in the case of hashmap.

Both provide key-value access to data. The Hashtable is one of the original collection classes in Java. HashMap is part of the new Collections Framework added with Java 2 v1.2.

The key difference between the two is that access to the Hashtable is synchronized on the table while access to the HashMap isn't. You can add it but it isn't there by default.

Another difference is that iterator in the HashMap is fail-safe while the enumerator for the Hashtable isn't. If you change the map while iterating you'll know.

And a third difference is that HashMap permits null values in it while Hashtable doesn't. (From jGuru)

Q:
Difference between Vector and ArrayList?
A: Vector is synchronized whereas arraylist is not.


Q:
Difference between Swing and Awt?
A: AWT are heavy-weight componenets. Swings are light-weight components. Hence swing works faster than AWT.

Q.
What is the difference between a constructor and a method?
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.


Q: What is an Iterator?
A: Some of the collection classes provide traversal of their contents via a java.util.Iterator interface. This interface allows you to walk through 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.

Q:
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.

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.


Q. Explain the modifiers public, protected and private.
public : all instances of other classes, irrespective of the module, namespace or package can have access to this artifact (class, method, variable)

protected : all instances of other classes in the same package and subclasses in other packages can have access to this (once child class inherits a protected method or variable, it becomes private with respect to child class). This cannot be applied to top level classes in java.

Private : only members of the same class can access this. Nobody from outside, even if from same package can access this.

Q:

What is an abstract class?
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.

Q.
What is static in java?
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.

Q:

What is final?
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).