What is the need for Interfaces, When there are Classes?

[Former Member]

Classes

Classes are the basic building block of every java application. Class stores related data and also defines the methods to work on that data. Every program needs one class with a main method. This class is the entry point for the program. The code in the main method executes first when the program starts.

Let's start with a very simple example:

// a simple Java class, it automatically extends the class java.lang.Object public class MyClass  {     public static int i = 5;      public static void main(String[] args)     {                         // this statement prints the value of i                        System.out.println(i);              } <a></a>

The static fields and methods of a class can be called by another program without creating an instance of the class.

The public static void  keywords conveys the following meaning to the Java interpreter:
    public  - Java Interpreter can call the program's main method to start the program.
    static  - instance of the class need not be created to call the main method
    void - the main method does not return data to the Java interpreter (void) when it ends.

the main calls the System.out.println method to display the value of i. The class Java.lang.System , has a static field out of type PrintStream that is used to invoke the println method.

A program must create an instance of a class to access its non-static fields and methods.

To illustrate this let's write some simple code:

//A simple class with one field of type integer and accessor methods    public class MyClass  {      //field to store integer     private int i = 5;      // gets the value of ingeter field i     public int getI()      {        return i;     }          // sets the value of integer field i         public void setI(int j)      {                 i = j;     }     public static void main(String[] args)     {        //creates a new instance of the class MyClass                 MyClass myClass = new MyClass();                 // retrieves the value of field i                int myInt = myClass.getI();        // this statement prints the retrived value of i                    System.out.println(myInt);     } ;

The class only describes the data and behavior. An instance of a class is an executable copy of the class. a class instance is needed to acquire and work on data.

Advantages of Using a class

• Use class to group related fields and methods.
• Use class to accelerate development by reducing redundant code entry, testing and bug fixing.
• Using a well-tested class or extending it will reduce, if not eliminate, the possibility of bugs propagating into the code.
• Using classes simplifies the relationships of interrelated data.

Inheritance

Inheritance provides a way in object oriented programming to define a new classes using predefined classes. The new class is known as derived class and the predefined class is referred as a base class. Inheritance helps in code reuse little or no modification.  Below is the example of Inheritance in Java:

public class MyBaseClass{      protected int data;       /**              * constructor for the class.         * It does not have a return type (not even void)        */            public MyClass()            {                  data = 1;       }       public int getData()       {                   return data;            }       public void setData(int d)       {          data = d;       }   }    /**     * MyClass is a class that extends MyBaseClass.       */     public class MyClass extends MyBaseClass {       

In the example above MyClass derived from the base class MyBaseClass and it inherits the properties and behavior of MyBaseClass, this demonstrates the reuse of the existing code.
 

Disadvantage of Inheritance:

• Inheritance locks your classes into a particular inheritance hierarchy as Java doesn't supports multiple inheritance

Interface

Interfaces form a contract between the class and the outside world. An interface is a group of related methods with empty bodies. If a class implements an interface, all methods defined by that interface must be defined in the class.

public interface Printable {    void print();  }    public class SayGoodMorning implements Printable{            void print(){               System.out.println("Good Morning");        }

Here class SayGoodMorning implements the interface Printable.

When to use interface:

• Design an interface when something in the design is going to change frequently
• You should generally favor interfaces over inheritance