interface

Interface

Interface looks like a class but it is not a class. An interface can have methods and variables just like the class but the methods declared in interface are by default abstract (only method signature, no body, see: Java abstract method). Also, the variables declared in an interface are public, static & final by default. We will cover this in detail, later in this guide.

What is the use of interface in Java?

As mentioned above they are used for full abstraction. Since methods in interfaces do not have body, they have to be implemented by the class before you can access them. The class that implements interface must implement all the methods of that interface. Also, java programming language does not allow you to extend more than one class, However you can implement more than one interfaces in your class.

Syntax:
Interfaces are declared by specifying a keyword “interface”. E.g.:

interface interface_name

{

   /* All the methods are public abstract by default

    * As you see they have no body

    */

   public void method1();

   public void method2();

}

Example

interface Bkec

{

  

   public void show();

   }

class cse implements bkec

{

  

   public void show()

   {

            System.out.println("welcome to cse");

   }

   public static void main(String arg[])

   {

            bkec obj = new cse();

            obj.show();

   }

}

Output:

Welcome to cse

Rules for using Interface

·         Methods inside Interface must not be static, final, native or strictfp.

·         All variables declared inside interface are implicitly public static final variables(constants).

·         All methods declared inside Java Interfaces are implicitly public and abstract, even if you don't use public or abstract keyword.

·         Interface can extend one or more other interface.

·         Interface cannot implement a class.

·         Interface can be nested inside another interface.

Multiple inheritance & interface

interface  cse{  

void print();  

}  

interface bkec{  

void show();  

}  

class bet implements cse,bkec

 {  

public void print()

{

System.out.println("Hello");}  

public void show(){

System.out.println("Welcome");}  

  

public static void main(String args[]){  

bet obj = new bet();  

obj.print();  

obj.show();  

 }  

Output:  Hello

         Welcome

Next Topics

Exception Handling



Package



Multi threading



Applet