Interface in Java

An interface in Java is a blueprint of a class.

It contains:

• method declarations
• constants

But usually it does not contain full method implementation.

Interface tells:

• What to do
• not
• How to do

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Syntax

interface Animal {

    void sound();

}

Here:

• Animal is interface
• sound() has no body

Implementing Interface

class Dog implements Animal {

    public void sound() {
        System.out.println("Bark");
    }

}

implements keyword is used.

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Why use Interface?

1. Achieve Abstraction

Hide internal details.

void sound();

Only rule is given.

2. Multiple Inheritance

Java does not support:

class A extends B, C

But interface allows multiple inheritance.

interface A {}
interface B {}

class Test implements A, B {}

Variables in interface are:

Automatically:

public static final

Example:

interface Test {

    int x = 10;

}

Same as:

public static final int x = 10;

Methods are automatically:

public abstract