Interface in Java

• An interface is a template that contains constant and method declaration. The class that inherits from an interface must implement those methods.
• An interface is declared using interface keyword followed by interface name.
• In UML diagram, an interface is represented by a circle or a rectangle having the text <<interface>>.
• You cannot apply protected or private access modifier to a top-level interface.
• Variables declared in interfaces are public, final, and static by default.
• Methods declared inside interfaces are implicitly public and abstract.
• In other words, the interface only supports public access modifier to all of its members. In case you apply any other access modifier, it will give compile time error.

Syntax

access-modifier interface InterfaceName
{
	//constants and method declarations
}

Relationship Between Class and Interface

• A class can extend another class using extends keyword.
• A class can implement an interface using implements keyword.
• An interface can extend another interface using extends keyword. It cannot extend a class.

Example of interface

interface Father
{
	int f = 1;
	void printFather();
}
interface Child extends Father
{
	int c = 2;
	void printChild();
}
class GrandSon implements Child
{
	public void printFather()
	{
		System.out.println("f = "+f);
	}
	public void printChild()
	{
		System.out.println("c = "+c);
	}
	public static void main(String args[])
	{
		GrandSon g = new GrandSon();
		g.printFather();
		g.printChild();
	}
}

Output

f = 1
c = 2

Multiple Inheritance in Java

In Java, you can achieve multiple inheritance with the help of interface. A class can implement one or more interface and an interface can extend one or more interface.

Example of Multiple Inheritance

interface Father
{
	int f = 1;
	void printFather();
}
interface Mother
{
	int m = 2;
	void printMother();
}
interface Child extends Father, Mother
{
	int c = 3;
	void printChild();	
}
class GrandSon implements Child
{
	public void printFather()
	{
		System.out.println("f = "+f);
	}
	public void printMother()
	{
		System.out.println("m = "+m);
	}
	public void printChild()
	{
		System.out.println("c = "+c);
	}
	public static void main(String args[])
	{
		GrandSon g = new GrandSon();
		g.printFather();
		g.printMother();
		g.printChild();
	}
}

Output

f = 1
m = 2
c = 3

Difference Between Abstract Class and Interface