Interfaces :

An interface in java is a blueprint of a class. It has static constants and abstract methods.The interface in Java is a mechanism to achieve abstraction  and multiple inheritance in Java.

Like a class, an interface can have methods and variables, but the methods declared in interface are by default abstract (only method signature, no body).  

To declare an interface, use interface keyword. It is used to provide total abstraction.

• Interfaces specify what a class must do and not how. It is the blueprint of the class.
• An Interface is about capabilities like a Player may be an interface and any class implementing Player must be able to (or must implement) move(). So it specifies a set of methods that the class has to implement.
• If a class implements an interface and does not provide method bodies for all functions specified in the interface, then class must be declared abstract.

• Java Interface also represents the IS-A relationship.

• It cannot be instantiated just like the abstract class.

• Since Java 8, we can have default and static methods in an interface.

• Since Java 9, we can have private methods in an interface.

Syntax :

interface <interface_name> 
{
    
    // declare constant fields
    // declare methods that abstract 
}

Why use Java Interface ? 

There are mainly three reasons to use interface. 

The relationship between classes and interfaces :

 A class extends another class, an interface extends another interface, but a class implements an interface.

Java Interface Example :

Here the Printable interface has only one method, and its implementation is provided in the Draw class.

//printable.java

interface printable
{  
   void print();  
}  

//Draw.java

class Draw implements printable
{  
    public void print()
    {
       System.out.println("Hello");
    }  
  
    public static void main(String args[])
    {  
       A6 obj = new A6();  
       obj.print();  
    }  
}