Functional Interface

1. If an interface has only one abstract method in it then such an interface is called as a functional interface
2. We can give the implementation using a lambda expression, anonymous inner class
3. We can also implement it using other interface
4. Using interface

interface jeevan{
    void disp();

}
class implementation implements jeevan{

     public void disp(){
        System.out.println("display");

    }
}
class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello, World!"); 
        implementation object = new implementation();
        object.disp();

    }
}

1. Functional Interface we use '@ functional interface' else we will get an error

Anonymous Inner Class

1. Types of Anonymous Inner Class

    a. Anonymous Inner class that extends a class
    b. Anonymous Inner class that implements an interface
    c. Anonymous Inner class that defines inside method/constructor argument
   

a. Anonymous inner class for implementing an interface example:

 @FunctionalInterface 
interface jeevan{
    void disp();

}
class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello, World!"); 
        jeevan object = new jeevan(){
            public void disp(){
                System.out.println("hi");
            }
        };
            object.disp();
    }
}

b. Anonymous Inner class that extends a class for method overriding-- use this when we only want to use the overridden method only once

example:

class jeevan{

public void disp(){
    System.out.println("jeevan");
}


}
class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello, World!"); 
        jeevan object = new jeevan(){
            public void disp(){
                System.out.println("hi");
            }
        };
            object.disp();

    }
}

Another method of doing this

interface computer{

    void disp();
}





class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello, World!"); 
        /*computer object =new computer(){
            public void disp(){
                System.out.println("hi");
            }
        };
        object.disp(); */ // why to write this we can use short cut called as  lambda function

        computer object = () -> 
        {
            System.out.println("short code");

        };

        object.disp();

    }
}

Note : We can use lambda function only with functional interface