Java 8 Functional Interface

Java 8 Functional Interface

Functional Interface is an interface with only single abstract method. As a functional interface can have only one abstract method that’s why it is also known as Single Abstract Method Interfaces or SAM Interfaces. We can either create our own functional interface or can use predefined functional interfaces provided by java.

Note:
When we creating our own functional interface we should use @FunctionalInterface annotation to mark it as functional interface. Although it is not mandatory to use it, but it’s good to use it with functional interfaces to avoid addition of extra methods accidentally. It will throw compile time error if we try to add more than one abstract method in a functional interface.

Rules to define a functional interface:

A functional interface can have only one abstract method. Along with the one abstract method, it can have any number of default and static methods. It can also have methods of object class.

A functional interface can extends another interface only when it does not have any abstract method.

Java functional interface example

package com.w3schools;

@FunctionalInterface  
interface AddInterface{  
    void add(int a, int b);  
}  

public class FunctionalInterfaceExample implements AddInterface {
	public void add(int a, int b){  
        System.out.println(a+b);  
    } 
	public static void main(String args[]){
		FunctionalInterfaceExample fie = new FunctionalInterfaceExample();  
        fie.add(10, 20);  
	}
}

Output

30

A functional interface can have methods of object class.

package com.w3schools;

@FunctionalInterface  
interface AddInterface{  
    void add(int a, int b);  
    
    //It can contain any number of Object class methods.  
    int hashCode();  
    String toString();  
    boolean equals(Object obj);  
}  

public class FunctionalInterfaceExample implements AddInterface {
	public void add(int a, int b){  
        System.out.println(a+b);  
    } 
	public static void main(String args[]){
		FunctionalInterfaceExample fie = new FunctionalInterfaceExample();  
        fie.add(10, 20);  
	}
}

Output

30

Predefined function interfaces