What are functional interfaces used for in Java 8?

Question

I came across a new term in Java 8: "functional interface". I could only find one use of it while working with lambda expressions.

Java 8 provides

Answer 1

An interface with only one abstract method is called Functional Interface. It is not mandatory to use @FunctionalInterface, but it’s best practice to use it with functional interfaces to avoid addition of extra methods accidentally. If the interface is annotated with @FunctionalInterface annotation and we try to have more than one abstract method, it throws compiler error.

package com.akhi;
    @FunctionalInterface
    public interface FucnctionalDemo {

      void letsDoSomething();
      //void letsGo();      //invalid because another abstract method does not allow
      public String toString();    // valid because toString from Object 
      public boolean equals(Object o); //valid

      public static int sum(int a,int b)   // valid because method static
        {   
            return a+b;
        }
        public default int sub(int a,int b)   //valid because method default
        {
            return a-b;
        }
    }