WHY DO I NEED A FUNCTIONAL INTERFACE IN JAVA ? code example

Example 1: how to implement a interface in java

interface methods{
 public static hey(); 
}

class scratch implements methods{
  // Required to implement all methods declared in an interface 
  // Or else the class becomes abstract
  public static hey(){
   System.out.println("Hey"); 
  }
}

Example 2: creating the functional interface in java

package com.concretepage;

@FunctionalInterface
public interface Calculator {
   long calculate(long num1, long num2);
}