java functional interfaces for lambda expressions? code example

Example 1: how to use lambda in java

StateOwner stateOwner = new StateOwner();

stateOwner.addStateListener(
    (oldState, newState) -> System.out.println("State changed")
);

Example 2: creating the functional interface in java

package com.concretepage;

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