Stefan Schuddinck - Lambda expressions and streams made easy: A quick guide to functional programming in Java (programming made easy Book 1)

The left side specifies the parameters required by the expression which could also be empty if no parameters are required. The right side is the lambda body which specifies the actions of the lambda expression:
(parameter1, parameter2) -> {code of the function}
The example below is a Lambda with 2 parameters that prints those parameters to the console:
( int number, char character) -> {System.out.println(number); System.out.println(character); }

• Parameters are optional in a Lambda
  (number, character) -> {System.out.println(number); System.out.println(character); }

• A Lambda that has 1 parameter => the parentheses are optional
  number -> {System.out.println(number); }

• A Lambda that hasnt any parameters => the parentheses are obliged
  () -> {System.out.println(Hello World); }

• A Lambda that has only one instruction => the curly brackets are optional
  number -> System.out.println(number);

• A Lambda that has only one instruction that is a return-instruction => the keyword return is optional
  number -> number * number;

The example below is such a single method interface: public interface EvenNumber { boolean isEven( int number ); } In Java7 you would have to use an anonymous interface implementation like this: public class Main { public static void main(String[] args ) { EvenNumber myEvenNumber = new EvenNumber() { public boolean isEven( int number ) { return number % 2 == 0; }}; System. out .println( myEvenNumber .isEven(7)); } } In Java8 you can use a Lambda expression that implements the functional interface: public class Main { public static void main(String[] args ) { EvenNumber myEvenNumber = number -> number % 2 == 0; System. out .println( myEvenNumber .isEven(7)); } }