Java 1.8 Lambda & Method Reference

One way to create thread is to pass a object that implements Runnable Interface to a Thread constructor as mentioned in Java Multithreading Approaches (Java Concurrency - Part 1):

​public class MyRunnable implements Runnable{
  // this method is called when a runnable thread starts
  public void run(){
    printMessage();
  }
}

public class ThreadDemo{
  public static void main(String args[]){
    Thread t1 = new Thread(new MyRunnable); 
    Thread t2 = new Thread(new MyRunnable);
    Thread t3 = new Thread(new MyRunnable);
    t1.start();    
    t2.start();
    t3.start();
  }

  public static void printMessage(){
    System.out.println("Hello");
  }
}

Java 1.8 introduce Functional Interface. Any interface with a SAM (Single Abstract Method) is a functional interface, and its implementation may be written as a lambda expression. A Functional Interface can have many default methods or static methods declared, but can only have one single abstract method. 

You may use Lambda Expression in place of a Functional Interface. For example, Runnable interface is a Functional Interface. Let me use the Lambda Expression instead. In this approach, I no longer need to define MyRunnable.

public class ThreadDemo{
  public static void main(String args[]){
    Thread t1 = new Thread(() -> printMessage()); 
    Thread t2 = new Thread(() -> printMessage());
    Thread t3 = new Thread(() -> printMessage());
    t1.start();    
    t2.start();
    t3.start();
  }

  public static void printMessage(){
    System.out.println("Hello");
  }
}

A method reference is the shorthand/alternative syntax for a lambda expression that executes just ONE method. Replace,

() -> printMessage();

with,

ThreadDemo::printMessage

and get:

public class ThreadDemo{
  public static void main(String args[]){
    Thread t1 = new Thread(ThreadDemo::printMessage); 
    Thread t2 = new Thread(ThreadDemo::printMessage);
    Thread t3 = new Thread(ThreadDemo::printMessage);
    t1.start();    
    t2.start();
    t3.start();
  }

  public static void printMessage(){
    System.out.println("Hello");
  }
}

To take the Method Reference syntax one step further, as another example, you can replace Lambda Expression:

(p) -> System.out.println(p)

with,

System.out::println

as in:

public class MethodReferenceExample{
  public static void main(String[] args){
    List<Person> people = Arrays.asList(
      new Person("Charles", "Dickens", 60),
      new Person(Lewis", "Carroll", 42)
    );
    performConditionally(people, p -> true, System.out::println);
  }

  private static void performConditionally(List<Person> people, Predicate<Person> predicate, Consumer<Person> consumer)
    for (Person p: people){
      if (predicate.test(p)){
        consumer.accept(p);
      }
    }
}