What does "->" mean in Java?

I am studying this tutorial. Some screenshots have the -> construction, for example:

vertx.deployVerticle("com.mycompany.MyOrderProcessorVerticle", res -> {
  if (res.succeeded()) {
    System.out.println("Deployment id is: " + res.result());
  } else {
    System.out.println("Deployment failed!");
  }
});

I've never encountered this before. Please tell me what it is and where you can read good materials about it.

1 answers

This lambda expression is an anonymous function. Simply put, it is a method without declaring (without access modifiers that return a value and a name).

Appeared in version 8 of Java.

Usage example

Let's write a simple example of the functional interface :

public interface Lambda {
    //Метод интерфейса  с отсутсвующей реализацией
    int getDoubleValue(int val); 

    //Метод интерфейса с реализацией по-умолчанию
    default void printVal(int val) { 
        System.out.println(val);
    }
}

A functional interface must have only one abstract method. You can read about the reasons for this restriction here.

Now let's create a class to use 

public class ClassForLambda {
    public static void main(String[] args) {
        //Объявляем ссылку на функциональный интерфейс
        Lambda lam;
        //Параметр для нашего абстрактногго метода
        int num =9;

        //Прописываем первый вариант реализации
        lam = (val) ->  val * 2;
        System.out.println(lam.getDoubleValue(num));

        //Прописываем второй вариант реализации
        lam = (val) ->  {
            System.out.println("Your number is "+val);
            return val * 2;
        };
        System.out.println(lam.getDoubleValue(num));
    }
}

As you can see, the reference to the method has not changed. Only the implementation was changed.

References:

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Author: Sanek Zhitnik, 2017-04-13 12:53:30