How to write lambda expression with EventHandler javafx

Question

I'm trying to rewrite this code

new EventHandler<MouseEvent>() {

    @Override
    public void handle(MouseEvent e) {
        System.out.println(e.hashCode());
    }
};

as

new EventHandler<MouseEvent>(e -> System.out.println(e.hashCode()));

and I get errors. What is my mistake here?

Solution

The lambda expression is here to replace the whole FunctionalInterface and not only its method, so it's not constructor + lambda but only lambda :

1. Use the EventHandler as parameter :

   someNode.addEventHandler(MouseEvent.MOUSE_CLICKED, 
                           new EventHandler<MouseEvent>() {
                              @Override
                              public void handle(MouseEvent event) {
                                 System.out.println(event.hashCode());
                              }
                           });
   

   Becomes :

    someNode.addEventHandler(MouseEvent.MOUSE_CLICKED, 
                             event ->  System.out.println(event.hashCode()));
   

---

2. Use the EventHandler in a variable :

   EventHandler<MouseEvent> eh = new EventHandler<MouseEvent>() {
                                      @Override
                                      public void handle(MouseEvent event) {
                                          System.out.println(event.hashCode());
                                      }
                               };
   

   It'll become :

   EventHandler<MouseEvent> eh = e -> System.out.println(e.hashCode());
   

---

---

It's exists various way to use lambda, with or without parameter, like :

Runnable r = () -> System.out.println("Here");

Comments

[James_D]

The lambda equivalent of the first code block is just event -> System.out.println(event.hashCode()) (without the call to a constructor, which is I think what you are trying to do).

[Dota2]

in simple words it is the entire functional interface that you replace with lambda and not the function within the interface. The lambda equivalent of ur first code snippet is event -> System.out.println(e.hashCode()). What you are doing is basically adding "lambda" in constructor. Invalid

[azro]

@Pascal Hoffenheimer if the answer satisfies you, you may accept it, it's how a forum works, if you didn't, people with same problem won't come see the solution because the post would appear as "unsolved"

[Pascal Hoffenheimer]

I understand my mistakes: 1. Constructor is to be left out. 2. Use parameter name. Don't rename it.

[Mahendran]

Assume am not going to use the event parameter at all, Any way I can just drop it or any special variable name (_) like in Kotlin?

[azro]

@mahemadhi not in this case, because the method of EventHandler does have a parameter, but for a Supplier/Runnable/... you can (see edit)

[Mahendran]

Am sorry to confuse you.. It's just another question