如何使用 EventHandler javafx 编写 lambda 表达式

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?

Answer 1

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()));

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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());

---

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

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