How do I replace an anonymous class with a lambda in Java?

Question

I've got this code but IntelliJ tells me to replace anonymous with lambda but I don't know how. can anyone help me with this? Here is my code:

soundVolume.valueProperty().addListener(new ChangeListener<Number>() {
    public void changed(ObservableValue<? extends Number> ov,
                     Number old_val, Number new_val) {
        main.setSoundVolume(new_val.doubleValue());
        main.getMediaPlayer().setVolume(main.getSoundVolume());
    }
}); 

Answer 1

Generally, something like this:

methodUsingYourClass(new YourClass() {
    public void uniqueMethod(Type1 parameter1, Type2 parameter2) {
        // body of function
    }
});

is replaced with

methodUsingYourClass((parameter1, parameter2) -> {
    // body of function
});

The types of the parameters can be inferred from usage, but there may be situations where specifying them is useful. This part from the above example

(parameter1, parameter2) -> {

would become this, if you decided to specify the types explicitly

(Type1 parameter1, Type2 parameter2) -> {

For your specific example, you can use:

soundVolume.valueProperty().addListener(
    (ov, old_val, new_val) -> {
        main.setSoundVolume(new_val.doubleValue());
        main.getMediaPlayer().setVolume(main.getSoundVolume());
    }
);

Note the replacement of an anonymous class with lambda is possible only if the anonymous class has one method. If the anonymous class has more methods then the substitution is not possible.

From the oracle documentation:

The previous section, Anonymous Classes, shows you how to implement a base class without giving it a name. Although this is often more concise than a named class, for classes with only one method, even an anonymous class seems a bit excessive and cumbersome. Lambda expressions let you express instances of single-method classes more compactly.