Java 8 Lambda Syntax change

问题:

I ran into an issue where the allowed syntax of a Lambda has changed between versions 1.8.0_05 and 1.8.0_20 (beta) of the java compiler.

Example:

package scratch;

import javafx.scene.control.MenuItem;

public class Test
{
    public void test()
    {
        MenuItem mi = new MenuItem();

        //This compiles anywhere
        mi.setOnAction(e -> System.out.println("hi"));

        //as does this
        mi.setOnAction(e -> {System.out.println("hi");});

        //This doesn't on build 1.8.0_20-ea-b13 - but does on build 1.8.0_05-b13
        mi.setOnAction(e -> (System.out.println("hi")));
    }
}

What I would like to know - is the last example a valid Lambda expression? And they have just tightened the compiler validation? Or is there a bug in the latest 1.8 compiler?

The error printed by the latest compiler is:

/scratch/src/scratch/Test.java:18: error: method setOnAction in class MenuItem cannot be applied to given types;
                mi.setOnAction(e -> (System.out.println("hi")));
                  ^
  required: EventHandler<ActionEvent>
  found: (e)->(Syst[...]hi"))
 reason: argument mismatch; bad return type in lambda expression
      missing return value
1 error

Edit (since I can't seem to format comments in replies):

The implementation of the setOnAction method is:

public final void setOnAction(EventHandler<ActionEvent> value) {
    onActionProperty().set( value);
}

And EventHandler:

@FunctionalInterface
public interface EventHandler<T extends Event> extends EventListener {
    /**
     * Invoked when a specific event of the type for which this handler is
     * registered happens.
     *
     * @param event the event which occurred
     */
    void handle(T event);
}

回答1:

In the Java programming language, a method invocation expression is an Expression Statement, a construct which can appear at both places, where an expression is required or where a statement is required.

Therefore you can use the simplified expression form param -> expression for the use case e -> System.out.println("hi") even if the method returns void. Since the function signature expected here is <T extends Event> T -> void, your lambda expression containing a single invocation of a void method is valid for this context.

Things change when you are trying to use the Expression Statement in a different context where an expression is required. Compare JLS §15.1:

An expression denotes nothing if and only if it is a method invocation (§15.12) that invokes a method that does not return a value, that is, a method declared void (§8.4). Such an expression can be used only as an expression statement (§14.8), because every other context in which an expression can appear requires the expression to denote something.

Applying this rule formally, even simply putting braces around it like in (System.out.println("hi")) is invalid as this is a compound expression trying to use the method invocation of a method declared void in a context where a “real expression” (returning a value) is required.

And so the lambda expression using the invalid expression as in mi.setOnAction(e -> (System.out.println("hi"))); can’t be valid either. The message is a bit misleading. It seems that the compiler focuses on the fact that an expression of the form ( whatever ), is a non-statement expression and therefore can’t be valid in a void context. However, reporting the initial error of putting a void method invocation in round braces would be more useful.

The rule that you can’t put ( … ) around a void method invocation didn’t change, so the error was the older compiler accepting this syntax which seems to have been fixed now.