How assign a method reference value to Runnable

问题

I hava a problem about Java 8 Runnable.

 public static void main(String[] args) {
    Runnable r1 = Test::t1;
    Runnable r2 = Test::t2;
    Runnable r3 = Test::t3;
}

public static void t1() {

}

public static String t2() {
    return "abc";
}

public static String t3(String t) {
    return t;
}

As the code show, I understand r1 is right and r3 is wrong, but I don't understand why r2 is also right. Can anybody help me understand it?

回答1:

r2 is fine due to section 15.13.2 of the JLS, which includes:

A method reference expression is congruent with a function type if both of the following are true:

The function type identifies a single compile-time declaration corresponding to the reference.

One of the following is true:

The result of the function type is void.

The result of the function type is R, and the result of applying capture conversion (§5.1.10) to the return type of the invocation type (§15.12.2.6) of the chosen compile-time declaration is R' (where R is the target type that may be used to infer R'), and neither R nor R' is void, and R' is compatible with R in an assignment context.

Basically, it would be valid to write t2(); and just ignore the return value, so it's valid to create a method reference which calls the method and ignores the return value.

t3 isn't valid, because you have to provide a parameter, and Runnable doesn't take a parameter, so there'd be nothing to "pass on" to the method.

来源：https://stackoverflow.com/questions/38118627/how-assign-a-method-reference-value-to-runnable

标签

java

runnable