java - 如何将方法引用值分配给 Runnable

Question

我有一个关于 Java 8 Runnable 的问题。

 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;
}

如代码所示，我理解r1是对的，r3是错的，但我不明白为什么r2也是对的.谁能帮我理解一下？

Answer 1

r2 由于 section 15.13.2 of the JLS 没问题，其中包括:

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.

基本上，编写 t2(); 并忽略返回值是有效的，因此创建调用方法并忽略返回值的方法引用是有效的。

t3 不是有效的，因为你必须提供一个参数，而Runnable 不是接受一个参数，所以没有什么可以“传递”给方法。