在空引用上创建方法引用不会引发异常 [英] Creating a method reference on a null-reference does not throw an exception

问题描述

为什么可以在一个对象上创建方法引用?Java 中的 null 引用?这样做可能永远不会正确，但可能会导致以后难以发现的错误:

Is there a reason why it is possible to create method references on a null reference in Java? Doing this is probably never correct but can result in errors which are hard to find later:

public class Test {
    void m() {
    }

    public static void main(String[] args) {
        Test test = null;
        Runnable fn = test::m; // no exception
        System.out.println(fn); // prints Test$$Lambda$1/791452441@1c20c684
        fn.run(); // throws a null pointer exception
    }
}

推荐答案

为什么可以在 Java 中的 null 引用上创建方法引用?

Is there a reason why it is possible to create method references on a null reference in Java?

不是，但显然 Eclipse 在这方面存在错误(自 已修复).根据规范，当您使用 JDK 的工具时，它会在 Runnable fn = test::m; 行上以 NPE 失败.

It isn't, but apparently there's a bug in Eclipse in this regard (edit: which has since been fixed). According to the specification, and when you use the JDK's tools, it fails with an NPE on the Runnable fn = test::m; line.

证明:http://ideone.com/APWXna(或使用javac 和 java 而不是 Eclipse)

Proof: http://ideone.com/APWXna (or compile and run it locally with javac and java rather than Eclipse)

理论:来自JLS §15.13.3:

首先，如果方法引用表达式以 ExpressionName 或 Primary 开头，则计算此子表达式.如果子表达式的计算结果为 null，则会引发 NullPointerException ，并且方法引用表达式会突然完成.

First, if the method reference expression begins with an ExpressionName or a Primary, this subexpression is evaluated. If the subexpression evaluates to null, a NullPointerException is raised, and the method reference expression completes abruptly.

(我的重点.)

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