在null-reference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

是否有可能在Java中使用
null 引用创建方法引用？这样做可能永远不会正确但可能导致以后很难找到的错误：

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

为什么有可能在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?

它不是，但显然Eclipse中存在这方面的错误（编辑：已有自已修复）。根据规范，当您使用JDK的工具时，它在 Runnable fn = test :: m; 行上的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 （或编译运行在本地使用 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 开头或者 Primary ，评估此子表达式。 如果子表达式求值为 null ，则会引发 NullPointerException ，并且方法引用表达式突然完成。

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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