使用null的方法重载选择 [英] Method overload selection with null

问题描述

鉴于此代码：

class Overloading
extends Object
{

static public void target(Object val, String chk) { System.out.println("Object["+val+"] :: Should be "+chk); }
static public void target(String val, String chk) { System.out.println("String["+val+"] :: Should be "+chk); }

static public void main(String[] args) {
    Object                              obj=null;

    target(null        ,"Object");
    target((Object)null,"Object");
    target(obj         ,"Object");
    }
}

输出（意外）如下：

String[null] :: Should be Object
Object[null] :: Should be Object
Object[null] :: Should be Object

问题出在第一行，我希望是第一行和其他两个一样。此外，我发誓，直到最近编译器才会给我一个模糊的调用警告，用于普通的 null 调用。然而，使用Java 5和6进行编译和测试会产生相同的结果。

The problem is with the first line, which I expect to be the same as the other two. Furthermore, I would swear that until recently the compiler would give me an ambiguous invocation warning for the plain null call. However compiling and testing with Java 5 and 6 yields the same results.

这对我来说是个重要问题，因为我有很多代码使用这种模式使用重载不同类型的default参数以选择返回类型并推断所需的转换/解析。任何人都可以解释这里发生了什么吗？

This is a significant issue for me since I have a lot of code which uses this pattern of using an overloaded "default" parameter of different types to select a return type and infer required conversion/parsing. Can anyone explain what is going on here?

推荐答案

Java总是以同样的方式工作：最具体适用的重载是总是选择。由于 String 是 Object 的子类，因此它更具体，而字符串选择了重载。如果重载是，例如 String 和 Integer ，并且您尝试传递 null ，那么你确实会得到编译时歧义错误，因为它们都处于同一继承层次结构的同一级别。

Java has always worked the same way: the "most specific" applicable overload is always chosen. Since String is a subclass of Object, it is "more specific", and the String overload is chosen. If the overloads were for, say String and Integer, and you tried to pass null, then you would indeed get a compile-time ambiguity error, since they are both at the same level of the same inheritance hierarchy.

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