使用单个空参数调用 Java varargs 方法? [英] Calling Java varargs method with single null argument?

问题描述

如果我有一个可变参数 Java 方法 foo(Object ...arg) 并且我调用 foo(null, null)，我有两个 arg[0] 和 arg[1] 作为 null s.但是如果我调用 foo(null)，arg 本身就是 null.为什么会这样?

If I have a vararg Java method foo(Object ...arg) and I call foo(null, null), I have both arg[0] and arg[1] as nulls. But if I call foo(null), arg itself is null. Why is this happening?

我应该如何调用 foo 使得 foo.length == 1 &&foo[0] == null 是 true 吗?

How should I call foo such that foo.length == 1 && foo[0] == null is true?

推荐答案

问题在于，当您使用文字 null 时，Java 不知道它应该是什么类型.它可能是一个空对象，也可能是一个空对象数组.对于单个参数，它假定后者.

The issue is that when you use the literal null, Java doesn't know what type it is supposed to be. It could be a null Object, or it could be a null Object array. For a single argument it assumes the latter.

你有两个选择.将 null 显式转换为 Object 或使用强类型变量调用该方法.请看下面的例子:

You have two choices. Cast the null explicitly to Object or call the method using a strongly typed variable. See the example below:

public class Temp{
   public static void main(String[] args){
      foo("a", "b", "c");
      foo(null, null);
      foo((Object)null);
      Object bar = null;
      foo(bar);
   }

   private static void foo(Object...args) {
      System.out.println("foo called, args: " + asList(args));
   }
}

输出:

foo called, args: [a, b, c]
foo called, args: [null, null]
foo called, args: [null]
foo called, args: [null]

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