使用单个null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vararg Java方法 foo(Object ... arg)
,我调用 foo(null,null)
,我有 arg [0]
和 arg [1]
as null
秒。但是,如果我调用 foo(null)
, arg
本身为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 null
s. But if I call foo(null)
, arg
itself is null. Why is this happening?
如何调用 foo
,以便 foo.length == 1&& ; foo [0] == null
是 true
?
推荐答案
问题是当你使用文字null时,Java不知道它应该是什么类型。它可以是null Object,也可以是null Object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.
您有两个选择。将null显式转换为Object或使用强类型变量调用该方法。请参阅下面的示例:
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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