Varargs在Java中使用null参数 [英] Varargs with null parameters in Java

问题描述

以下代码只使用空引用作为varargs参数。

The following code simply uses a null reference as a varargs parameter.

package currenttime;

import java.util.Arrays;

public class Main
{
    private static void temp(String...str)
    {
        System.out.println(Arrays.asList(str));
    }

    public static void main(String[] args)
    {
        temp(null,null);
        temp(null);
    }
}

第一次调用方法 temp（null，null）; 显示 [null，null] 表示 str [0] = null 和 str [1] = null 。

The first call to the method temp(null, null); displays [null, null] means that str[0]=null and str[1]=null.

但稍后调用 temp（null）; 导致 NullPointerException 被抛出，看起来 str 本身是 null 。

but the later call to temp(null); causes the NullPointerException to be thrown which appears that the str itself is null.

如果它的类型转换为 String 类似这样的 temp（（String）null）; ，它可以工作并显示 [null] 。

If it's type cast to String something like this temp((String)null);, it works and displays [null].

为什么在最后一次调用中，需要显式类型转换？ 在我看来，它被认为是一个带有 null 引用的字符串数组，它与第一次调用不同。什么是正确的答案？

Why in the last call, an explicit type cast is required? It seems to me that it's considered to be a string array with a null reference which is different from the first call. What is the correct answer?

推荐答案

在我看来，它被认为是一个带有$ b $的字符串数组b与第一次调用不同的参考

It seems to me that it's considered to be a string array with a null reference which is different from the first call

完全正确。这就是发生的事情。

Exactly. This is what happening.

实际上，它只是优先级之间： - exact-匹配， var-args ，拳击和类型转换。

Actually, it's just about precedence between: - exact-match, var-args, boxing and type-casting.

在检查要调用的方法或参数的传递方式时，编译器遵循以下优先顺序： -

Compiler follow the following precedence when checking for methods to be called, or how the argument will be passed: -

完全匹配>类型投射>拳击>变种参数

Exact-Match > Type-Cast > Boxing > Var-args

所以，你可以看到， var-args 具有最低优先级， exact-match 具有最高优先权。这意味着，如果参数 足够，完全匹配那么它将被视为这样。

So, you can see that, var-args has the lowest precedence, and exact-match has the highest precedence. That means, if an argument is good-enough for an exact-match then it will be considered as such.

现在，当你传递 null 作为参数时，那么 null 可以直接传递给你的 var-args 参数作为的引用值。这个参数是完全匹配。

所以，你需要 typecast 它显式地 String 告诉它实际上是你的 var-args 参数的第一个元素

Now, when you pass null as an argument, then null can directly be passed to your var-args parameter as the value of reference. It is an exact match for the parameter.
So, you need to typecast it explicitly to String to tell that its actually the first element of your var-args parameter

然而，在 null，null 的情况下，它将被视为两个元素您的 var-args 。因为它不能是的参考值。

Whereas, in case of null, null, it will be taken as two elements of your var-args. As it cannot be a value of reference.

这篇关于Varargs在Java中使用null参数的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！