List.toArray奇仿制药的行为（T []） [英] Odd generics behaviour of List.toArray(T[])

问题描述

我遇到现在的东西非常基本但非常令人困惑来了。我需要一个列表转换为数组。该列表包含字符串实例。采用完美的例子 List.toArray（T []），因为我想一个的String [] 实例。这是行不通的，但是，如果没有结果明确铸造到的String [] 。

I came across something very basic but extremely bewildering today. I needed to convert a list to an array. The list contained String instances. Perfect example of using List.toArray(T[]), since I wanted a String[] instance. It would not work, however, without explicitly casting the result to String[].

作为测试场景中，我用下面的code：

As a test scenario, I used the following code:

import java.util.Arrays;
import java.util.List;

public class MainClass {
    public static void main(String args[]) {
        List l = Arrays.asList("a", "b", "c");
        String stuff[] = l.toArray(new String[0]);
        System.err.println(Arrays.asList(stuff));
    }
}

不编译。它在的的javadoc ，但是编译器说以下内容：

which does not compile. It's nearly an exact copy of the example in the javadoc, yet the compiler says the following:

MainClass.java:7: incompatible types
found   : java.lang.Object[]
required: java.lang.String[]
    String stuff[] = l.toArray(new String[0]);
                          ^

如果我加入强制转换为的String [] 将编译和完美运行。但是，这不是我所期望的时候我看着的toArray方法的签名：

If I add a cast to String[] it will compile AND run perfectly. But that is not what I expect when I looked at the signature of the toArray method:

<T> T[] toArray(T[] a)

这告诉我，我不应该投。这是怎么回事？

This tells me that I shouldn't have to cast. What is going on?

编辑：

奇怪的是，如果我更改列表声明：

Curiously, if I change the list declaration to:

List<?> l = Arrays.asList("a", "b", "c");

它也可以。或列表与LT;对象＆gt; 。所以它并没有成为一个列表与LT;弦乐＆GT; 的建议。我开始觉得用原料列表键入也怎么变化这个类里面工作泛型方法。

it also works. Or List<Object>. So it doesn't have to be a List<String> as was suggested. I am beginning to think that using the raw List type also changes how generic methods inside that class work.

第二个编辑：

我觉得我得到它了。什么汤姆Hawtin在评论中写道下面似乎是最好的解释。如果您在原始的方式使用泛型类型，从该实例的所有泛型信息将被编译器删除。

I think I get it now. What Tom Hawtin wrote in a comment below seems to be the best explanation. If you use a generic type in the raw way, all generics information from that instance will be erased by the compiler.

推荐答案

您忘了指定类型参数列表：

you forgot to specify the type parameter for your List:

List<String> l = Arrays.asList("a", "b", "c");

在这种情况下，你可以写安全性：

in this case you can write safety:

String[] a = l.toArray(new String[0]);

无需任何转换。

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