为什么 Java 编译器抱怨使用原始类型的 foreach? [英] Why does the Java Compiler complain on using foreach with a raw type?

问题描述

在 Java 的 for-each 循环中使用泛型时，我遇到了一个奇怪的编译器错误.这是 Java 编译器的错误，还是我真的在这里遗漏了什么?

I got a strange compiler error when using generics within a for-each loop in Java. Is this a Java compiler bug, or am I really missing something here?

这是我的全班:

public class Generics<T extends Object> {
  public Generics(T myObject){
    // I didn't really need myObject
  }

  public List<String> getList(){
    List<String> list = new ArrayList<String>();
    list.add("w00t StackOverflow");
    return list;
  }

  public static void main(String...a){
    Generics generics = new Generics(new Object());
    for(String s : generics.getList()){
      System.out.println(s);
    }
  }
}

编译器抱怨带有 for-each 的行:类型不匹配无法从元素类型 Object 转换为 String."
如果我进行这个细微的更改，它会编译:

The compiler is complaining about the line with the for-each: "Type mismatch cannot convert from element type Object to String."
If I make this subtle change, it compiles:

public static void main(String...a){
  Generics<?> generics = new Generics(new Object());
  for(String s : generics.getList()){
    System.out.println(s);
  }
}

我知道 getList() 确实使用泛型，但它以我认为完全不相关的方式使用它们.如果我试图迭代 T 类型的内容并且 getList() 返回一个 List 或其他内容，我可以理解这一点，但这里的情况并非如此.getList() 的返回类型应该与 T 完全无关，并且不应该关心我是否为泛型对象使用原始类型......对吗?这些不应该是完全无关的，还是我真的在这里遗漏了什么?

I know getList() does use generics, but it uses them in what I thought was a completely unrelated way. I could understand this if I were trying to iterate over something of type T and getList() returned a List<T> or something, but that's not the case here. The return type of getList() should have absolutely nothing to do with T and shouldn't care whether I use the raw type for my Generics object or not...right? Shouldn't these be completely unrelated, or am I really missing something here?

请注意，如果我这样做，代码也会编译，我认为这也应该与第一个相同:

Note that the code also compiles if I do this, which I thought should have been equivalent to the first as well:

public static void main(String...a){
  Generics generics = new Generics(new Object());
  List<String> list = generics.getList();
  for(String s : list){
    System.out.println(s);
  }
}

推荐答案

不同的是，当你使用原始类型时，所有成员签名中的泛型引用也被转换为它们的原始形式.如此有效地调用了一个现在具有如下签名的方法:

The difference is that when you use the raw type, all the generic references within the member signatures are converted to their raw forms too. So effectively you're calling a method which now has a signature like this:

List getList()

现在至于为什么你的最终版本会编译——虽然它会编译，但如果你使用 -Xlint 会有一个警告:

Now as for why your final version compiles - although it does, there's a warning if you use -Xlint:

Generics.java:16: warning: [unchecked] unchecked conversion
    List<String> list = generics.getList();
                                        ^

这类似于:

 List list = new ArrayList();
 List<String> strings = list;

... 也可以编译，但在 -Xlint 下有警告.

... which also compiles, but with a warning under -Xlint.

故事的寓意:不要使用原始类型！

The moral of the story: don't use raw types!

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