为什么 Collection.toArray(T[]) 不采用 E[] 代替

问题描述

toArray 方法(让我们在 java.util.ArrayList) 如下:

The toArray method (lets pick the implementation in java.util.ArrayList) is the following:

class ArrayList<E> ....{
    public <T> T[] toArray(T[] a){
        if(a.length < size)
            return (T[]) Arrays.copyof(elementData, size, a.getClass());
        System.arraycopy(elementData, 0, a, 0, size);
        if(a.length > size)
            a[size] = null;
        return a;
    }
}

我想知道在这种情况下我们可以使用 而不是 吗?喜欢

I am wondering could we use <E> instead of <T> in this case ? like

public E[] toArray(E[] a){
      if(a.length < size)
             return (E[]) Arrays.copyof(elementData, size, a.getClass());
      System.arraycopy(elementData, 0, a, 0, size);
      if(a.length > size)
            a[size] = null;
      return a;
}

既然 ArrayList 类本身已经是 <E> 的泛型，那么我们可以使用它来代替新的泛型类型 <T> 吗?

Since the ArrayList class iteself is already generic to <E>, so could we use that instead of a new generic type <T> ?

推荐答案

<T> 的重点是如果想要的数组是 E.例如，如果 E 是 HashMap 但所需的数组是 Map[].如果 toArray 被锁定到 E 这将是不可能的.

The point of the <T> is if the array desired is of a base class of E. For example if E is HashMap but the desired array was Map[]. If toArray were locked down to E this would not be possible.

由于类型擦除，泛型集合/类型中不需要这种类型的东西.但是数组没有类型擦除，因此数组的类型可能非常重要.

This type of thing is not needed in generic collections / types due to type-erasure. But there is no type-erasure with arrays so the type of the array can be very important.

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