快速Java问题：将一个对象数组转换成我想要的类的数组 [英] Quick Java question: Casting an array of Objects into an array of my intended class

问题描述

只是为了审查，有人可以快速解释阻止这种工作（在编译时）：

  private HashSet Data; 
 
 ... 
 
 public DataObject [] getDataObjects（）
 {
 return（DataObject []）Data.toArray 
} 
  

...这就是DOES的工作方式：

  public DataObject [] getDataObjects（）
 {
 return（DataObject []）Data.toArray Data.size（）]）; 
} 
  

我不清楚使用铸造的机制因为 toArray（）创建了一个一个Object数组，你不能通过转换它来使 Object [] 进入 DataObject [] 。 toArray（DataObject []）创建一个 DataObject 数组

是的，它是Collections类的一个缺点，以及Generics被引入Java的方式。你会期望 Collection< E> .toArray（）可能返回一个E数组，但它不会。

有关 toArray（DataObject []）调用的有趣事情：你不必使a数组足够大，因此可以调用它 toArray（new DataObject [0]）如果你喜欢。

调用它像 toArray （new DateObject [0]）实际上更好，如果你使用 .length 以后得到数组长度。如果初始长度很大，并且返回了您传递的相同数组对象，那么您可能会稍后 NullPointerException

我之前提过一个关于Java泛型的问题，并指出这个常见问题解答非常有用： http： //www.angelikalanger.com/GenericsFAQ/JavaGenericsFAQ.html

Just for review, can someone quickly explain what prevents this from working (on compile):

private HashSet Data;

...

public DataObject[] getDataObjects( )
{
    return (DataObject[]) Data.toArray();
}

...and what makes this the way that DOES work:

public DataObject[] getDataObjects( )
{
    return (DataObject[]) Data.toArray( new DataObject[ Data.size() ] );
}

I'm not clear on the mechanism at work with casting (or whatever it is) that makes this so.

解决方案

Because toArray() creates an array of Object, and you can't make Object[] into DataObject[] just by casting it. toArray(DataObject[]) creates an array of DataObject.

And yes, it is a shortcoming of the Collections class and the way Generics were shoehorned into Java. You'd expect that Collection<E>.toArray() could return an array of E, but it doesn't.

Interesting thing about the toArray(DataObject[]) call: you don't have to make the "a" array big enough, so you can call it with toArray(new DataObject[0]) if you like.

Calling it like toArray(new DateObject[0]) is actually better if you use .length later to get the array length. if the initial length was large and the same array object you passed was returned then you may face NullPointerExceptions later

I asked a question earlier about Java generics, and was pointed to this FAQ that was very helpful: http://www.angelikalanger.com/GenericsFAQ/JavaGenericsFAQ.html

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