Java泛型返回数组 [英] java generics return an array

问题描述

编辑：我没有看到其他的问题如何回答我的。我已经有一个明确的CAST（E []），它的另一个问题笔者所没有的。
我也尝试过类似SEQ1 =（整数[]）iw.result（）;但这并不能工作。

edit: I don't see how the other question answers mine. I already have an explicit cast (E[]) which the author of the other question doesn't have. I also tried something like seq1 = (Integer[]) iw.result(); but that doesn't work either.

我有以下的code：

public class Something<E extends Comparable<E>> {
E[] seq;
public Something() {
    seq = (E[]) new Object[100];
}

public E[] result() {
    return sequence;
}

public static void main(String[] args) {
    Something<Integer> iw = new Something<Integer>();
    Integer[] seq1 = new Integer[100];
    seq1 = iw.result();
}}

我得到的方式的errormessage的：[对象不能转换为[可比

I get an errormessage in the way of: [Object can't be cast to [Comparable

所以我改变的东西 - 构造器：

So I change the Something-Constructor to:

seq = (E[]) new Comparable[100];

现在我得到的方式的errormessage的：[可比不能转换为[整数

Now I get an errormessage in the way of: [Comparable can't be cast to [Integer

有没有什么办法，使上述code的工作？我知道我会过得更好使用集合在这里，但我只是好奇，这有什么错我的code。

Is there any way to make the above code work? I know I would be better off working with Collections here, but I'm just curious what's wrong with my code.

推荐答案

一对夫妇在这里的问题。

A couple of issues here.

首先，这不是一个无界的泛型参数，这是有一个必然的可比＆LT; E＆GT; 连接到它

First, that's not an unbounded generic parameter, that's got a bound of Comparable<E> attached to it.

在类型擦除，用类声明为：

During type erasure, with your class declared as:

public class Something<E extends Comparable<E>> {
    E[] seq;
}

... 电子势必可比。

public class Something {
    Comparable[] seq;
}

的这的是，为什么你的铸件行不通的，因为一个对象不是可比。你想用新可比代替。

This is why your casting isn't going to work, since an Object is not a Comparable. You would want to use new Comparable instead.

public class Something<E extends Comparable<E>> {
    E[] seq;

    public Something() {
        seq = (E[]) new Comparable[100];
    }
}

现在，Java是和的应的突破的最后两个语句。

Now, Java is and should break on the last two statements.

Integer[] seq1 = new Integer[100];
seq1 = iw.result();

iw.result（）势必可比[] ，而不是整数[ ] 。 A 可比[] 永远不能成为一个整数[] 。

iw.result() is bound to Comparable[], not Integer[]. A Comparable[] can never become an Integer[].

您可以做到这一点，而不是避开了 ClassCastException异常而不是：

You can do this instead to eschew the ClassCastException instead:

Comparable[] seq1 = new Integer[100];

因为一个整数这将工作是可比。这是由于这一事实，即数组是协变的（也就是说，一个整数[] 是一个亚型可比[] ）。

This will work since an Integer is Comparable. This is due to the fact that arrays are covariant (that is to say, an Integer[] is a subtype of a Comparable[]).

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