如何将整数元素添加到泛型通配符的 ArrayList 中?

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问题描述

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我有一个 ArrayList 通用通配符类型，它将 Number 作为扩展.我正在尝试将整数值添加到 ArrayList.

但是它给了我一个错误说

ArrayList<?扩展数>numberList = new ArrayList();numberList = new ArrayList();numberList.add(100);

<块引用>

ArrayList 类型中的 add(int, capture#2-of ?) 方法不适用于参数 (int).

我也尝试过这种方式，但给了我同样的错误

ArrayList<?>numberList = new ArrayList();numberList = new ArrayList();numberList.add(100);

错误是:

<块引用>

ArrayList 类型中的 add(int, capture#2-of ?) 方法不适用于参数 (int)

解决方案

你不能.<代码>?extends 部分基本上告诉编译器:它是某种类型，我不知道，但它扩展了 Number.

所以编译器不能保证你要添加的任何类型与未知类型兼容.因此，您无法向此类集合添加任何内容.

I have a ArrayList generic wildcards type, which is taking Number as extends. I am trying to add the integer values to the ArrayList.

Buts it's giving me an error saying that

ArrayList<? extends Number> numberList = new ArrayList<Number>();
    numberList = new ArrayList<Integer>();
    numberList.add(100);

The method add(int, capture#2-of ?) in the type ArrayList<capture#2-of ?> is not applicable for the arguments (int).

I have tried with this way also, but is giving me the same error

ArrayList<?> numberList = new ArrayList<Number>();
    numberList = new ArrayList<Integer>();
    numberList.add(100);

The error is :

The method add(int, capture#2-of ?) in the type ArrayList<capture#2-of ?> is not applicable for the arguments (int)

解决方案

You can't. the ? extends part basically tells the compiler: it is of some type, which I don't know, but which extends Number.

So the compiler can't guarantee that the type of whatever you want to add is compatible to the unknown type. Therefor you can't add anything to such a collection.

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