为什么 HashMap 值不在列表中转换? [英] why HashMap Values are not cast in List?

问题描述

我将值放入形式为的哈希图中，

I'm putting values into the hashmap which is of the form,

Map<Long, Double> highLowValueMap=new HashMap<Long, Double>();
highLowValueMap.put(1l, 10.0);
highLowValueMap.put(2l, 20.0);

我想使用地图的values()方法创建一个列表.

I want to create a list by using values() method of map.

List<Double> valuesToMatch=new ArrayList<>();
valuesToMatch=(List<Double>) highLowValueMap.values();

或

List<Double> valuesToMatch=(List<Double>) highLowValueMap.values();

然而，它抛出了一个异常:

However, it throws an exception:

线程main"中的异常java.lang.ClassCastException:
java.util.HashMap$Values 不能转换为 java.util.List

Exception in thread "main" java.lang.ClassCastException:
java.util.HashMap$Values cannot be cast to java.util.List

但它允许我将其传递给列表的创建:

But it allows me to pass it in to the creation of a list:

List<Double> valuesToMatch  = new ArrayList<Double>( highLowValueMap.values());

推荐答案

TL;DR

List<V> al = new ArrayList<V>(hashMapVar.values());

说明

因为 HashMap#values() 返回一个 java.util.Collection 并且您不能将 Collection 转换为ArrayList，因此你得到 ClassCastException.

Explanation

Because HashMap#values() returns a java.util.Collection<V> and you can't cast a Collection into an ArrayList, thus you get ClassCastException.

我建议使用 ArrayList(Collection) 构造函数.这个构造函数接受一个实现 Collection 作为参数.当你像这样传递 HashMap.values() 的结果时，你不会得到 ClassCastException:

I'd suggest using ArrayList(Collection<? extends V>) constructor. This constructor accepts an object which implements Collection<? extends V> as an argument. You won't get ClassCastException when you pass the result of HashMap.values() like this:

List<V> al = new ArrayList<V>(hashMapVar.values());

深入了解 Java API 源代码

HashMap#values(): 检查源代码中的返回类型，问问自己，是否可以将 java.util.Collection 转换为 java.util.ArrayList?否

Going further into the Java API source code

HashMap#values(): Check the return type in the source, and ask yourself, can a java.util.Collection be casted into java.util.ArrayList? No

public Collection<V> values() {
    Collection<V> vs = values;
    return (vs != null ? vs : (values = new Values()));
}

ArrayList(Collection): 检查源中的参数类型.参数是超类型的方法可以接受子类型吗?是的

ArrayList(Collection): Check the argument type in the source. Can a method which argument is a super type accepts sub type? Yes

public ArrayList(Collection<? extends E> c) {
    elementData = c.toArray();
    size = elementData.length;
    // c.toArray might (incorrectly) not return Object[] (see 6260652)
    if (elementData.getClass() != Object[].class)
        elementData = Arrays.copyOf(elementData, size, Object[].class);
}

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