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 map 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();
但是，它会引发异常:
线程主"java.lang.ClassCastException 中的异常:
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创建列表:
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.CollectionCollection 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<? extends V>) 构造函数.这个构造函数接受一个实现 Collection<? 的对象.扩展 V> 作为参数.当您像这样传递 HashMap.values() 的结果时，您不会得到 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，并问自己，java.util.Collection是否可以强制转换为java.util.ArrayList?没有
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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