为什么HashMap值不能在List中转换? [英] why HashMap Values are not cast in List?
问题描述
我将值放入hashmap中,形式为
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 values()创建一个列表
地图方法。
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main中的异常java.lang.ClassCastException:
java.util.HashMap $值不能转换为java.util.List
Exception in thread "main" java.lang.ClassCastException:
java.util.HashMap$Values cannot be cast to java.util.List
但它允许我将它传递给一个列表的创建:
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
TL;DR
List<V> al = new ArrayList<V>(hashMapVar.values());
说明
c> HashMap#values()返回 java.util.Collection< V>
,并且不能转换集合到
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< ;?扩展V>
作为参数。当你传递 HashMap.values()
的结果时,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Java API源代码
HashMap#values():检查源中的返回类型,并问自己,是否可以将 java.util.Collection
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):在源。参数是超类型的方法可以接受子类型吗?是
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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