java Map.Entry<K,V> 的比较器
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comparator for Map.Entry<K,V>
提问by Raghava
I have a Map with an enumeration type as the key and Double as the value. I want to sort this based on the Double values. So I got the entry set and want to use Collections.sort()with a comparator. I have the following code for the comparator
我有一个以枚举类型为键、以 Double 为值的 Map。我想根据 Double 值对其进行排序。所以我得到了条目集并想Collections.sort()与比较器一起使用。我有以下比较器代码
class ScoreComparator<Map.Entry<K, V>> implements Comparator<Map.Entry<K, V>> {
public int compare(Map.Entry<K, V> o1, Map.Entry<K, V> o2) {
return o1.getValue().compareTo(o2.getValue());
}
}
I am getting the following error messages
我收到以下错误消息
Syntax error on token ".", extends expected (line 1).The type parameter Map is hiding the type Map<K,V> (line 1).- Because of the above two errors, K and V cannot be resolved to a type (lines 3,4).
Syntax error on token ".", extends expected (line 1).The type parameter Map is hiding the type Map<K,V> (line 1).- 由于上述两个错误，K 和 V 无法解析为类型（第 3,4 行）。
I am unable to resolve this. Any help is highly appreciated. Thanks in advance.
我无法解决这个问题。任何帮助都受到高度赞赏。提前致谢。
回答by Lukas Eder
You probably want this:
你可能想要这个：
// Declare K and V as generic type parameters to ScoreComparator
class ScoreComparator<K, V extends Comparable<V>>
// Let your class implement Comparator<T>, binding Map.Entry<K, V> to T
implements Comparator<Map.Entry<K, V>> {
public int compare(Map.Entry<K, V> o1, Map.Entry<K, V> o2) {
// Call compareTo() on V, which is known to be a Comparable<V>
return o1.getValue().compareTo(o2.getValue());
}
}
ScoreComparatortakes two generic type arguments Kand V. Map.Entry<K, V>is not a valid generic type definition, but you may well use it to bind to Comparator<T>'s Ttype.
ScoreComparator接受两个泛型类型参数K和V. Map.Entry<K, V>不是有效的泛型类型定义，但您可以很好地使用它来绑定到Comparator<T>的T类型。
Note that Vmust extend Comparable<V>, in order to be able to call compareTo()on o1.getValue().
需要注意的是V必须扩展Comparable<V>，为了能够调用compareTo()上o1.getValue()。
You can now use the above ScoreComparatoras such:
您现在可以使用上述内容ScoreComparator：
new ScoreComparator<String, String>();
new ScoreComparator<Long, Integer>();
// etc...
Note, from your current implementation, you probably don't even need the Kparameter. An alternative:
请注意，从您当前的实现来看，您可能甚至不需要该K参数。替代：
class ScoreComparator<V extends Comparable<V>>
implements Comparator<Map.Entry<?, V>> {
public int compare(Map.Entry<?, V> o1, Map.Entry<?, V> o2) {
// Call compareTo() on V, which is known to be a Comparable<V>
return o1.getValue().compareTo(o2.getValue());
}
}
