I'm trying to sort a java.util.Map
as follows.
public final class SortMapByValue <K, V extends Comparable<? super V>> implements Comparator<Map.Entry<K, V>>
{
@Override
public int compare(Entry<K, V> o1, Entry<K, V> o2) {
return (o1.getValue()).compareTo(o2.getValue());
}
public static <K, V extends Comparable<? super V>> Map<K, V> sortMapByValue(Map<K, V> unsortedMap)
{
List<Map.Entry<K, V>> list = new LinkedList<Map.Entry<K, V>>(unsortedMap.entrySet());
Collections.sort(list); //Compiler error here.
Map<K, V> sortedMap = new LinkedHashMap<K, V>();
for (Map.Entry<K, V> entry : list) {
sortedMap.put(entry.getKey(), entry.getValue());
}
return sortedMap;
}
}
As commented in the code, it issues a compile-time error as follows.
no suitable method found for sort(List<Entry<K,V>>)
method Collections.<T#1>sort(List<T#1>,Comparator<? super T#1>) is not applicable
(cannot instantiate from arguments because actual and formal argument lists differ in length)
method Collections.<T#2>sort(List<T#2>) is not applicable
(inferred type does not conform to declared bound(s)
inferred: Entry<K,V>
bound(s): Comparable<? super Entry<K,V>>)
where K,V,T#1,T#2 are type-variables:
K extends Object declared in method <K,V>sortMapByValue(Map<K,V>)
V extends Comparable<? super V> declared in method <K,V>sortMapByValue(Map<K,V>)
T#1 extends Object declared in method <T#1>sort(List<T#1>,Comparator<? super T#1>)
T#2 extends Comparable<? super T#2> declared in method <T#2>sort(List<T#2>)
This could also be done in the following way in the sortMapByValue()
method as follows.
Collections.sort(list, new Comparator<Map.Entry<K, V>>()
{
@Override
public int compare(Map.Entry<K, V> o1, Map.Entry<K, V> o2) {
return (o1.getValue()).compareTo(o2.getValue());
}
});
But instead, I would like fixing the the error to follow that way (avoiding this anonymous comparator). How to fix that error?
Map.Entry
does not implement Comparable
, so Collections.sort(List<Entry>)
has no way to know how the entries should be sorted. So you have to provide a Comparator.
But since your SortMapByValue
already implements the Comparator interface, you can simply use an instance of that class:
Collections.sort(list, new SortMapByValue<>());
Also note that with Java 8 you can significantly reduce the length of the code:
public static <K, V extends Comparable<? super V>> Map<K, V> sortMapByValue(Map<K, V> unsortedMap) {
return unsortedMap.entrySet().stream()
.sorted(comparing(Entry::getValue))
.collect(toMap(Entry::getKey, Entry::getValue, (e1,e2) -> e1, LinkedHashMap::new));
}
The technical post webpages of this site follow the CC BY-SA 4.0 protocol. If you need to reprint, please indicate the site URL or the original address.Any question please contact:yoyou2525@163.com.