从 HashMap 中，如何删除 k 个具有最高值的元素（Java）？

Question

myMap = {1=3, 2=2, 3=6, 4=8, 5=2, 6=1}

k = 3

so I'll delete the (3) highest map items with the highest values.

What can I do in Java so the map becomes?

myMap = {2=2, 5=2, 6=1}

I tried a for loop that iterates (3) times

and within it, a nested for loop that uses map.entry to get the max value and remove it. I keep getting an error.

     int k =3; // number of large value pairs to remove

 int max = Collections.max(myMap.values()); // find max value
   

 for(int i =0; i<k ;i++) {  // iterate 3 times to delete (3) sets

    for (Map.Entry<Integer, Integer> entry : myMap.entrySet()) {
        Integer key = entry.getKey(); // find key associated with current largest value 
        Integer value = entry.getValue();// find largest value 
        if(value==max) { // test retrieved vale with max value 
            hMap.remove(key);// remove set from HashMap
        }
        max = Collections.max(myMap.values()); // find new maximum
    }      
 }

Error:

Exception in thread "main" java.util.ConcurrentModificationException
    at java.base/java.util.HashMap$HashIterator.nextNode(HashMap.java:XXXX)
    at java.base/java.util.HashMap$EntryIterator.next(HashMap.java:XXXX)
    at java.base/java.util.HashMap$EntryIterator.next(HashMap.java:XXXX)

Answer 1

form jdk17 myMap.entrySet() -> hashMap's implements it will create a new EntryIterator() object,then will use super class HashIterator's constructors without arguments, When you use hashMap's put() or remove() method, modCount will + 1,As a result HashIterator's nextNode() enter image description here

Answer 2

There are multiple issues with your approach.

• It is very inefficient to loop through the values and keep finding the max after each delete
• You are not allowed to modify the source map by calling the remove method from within the forEach loop of it's entry set.

Instead, get the keys corresponding to the top k values in your map and call the remove method on the map on each key, such as below..

Map<Integer, Integer> map = new HashMap<>(Map.of(1, 3, 2, 2, 3, 6, 4, 8, 5, 2, 6, 1));
int k = 3;

map.entrySet().stream().sorted((e1, e2) -> Integer.compare(e2.getValue(), e1.getValue())).map(e -> e.getKey())
            .limit(k).forEach(map::remove);

System.out.println(map);

prints: {2=2, 5=2, 6=1}

Answer 3

Using for(element e: list) loops uses the iterator of the map's entry set. This iterator can only exist once at a time. The problem can be fixed by using a traditional for loop instead: for (int i = 0; i < myMap.entrySet().size(); i++) or similar.