TreeMap Customized Comparator

Question

So I want to use TreeMap with a customized comparator My key is a String: id, my value is an int: count; I NEED TO COMPARE THE COUNT, AS THE VALUE(INTEGER) IN THE TREEMAP So I have:

In one class:

import java.util.*;

public TreeMap<String, Integer> tm = new TreeMap<String, Integer>(new SortIdCount<Integer>());

In another class:

import java.util.Comparator;

public class SortIdCount implements Comparator<Integer>{

    public int compare(Integer count1, Integer count2) {
        return count1.compareTo(count2);
    }

}

It shows error in eclipse:

The type SortIdCount is not generic; it cannot be parameterized with arguments <Integer>

Answer 1

The type SortIdCount is not generic; it cannot be parameterized with arguments < Integer >

Reason : Class SortIdCount is not genric type so you can not pass parameterized argument. Error At Line : (new SortIdCount<Integer>()

Note : A TreeMap is always sorted based on its keys, however if you want to sort it based on its values then you can build a logic to do this using comparator. Below is a complete code of sorting a TreeMap by values.

To Sort on Values You can refer below code snippet.

import java.util.*;

public class TreeMapDemo {
  //Method for sorting the TreeMap based on values
  public static <K, V extends Comparable<V>> Map<K, V> 
    sortByValues(final Map<K, V> map) {
    Comparator<K> valueComparator = 
             new Comparator<K>() {
      public int compare(K k1, K k2) {
        int compare = 
              map.get(k1).compareTo(map.get(k2));
        if (compare == 0) 
          return 1;
        else 
          return compare;
      }
    };

    Map<K, V> sortedByValues = 
      new TreeMap<>(valueComparator);
    sortedByValues.putAll(map);
    return sortedByValues;
  }
  public static void main(String args[]) {

    TreeMap<String, Integer> treemap = new TreeMap<>();

    // Put elements to the map
    treemap.put("Key1", 5);
    treemap.put("Key2", 4);
    treemap.put("Key3", 3);
    treemap.put("Key4", 2);
    treemap.put("Key5", 1);

    // Calling the method sortByvalues
    Map sortedMap = sortByValues(treemap);

    // Get a set of the entries on the sorted map
    Set set = sortedMap.entrySet();

    // Get an iterator
    Iterator i = set.iterator();

    // Display elements
    while(i.hasNext()) {
      Map.Entry me = (Map.Entry)i.next();
      System.out.print(me.getKey() + ": ");
      System.out.println(me.getValue());
    }
  }
}

For more details refer this answer

Answer 2

As others might already have mentioned, that the Comparator used in the TreeMap constructor is used to sort the map by key.

public TreeMap(Comparator comparator)

... ordered according to the given comparator. All keys inserted into the map must be mutually comparable by the given comparator: comparator.compare(k1, k2) must not throw a ClassCastException for any keys k1 and k2 in the map...

But if you want to sort a map by the value, still we have a solution using LinkedHashMap as:

    Map<String, Integer> treeMap = new TreeMap<>();
    treeMap.put("hi", 1);
    treeMap.put("there", 3);
    treeMap.put("hey", 2);
    treeMap = treeMap.entrySet().stream().sorted((e1, e2) -> e2.getValue().compareTo(e1.getValue()))
            .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (oldV, newV) -> oldV, LinkedHashMap::new));
    System.out.println(treeMap);

Output:

{there=3, hey=2, hi=1}