Can anyone show how to implement SortByValue in Tree Map for Java ?

0 votes

I want to sort a TreeMap by value instead of the default natural ordering using a comparator.

This is the code I have written but it is not working:

import java.util.*;

class treeMap {
    public static void main(String[] args) {
        System.out.println("the main");
        byValue cmp = new byValue();
        Map<String, Integer> map = new TreeMap<String, Integer>(cmp);
        map.put("de",10);
        map.put("ab", 20);
        map.put("a",5);

        for (Map.Entry<String,Integer> pair: map.entrySet()) {
            System.out.println(pair.getKey()+":"+pair.getValue());
        }
    }
}

class byValue implements Comparator<Map.Entry<String,Integer>> {
    public int compare(Map.Entry<String,Integer> e1, Map.Entry<String,Integer> e2) {
        if (e1.getValue() < e2.getValue()){
            return 1;
        } else if (e1.getValue() == e2.getValue()) {
            return 0;
        } else {
            return -1;
        }
    }
}

Please tell me how can I get a Map.Entry passed to the comparator?

Dec 10, 2018 in Java by Rishabh
• 3,580 points

edited Dec 10, 2018 by Rishabh 133 views

1 answer to this question.

0 votes

You can't have the TreeMap itself sort on the values, since that defies the SortedMap specification:

A Map that further provides a total ordering on its keys.

However, using an external collection, you can always sort Map.entrySet() however you wish, either by keys, values, or even a combination(!!) of the two.

Given below is a generic method that returns a SortedSet of Map.Entry, given a Map whose values are Comparable:

static <K,V extends Comparable<? super V>>
SortedSet<Map.Entry<K,V>> entriesSortedByValues(Map<K,V> map) {
    SortedSet<Map.Entry<K,V>> sortedEntries = new TreeSet<Map.Entry<K,V>>(
        new Comparator<Map.Entry<K,V>>() {
            @Override public int compare(Map.Entry<K,V> e1, Map.Entry<K,V> e2) {
                int res = e1.getValue().compareTo(e2.getValue());
                return res != 0 ? res : 1;
            }
        }
    );
    sortedEntries.addAll(map.entrySet());
    return sortedEntries;
}

The next step is to do the following:

    Map<String,Integer> map = new TreeMap<String,Integer>();
    map.put("A", 3);
    map.put("B", 2);
    map.put("C", 1);   

    System.out.println(map);
    // prints "{A=3, B=2, C=1}"
    System.out.println(entriesSortedByValues(map));
    // prints "[C=1, B=2, A=3]"

Note that funky stuff will happen if you try to modify either the SortedSet itself, or the Map.Entrywithin, because this is no longer a "view" of the original map like entrySet() is.


Note on == for Integer

Your original comparator compares Integer using ==. This is almost always wrong, since == with Integer operands is a reference equality, not value equality.

    System.out.println(new Integer(0) == new Integer(0)); // prints "false"!!!
answered Dec 10, 2018 by Sushmita
• 6,890 points

edited Dec 10, 2018 by Sushmita

Related Questions In Java

0 votes
2 answers

How can I convert a String variable to a primitive int in Java

 Here are two ways illustrating this: Integer x ...READ MORE

answered Aug 20, 2019 in Java by Sirajul
• 55,300 points
442 views
0 votes
1 answer

I am learning looping statements. Can you tell me how 'for-each' works in Java?

While programming we often write code that ...READ MORE

answered Apr 17, 2018 in Java by Rishabh
• 3,580 points
129 views
+15 votes
21 answers

How can I convert String to JSON object in Java?

Hi @Daisy You can use Google gson  for more ...READ MORE

answered Feb 7, 2019 in Java by Suresh
• 720 points
173,550 views
0 votes
2 answers

How can we add leading zeros to the number in Java?

From Java 1.5 you can use the String.format method. ...READ MORE

answered Aug 26, 2019 in Java by Sirajul
• 55,300 points
1,134 views
0 votes
2 answers

Ho do I Iterate through a HashMap which contains duplicate values

for (Map.Entry<String, String> item : params.entrySet()) { ...READ MORE

answered Jul 24, 2018 in Java by samarth295
• 2,190 points
609 views
0 votes
2 answers

Performing Iteration over each entry in a 'Map'

In Java 8 you can do it ...READ MORE

answered Oct 24, 2018 in Java by Sushmita
• 6,890 points
123 views
0 votes
2 answers
0 votes
1 answer

How can I Sort an ArrayList in Java

You can sort the ArrayList in 2 ...READ MORE

answered Apr 23, 2018 in Java by sharth
• 3,370 points
161 views
0 votes
3 answers

How can I add new elements to an Array in Java

String[] source = new String[] { "a", ...READ MORE

answered Sep 19, 2018 in Java by Sushmita
• 6,890 points
342 views
0 votes
2 answers

How can I sort values of a Map in Java using its key

Assuming TreeMap is not good for you ...READ MORE

answered Oct 10, 2018 in Java by Sushmita
• 6,890 points
120 views