How to Sort a Map by Value in Java 8+ (original) (raw)

This has been one of the most frequently asked questions in Java interviews. With streams introduced after Java 8, this can be achieved in an elegant and functional way.

For example, let us consider I map a word and its corresponding counts in a particular document as shown below:

final Map<String, Integer> wordCounts = new HashMap<>();
wordCounts.put("USA", 100);
wordCounts.put("jobs", 200);
wordCounts.put("software", 50);
wordCounts.put("technology", 70);
wordCounts.put("opportunity", 200);

Now, if I have to sort this map with value in ascending order, then it would be simplest and readable as below:

final Map<String, Integer> sortedByCount = wordCounts.entrySet()
                .stream()
                .sorted(Map.Entry.comparingByValue())
                .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (e1, e2) -> e1, LinkedHashMap::new));

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Here, I am usingLinkedHashMap to store the sorted result to preserve the order of the elements in the resulting map.

The advantages of this approach are:

1. It doesn't modify the original data wordCounts, making it more thread safe.
2. It is more readable.

If you want to sort a map in reverse order, then you just need to specify comparing the value as reversed order as:

final Map<String, Integer> sortedByCount = wordCounts.entrySet()
                .stream()
                .sorted((Map.Entry.<String, Integer>comparingByValue().reversed()))
                .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (e1, e2) -> e1, LinkedHashMap::new));

The complete program for sorting in reverse order is:

public class SortMapByValueExample {
    public static Map<String, Integer> sortByValue(final Map<String, Integer> wordCounts) {

        return wordCounts.entrySet()
                .stream()
                .sorted((Map.Entry.<String, Integer>comparingByValue().reversed()))
                .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (e1, e2) -> e1, LinkedHashMap::new));
    }

    public static void main(String[] args) {
        final Map<String, Integer> wordCounts = new HashMap<>();
        wordCounts.put("USA", 100);
        wordCounts.put("jobs", 200);
        wordCounts.put("software", 50);
        wordCounts.put("technology", 70);
        wordCounts.put("opportunity", 200);

        final Map<String, Integer> sortedByCount = sortByValue(wordCounts);

        System.out.println(sortedByCount);
    }
}

The output of the program is:

{jobs=200, opportunity=200, USA=100, technology=70, software=50}

You can see that the sorted() method takesComparator as an argument, making it possible to sort a map with any kind of value. For example, the above sort can be written with the Comparator as:

public static Map<String, Integer> sortByValue(final Map<String, Integer> wordCounts) {

        return wordCounts.entrySet()
                .stream()
                .sorted((e1, e2) -> e2.getValue().compareTo(e1.getValue()))
                .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (e1, e2) -> e1, LinkedHashMap::new));
    }

You can implement Comparator with any kind of object. Isn't this a cool feature?

If you enjoyed this article and want to learn more about Java Collections, check out this collection of tutorials and articles on all things Java Collections.