What is the easiest way to sort maps according to values in Java?

Question

I want my hash to sort in descending order according to the values. How do I do that in Java?

Answer 1

A HashMap (and its legacy predecesor Hashtable ) is by nature unordered. Even if you sort it, it will remain unordered. If you want to maintain insertion order, then use LinkedHashMap instead. If you want an automatic sort on keys , regardless of insertion order, then use SortedMap instead.

If you want to sort a Map on values , then you basically need to put the key/value pairs in another kind of a sortable data structure, eg List<Entry<K, V>> , then sort it using Collections#sort() with help of a Compatator<Entry<K, V>> and finally repopulate a LinkedHashMap with it (not a HashMap or you will lose the ordering again).

Here's a basic example (leaving obvious runtime exception handling aside):

// Prepare.
Map<String, String> map = new HashMap<String, String>();
map.put("foo", "bar");
map.put("bar", "waa");
map.put("waa", "foo");
System.out.println(map); // My JVM shows {waa=foo, foo=bar, bar=waa}

// Get entries and sort them.
List<Entry<String, String>> entries = new ArrayList<Entry<String, String>>(map.entrySet());
Collections.sort(entries, new Comparator<Entry<String, String>>() {
    public int compare(Entry<String, String> e1, Entry<String, String> e2) {
        return e1.getValue().compareTo(e2.getValue());
    }
});

// Put entries back in an ordered map.
Map<String, String> orderedMap = new LinkedHashMap<String, String>();
for (Entry<String, String> entry : entries) {
    orderedMap.put(entry.getKey(), entry.getValue());
}

System.out.println(orderedMap); // {foo=bar, waa=foo, bar=waa}

To sort it descencing , use the following Comparator . Basically just swap the entries to compare:

Collections.sort(entries, new Comparator<Entry<String, String>>() {
    public int compare(Entry<String, String> e1, Entry<String, String> e2) {
        return e2.getValue().compareTo(e1.getValue()); // Sorts descending.
    }
});

Answer 2

使用Collections.reverseOrder（） 。

Answer 3

Here is how I do it:

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(k2).compareTo(map.get(k1));
        if (compare == 0) return 1;
        else return compare;
    }
};
Map<K, V> sortedByValues = new TreeMap<K, V>(valueComparator);
sortedByValues.putAll(map);
return sortedByValues;

}