Using HashMap and Sort it using TreeMap with Descending Key Values

Question

I have this code to populate a HashMap and pass it to TreeMap to sort it in natural order in Key values.

    Map<Integer, String[]> hashMap = new HashMap<Integer, String[]>();
    hashMap.put(3, new String[]{"1","2"});
    hashMap.put(2, new String[]{"1","2"});
    hashMap.put(4, new String[]{"1","2"});
    hashMap.put(1, new String[]{"1","2"});

    System.out.println(hashMap);

    Map<Integer, String[]> treeMap = new TreeMap<Integer, String[]>(hashMap);

    System.out.println(hashMap); // Natural Order, Ascending

Now my problems is, How can I sort my treeMap in Descending order? I've prepared my Comparator class named KeyComparator that sort Key to descending order. Here is my code below:

public class KeyComparator implements Comparator<Integer> {
    @Override
    public int compare(Integer o1, Integer o2) {
        if (o1 < o2) {
            return 1;
        } else if (o1 > o2) {
            return -1;
        } else {
            return 0;
        }
    }
}

The TreeMap has no 2 parameterized Constructor like for example TreeMap(new KeyComparator(),hashMap). How can I use my KeyComparator class at the same time use to load the hashMap into my treeMap.

If all you need is descending natural order then you could use new TreeMap<Integer, String[]>(hashMap).descendingMap() — Thomas Kläger
– Thomas Kläger, Commented Nov 29, 2016 at 11:33

Mritunjay · Accepted Answer · 2016-11-29 11:26:04Z

3

Do it like below, Create a treemap with Comparator and then use putAll method to pass the hashmap to it.

Map<Integer, String[]> treeMap = new TreeMap<Integer, String[]>(new Comparator<Integer>() {
    @Override
    public int compare(Integer o1, Integer o2) {
        if (o1 < o2) {
            return 1;
        } else if (o1 > o2) {
            return -1;
        } else {
            return 0;
        }
    }
});
treeMap.putAll(hashMap);

System.out.println(treeMap); // Descending

Note:- In the example above, I have passed an anonymous implementation of Comparator. You can always pass the KeyComparator in the constructor.

answered Nov 29, 2016 at 11:26

Mritunjay

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1 Comment

msagala25 Over a year ago

treeMap.putAll really helps. This is the first time I used it. thanks.

guest1423 · Accepted Answer · 2016-11-29 11:28:42Z

1

While this may not answer your question, why not simply use an Array to sort it?

System.out.println("map: " + hashMap);
List<Integer> keys = new ArrayList<Integer>(hashMap.keySet());
System.out.println("unsorted keys: " + keys);
Collections.sort(keys);
System.out.println("sorted (asc) keys: " + keys);
Collections.reverse(keys);
System.out.println("sorted (desc) keys: " + keys);

answered Nov 29, 2016 at 11:28

guest1423

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1 Comment

msagala25 Over a year ago

Sorry but I need it in a TreeMap way, btw thanks for some help.

Nicolas Filotto · Accepted Answer · 2016-11-29 11:37:00Z

1

As already mentioned, you need first to use the constructor with the key comparator which is TreeMap(Comparator<? super K> comparator) then use putAll(Map<? extends K,? extends V> m) to add all entries.

If you use Java 8 no need to reimplement it, simply use Comparator.reverseOrder() to get the reverse of the natural ordering as next:

Map<Integer, String[]> treeMap = new TreeMap<>(Comparator.reverseOrder());
treeMap.putAll(hashMap);
System.out.println(treeMap);

answered Nov 29, 2016 at 11:37

Nicolas Filotto

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1 Comment

msagala25 Over a year ago

Im using Java 7. treeMap.putAll is all I need, thanks by the way to overview me in Java 8.

n247s · Accepted Answer · 2016-11-29 11:38:50Z

0

The TreeMap has a nice method for this. You can do

 TreeMap<Integer, String[]> map = new TreeMap<>(hashMap);
 NavigleMap<Integer, String[]> reversedMap = map.descendingMap();

answered Nov 29, 2016 at 11:38

n247s

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Comments

gipsy · Accepted Answer · 2016-11-29 11:57:53Z

0

Create an empty TreeMap with your comparator first

Map<Integer, String[]> treeMap = new TreeMap<Integer, String[]>(new KeyComparator());

treeMap.putAll(hashMap);

http://docs.oracle.com/javase/7/docs/api/java/util/TreeMap.html#TreeMap(java.util.Comparator)

You may have to tweak the syntax

answered Nov 29, 2016 at 11:57

gipsy

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