Java.util.TreeMap.descendingMap() and descendingKeyset() in Java

There are two variants of descending() in Java.util.TreeMap, both are discussed in this article.

1. descendingKeySet(): It returns a reverse order Navigable Set view of the keys contained in the map.

Syntax : 
public NavigableSet descendingKeySet()
Parameters:
NA
Return Value:
It returns a reverse order navigable set view of the keys in this map.
Exception:
NA

// Java code to demonstrate the working
// descendingKeySet()
import java.io.*;
import java.util.*;
public class descendingKeySet1 {
public static void main(String[] args)
    {

        // Declaring the tree map of Integer and String
        TreeMap<Integer, String> treemap = new TreeMap<Integer, String>();

        // assigning the values in the tree map
        // using put()
        treemap.put(2, "two");
        treemap.put(0, "zero");
        treemap.put(3, "three");
        treemap.put(6, "six");
        treemap.put(9, "nine");
        treemap.put(7, "seven");

        // putting values in navigable set
        // use of descendingKeySet
        NavigableSet set1 = treemap.descendingKeySet();

        System.out.println("Navigable set values are: " + set1);
    }
}

Output:

Navigable set values are: [9, 7, 6, 3, 2, 0]

2. descendingMap() : It returns a reverse order view of the mappings contained in the map.

Syntax : 
public NavigableMap descendingMap()
Parameters:
NA
Return Value
It returns a reverse order view of the map.
Exception:
NA

// Java code to demonstrate the working
// of descendingMap()
import java.io.*;
import java.util.*;
public class descendingMap {
public static void main(String[] args)
    {

        // Declaring the tree map of Integer and String
        TreeMap<Integer, String> treemap = new TreeMap<Integer, String>();

        // assigning the values in the tree map
        // using put()
        treemap.put(2, "two");
        treemap.put(0, "zero");
        treemap.put(3, "three");
        treemap.put(6, "six");
        treemap.put(9, "nine");
        treemap.put(7, "seven");

        // putting values in navigable map
        // use of descendingMap()
        NavigableMap map1 = treemap.descendingMap();

        System.out.println("Navigable map values are: " + map1);
    }
}

Output:

Navigable map values are: {9=nine, 7=seven, 6=six, 3=three, 2=two, 0=zero}

Practical Application : There are many applications possible of descending functions explained in this article. Some among them are priority scheduling, or designing a ranking system. Latter one is demonstrated in the code below.

// Java code to demonstrate the application
// of descendingMap() and descendingKetSet()
import java.io.*;
import java.util.*;
public class descendingAppli {
public static void main(String[] args)
    {

        // Declaring the tree map of Integer and String
        // to store participants info of scores with name
        TreeMap<Integer, String> participants = new TreeMap<Integer, String>();

        // assigning score of participants
        // using put()
        participants.put(30, "Ashty");
        participants.put(45, "Shavi");
        participants.put(16, "Vaish");
        participants.put(15, "Kil");
        participants.put(11, "Manju");

        // putting ranks in NavigableMap
        // use of descendingMap() to assign 1st to
        // maximum values and so on
        NavigableMap<Integer, String> Ranks = participants.descendingMap();

        System.out.println("The ranks according to scores are : ");

        // Printing values rankwise
        int count = 0;
        for (NavigableMap.Entry<Integer, String> entry : Ranks.entrySet()) {
            count++;
            String str = Integer.toString(count);
            System.out.println("Rank " + str + ": " + entry.getValue());
        }
    }
}

Output:

The ranks according to scores are : 
Rank 1: Shavi
Rank 2: Ashty
Rank 3: Vaish
Rank 4: Kil
Rank 5: Manju