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

Last Updated : 30 Oct, 2017

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

This article is contributed by Shambhavi Singh. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.
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