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