Java TreeMap descendingKeySet() Method

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Description

The Java TreeMap descendingKeySet() method is used to return a reverse order NavigableSet view of the keys contained in this map. The set's iterator returns the keys in descending order. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa.

Declaration

Following is the declaration for java.util.TreeMap.descendingKeySet() method.

public NavigableSet<K> descendingKeySet()

Parameters

NA

Return Value

The method call returns a reverse order navigable set view of the keys in this map.

Exception

NA

Getting a Navigable Set of Keys in Descending Order from a TreeMap of Integer,Integer Pairs Example

The following example shows the usage of Java TreeMap descendingKeySet() method to get a navigable set of keys ordered in a descending order. We've created a TreeMap object of Integer,Integer pairs. Then few entries are added to map and using descendingKeySet() we've retrieved the navigable set of keys and then printed the same.

package com.tutorialspoint;

import java.util.NavigableSet;
import java.util.TreeMap;

public class TreeMapDemo {
   public static void main(String[] args) {
      
      // creating tree map 
      TreeMap<Integer, Integer> treemap = new TreeMap<>();

      // populating tree map
      treemap.put(2, 2);
      treemap.put(1, 1);
      treemap.put(3, 3);
      treemap.put(6, 6);
      treemap.put(5, 5);

      // putting values in navigable set
      NavigableSet nset = treemap.descendingKeySet();

      System.out.println("Checking value");
      System.out.println("Navigable set values: "+nset);
   }    
}

Output

Let us compile and run the above program, this will produce the following result.

Checking value
Navigable set values: [6, 5, 3, 2, 1]

Getting a Navigable Set of Keys in Descending Order from a TreeMap of Integer,String Pairs Example

The following example shows the usage of Java TreeMap descendingKeySet() method to get a navigable set of keys ordered in a descending order. We've created a TreeMap object of Integer,String pairs. Then few entries are added to map and using descendingKeySet() we've retrieved the navigable set of keys and then printed the same.

package com.tutorialspoint;

import java.util.NavigableSet;
import java.util.TreeMap;

public class TreeMapDemo {
   public static void main(String[] args) {
      
      // creating tree map 
      TreeMap<Integer, String> treemap = new TreeMap<>();

      // populating tree map
      treemap.put(2, "two");
      treemap.put(1, "one");
      treemap.put(3, "three");
      treemap.put(6, "six");
      treemap.put(5, "five");

      // putting values in navigable set
      NavigableSet nset = treemap.descendingKeySet();

      System.out.println("Checking value");
      System.out.println("Navigable set values: "+nset);
   }    
}

Output

Let us compile and run the above program, this will produce the following result.

Checking value
Navigable set values: [6, 5, 3, 2, 1]

Getting a Navigable Set of Keys in Descending Order from a TreeMap of Integer,Object Pairs Example

The following example shows the usage of Java TreeMap descendingKeySet() method to get a navigable set of keys ordered in a descending order. We've created a TreeMap object of Integer,Student pairs. Then few entries are added to map and using descendingKeySet() we've retrieved the navigable set of keys and then printed the same.

package com.tutorialspoint;

import java.util.NavigableSet;
import java.util.TreeMap;

public class TreeMapDemo {
   public static void main(String[] args) {
      
      // creating tree map 
      TreeMap<Integer, Student> treemap = new TreeMap<>();

      // populating tree map
      treemap.put(2, new Student(2, "Robert"));
      treemap.put(1, new Student(1, "Julie"));  
      treemap.put(3, new Student(3, "Adam"));
      treemap.put(6, new Student(6, "Julia"));
      treemap.put(5, new Student(5, "Tom"));

      // putting values in navigable set
      NavigableSet nset = treemap.descendingKeySet();

      System.out.println("Checking value");
      System.out.println("Navigable set values: "+nset);
   }    
}
class Student {
   int rollNo;
   String name;

   Student(int rollNo, String name){
      this.rollNo = rollNo;
      this.name = name;
   }

   @Override
   public String toString() {
      return "[ " + this.rollNo + ", " + this.name + " ]";
   }
   @Override
   public boolean equals(Object obj) {
      if(obj == null) return false;
      Student s = (Student)obj;
      return this.rollNo == s.rollNo && this.name.equalsIgnoreCase(s.name);
   }
}

Output

Let us compile and run the above program, this will produce the following result.

Checking value
Navigable set values: [6, 5, 3, 2, 1]