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TreeMap keySet() Method in Java
  • Last Updated : 09 Jul, 2018

The java.util.TreeMap.keySet() method in Java is used to create a set out of the key elements contained in the treemap. It basically returns a set view of the keys or we can create a new set and store the key elements in them in an ascending order. Since the set is backed by the map, any changes made to the map are reflected in the set, and vice-versa.

Syntax:

tree_map.keySet()

Parameters: The method does not take any parameter.

Return Value: The method returns a set having the keys of the treemap in ascending order.

Below programs are used to illustrate the working of java.util.TreeMap.keySet() Method:
Program 1: Mapping String Values to Integer Keys.






// Java code to illustrate the keySet() method
import java.util.*;
  
public class Tree_Map_Demo {
    public static void main(String[] args)
    {
  
        // Creating an empty TreeMap
        TreeMap<Integer, String> tree_map = 
                  new TreeMap<Integer, String>();
  
        // Mapping string values to int keys
        tree_map.put(10, "Geeks");
        tree_map.put(15, "4");
        tree_map.put(20, "Geeks");
        tree_map.put(25, "Welcomes");
        tree_map.put(30, "You");
  
        // Displaying the TreeMap
        System.out.println("Initial Mappings are: " +
                             tree_map);
  
        // Using keySet() to get the set view of keys
        System.out.println("The set is: "
                            tree_map.keySet());
    }
}
Output:
Initial Mappings are: {10=Geeks, 15=4, 20=Geeks, 25=Welcomes, 30=You}
The set is: [10, 15, 20, 25, 30]

Program 2: Mapping Integer Values to String Keys.




// Java code to illustrate the keySet() method
import java.util.*;
  
public class Tree_Map_Demo {
    public static void main(String[] args)
    {
  
        // Creating an empty TreeMap
        TreeMap<String, Integer> tree_map = new 
                      TreeMap<String, Integer>();
  
        // Mapping int values to string keys
        tree_map.put("Geeks", 10);
        tree_map.put("4", 15);
        tree_map.put("Geeks", 20);
        tree_map.put("Welcomes", 25);
        tree_map.put("You", 30);
  
        // Displaying the TreeMap
        System.out.println("Initial Mappings are: " +
                            tree_map);
  
        // Using keySet() to get the set view of keys
        System.out.println("The set is: "
                             tree_map.keySet());
    }
}
Output:
Initial Mappings are: {4=15, Geeks=20, Welcomes=25, You=30}
The set is: [4, Geeks, Welcomes, You]

Note: The same operation can be performed with any type of Mappings with variation and combination of different data types.

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