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HashMap keySet() Method in Java

  • Difficulty Level : Basic
  • Last Updated : 26 Nov, 2018

The java.util.HashMap.keySet() method in Java is used to create a set out of the key elements contained in the hash map. It basically returns a set view of the keys or we can create a new set and store the key elements in them.

Syntax:

hash_map.keySet()

Parameters: The method does not take any parameter.

Return Value: The method returns a set having the keys of the hash map.

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






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

Program 2: Mapping Integer Values to String Keys.




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

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

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