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Showing content from https://www.geeksforgeeks.org/java/concurrenthashmap-keyset-method-in-java-with-examples/ below:

ConcurrentHashMap keySet() method in Java with Examples

Last Updated : 30 Oct, 2018

The

keySet()

method of

ConcurentHashMap class

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:

ConcurrentHashMap.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 keySet() Method:

Program 1:

Mapping String Values to Integer Keys.

Java


 // Java code to illustrate the keys() method

import java.util.*;
import java.util.concurrent.*;

public class ConcurrentHashMapDemo {
    public static void main(String[] args)
    {

        // Creating an empty ConcurrentHashMap
        ConcurrentHashMap<Integer, String> hash_map
            = new ConcurrentHashMap<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


 // Java code to illustrate the keys() method

import java.util.*;
import java.util.concurrent.*;

public class ConcurrentHashMapDemo {
    public static void main(String[] args)
    {

        // Creating an empty ConcurrentHashMap
        ConcurrentHashMap<String, Integer>
            hash_map = new ConcurrentHashMap<String,
                                             Integer>();

        // Inserting elements into the table
        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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