HashMap containsKey() Method in Java

The java.util.HashMap.containsKey() method is used to check whether a particular key is being mapped into the HashMap or not. It takes the key element as a parameter and returns True if that element is mapped in the map.

Syntax:

Hash_Map.containsKey(key_element)

Parameters: The method takes just one parameter key_element that refers to the key whose mapping is supposed to be checked inside a map.
Return Value: The method returns boolean true if the presence of the key is detected else false .
Below programs are used to illustrate the working of java.util.HashMap.containsKey() Method: 
Program 1: Mapping String Values to Integer Keys. 

Java

// Java code to illustrate the containsKey() 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);

        // Checking for the key_element '20'
        System.out.println("Is the key '20' present? " +
        hash_map.containsKey(20));

        // Checking for the key_element '5'
        System.out.println("Is the key '5' present? " +
        hash_map.containsKey(5));
    }
}

Output:

Initial Mappings are: {20=Geeks, 25=Welcomes, 10=Geeks, 30=You, 15=4}
Is the key '20' present? true
Is the key '5' present? false

Program 2: Mapping Integer Values to String Keys. 

Java

// Java code to illustrate the containsKey() 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);

        // Checking for the key_element 'Welcomes'
        System.out.println("Is the key 'Welcomes' present? " +
        hash_map.containsKey("Welcomes"));

        // Checking for the key_element 'World'
        System.out.println("Is the key 'World' present? " +
        hash_map.containsKey("World"));
    }
}

Output:

Initial Mappings are: {4=15, Geeks=20, You=30, Welcomes=25}
Is the key 'Welcomes' present? true
Is the key 'World' present? false

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

Time Complexity:

Time Complexity of HashMap.containsKey() is O(1) in Average Case, and O(n) in worst case.