Usage and examples of Java HashMap containsKey()

Java HashMap Methods

The Java HashMap containsKey() method checks if there is a mapping for the specified key in the hash map.

The syntax of containsKey() method is:

hashmap.containsKey(Object key)

containsKey() parameter

• key -　 Check for key mapping in hash map

Returns value of containsKey()

• Returns true if there is a mapping for the specified key in the hash map
  

• Returns false if there is no mapping for the specified key in the hash map
  

Example1: Java HashMap containsKey()

import java.util.HashMap;
class Main {
　　public static void main(String[] args){
　　　　//Create HashMap
　　　　HashMap<String, String> details = new HashMap<>();
　　　　//Add the mapping to the HashMap
　　　　details.put("Name", "w3codebox");
　　　　details.put("Domain", "oldtoolbag.com");
　　　　details.put("Location", "Nepal");
　　　　System.out.println("w3codebox details: 
"　+　details);
　　　　//Check if the key "Domain" exists
　　　　if(details.containsKey("Domain")) {
　　　　　　System.out.println("Domain exists in Hashmap");
　　　　}
　　}
}

Output Result

w3codebox details:　
{Domain=oldtoolbag.com, Name=w3codebox, Location=Nepal}
Domain exists in Hashmap

In the above example, we created a hash map. Note these expressions,

details.containsKey("Domain")　//　Return true

　 Here, hashmap contains a mapping with the key of Domain. Therefore, the if statement block is executed, and the containsKey() method will return true and the statement.

Example2：If the key does not exist, add the entry to the HashMap

import java.util.HashMap;
class Main {
　　public static void main(String[] args){
　　　　//　Create a HashMap
　　　　HashMap<String, String> countries = new HashMap<>();
　　　　//　Add the mapping to the HashMap
　　　　countries.put("USA", "Washington");
　　　　countries.put("Australia", "Canberra");
　　　　System.out.println("HashMap:\n"　+　countries);
　　　　//　Check if the key "Spain" exists
　　　　if(!countries.containsKey("Spain")) {
　　　　　　//　If the key does not exist, add the entry
　　　　　　countries.put("Spain", "Madrid");
　　　　}
　　　　System.out.println("Updated HashMap:\n"　+　countries);
　　}
}

Output Result

HashMap:
{USA=Washington, Australia=Canberra}
Updated HashMap:
{USA=Washington, Australia=Canberra, Spain=Madrid}

In the above example, please note the following expression:

if(!countries.containsKey("Spain")) {..}

Here, we used the containsKey() method to check if there is a mapping for Spain in the hash map. Since we used the negation symbol (!), if this method returns false, the if block will be executed.

　Therefore, a new mapping is only added if there is no specified key mapping in the hashmap.

Note: We can also useHashMap putIfAbsent()Methods perform the same task.

Java HashMap Methods