Java HashMap putIfAbsent() usage and example

Java HashMap Methods

If the specified key does not appear in the HashMap, the Java HashMap putIfAbsent() method will put the specified key/The value mapping is inserted into the HashMap.

The syntax of the putIfAbsent() method is:

hashmap.putIfAbsent(K key, V value)

putIfAbsent() parameters

The putIfAbsent() method has two parameters.

• key - The specified value is mapped to key

• value - Value associated with key

putIfAbsent() return value

• 　If the specified key already exists in the hash map, return the value associated with the key.

• If the specified key does not exist in the hash map, return null
  

Note：If a null value was specified previously, return null value.

Example1：Java HashMap putIfAbsent()

import java.util.HashMap;
class Main {
　　public static void main(String[] args){
　　　　//　Create HashMap
　　　　HashMap<Integer, String> languages = new HashMap<>();
　　　　//　add mappings to HashMap
　　　　languages.put(1, "Python");
　　　　languages.put(2, "C");
　　　　languages.put(3, "Java");
　　　　System.out.println("Languages: ",　+　languages);
　　　　//The key does not exist in the HashMap
　　　　languages.putIfAbsent(4, "JavaScript");
　　　　//The key appears in the HashMap
　　　　languages.putIfAbsent(2, "Swift");
　　　　System.out.println("Updated Languages: ",　+　languages);
　　}
}

Output Result

Languages: {1=Python,　2=C,　3=Java}
Updated Languages: {1=Python,　2=C,　3=Java,　4=JavaScript}

In the above example, we created a hash map named languages. Note this line,

languages.putIfAbsent(4, "JavaScript");

Here, the key4Not associated with any value. Therefore, the putifAbsent() method will add the mapping {4 = JavaScript} added to the hash map.

Note this line,

languages.putIfAbsent(2, "Swift");

Here, the key2Already associated with Java. Therefore, the putIfAbsent() method will not add the mapping {2 = Swift} added to the hash map.

NoteWe have used the put() method to add a single mapping to the hash map. For more information, please visitJava HashMap put().

Java HashMap Methods