Java HashMap computeIfAbsent() Method

Last Updated : 11 Apr, 2025

In Java, the computeIfAbsent() method of the HashMap class is used to compute and insert a value for a specific key to a HashMap only if the key does not already have a value.

Example 1: Here, the computeIfAbsent() adds a value for a key if it is not present in the map and does nothing if the key is present in the map.

Java

// Java program to demonstrate the 
// working of computeIfAbsent()
import java.util.HashMap;

public class Geeks {
    public static void main(String[] args) {
      
        HashMap<String, Integer> hm = new HashMap<>();

        // Adding initial values to the map
        hm.put("A", 1);
        hm.put("B", 2);

        System.out.println("Initial Map: " + hm);

            // Using computeIfAbsent()
            Integer i
            = hm.computeIfAbsent("C", key -> key.length());
        Integer j
            = hm.computeIfAbsent("A", key -> key.length());

        // the key "C" is not present in the
        //  map the value is computed
        System.out.println("Value for C: " + i);

        // the key "A" is present in the 
        // map the value is not recomputed
        System.out.println("Value for A: " + j);
        System.out.println("Updated Map: " + hm);
    }
}

Output

Initial Map: {A=1, B=2}
Value for C: 1
Value for A: 1
Updated Map: {A=1, B=2, C=1}

Syntax of computeIfAbsent() Method

public V computeIfAbsent(K key, Function mappingFunction)

Parameters:

Key: The key whose associated value is to be calculated.
mappingFunction: It is a function that computes the value if the key is not present.

Return Type:

If the key is present, then it returns the existing value.
If the key is not present, it calculates the value using mappingFunction and then returns the calculated value.

Key Points:

If remapping function returns null for a key, then no mapping is recorded for that key.
At time of computation if remapping function throws an exception, the exception is rethrown, and the no mapping is recorded.
This method will throw a ConcurrentModificationException if the remapping function modified this map during computation.

Example 2: Here, the computeIfAbsent() return the existing value for a key if it is already present in the map.

Java

// Handling Existing Keys
import java.util.HashMap;

public class Geeks {
    public static void main(String[] args)
    {
        HashMap<String, String> hm = new HashMap<>();

        // Adding initial values to the map
        hm.put("A", "1");
        hm.put("B", "2");

        System.out.println("Initial Map: " + hm);

            // Use computeIfAbsent for an existing key
            String e
            = hm.computeIfAbsent("A", key -> "New Value");

        // Use computeIfAbsent for a new key
        String n = hm.computeIfAbsent("C", key -> "3");

        System.out.println("Value for existing key 'A': "
                           + e);
        System.out.println("Value for new key 'C': " + n);

        System.out.println("Updated Map: " + hm);
    }
}

Output

Initial Map: {A=1, B=2}
Value for existing key 'A': 1
Value for new key 'C': 3
Updated Map: {A=1, B=2, C=3}

Java HashMap computeIfAbsent() Method

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