HashMap computeIfAbsent() method in Java with Examples

The computeIfAbsent(Key, Function) method of HashMap class is used to compute value for a given key using the given mapping function, if key is not already associated with a value (or is mapped to null) and enter that computed value in Hashmap else null.

• If mapping function of this method returns null, 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.
• During computation, modification this map using this method is not allowed.
• This method will throw a ConcurrentModificationException if the remapping function modified this map during computation.

Syntax:

public V 
       computeIfAbsent(K key,
             Function<? super K, ? extends V> remappingFunction)

Parameters: This method accepts two parameters:

key : key for which we want to compute value using mapping.

remappingFunction : function to do the operation on value.

Returns: This method returns current (existing or computed) value associated with the specified key, or null if mapping returns null.

// Java program to demonstrate 
// computeIfAbsent(Key, Function) method. 

import java.util.*; 

public class GFG { 

    // Main method 
    public static void main(String[] args) 
    { 

        // create a HashMap and add some values 
        HashMap<String, Integer> map 
            = new HashMap<>(); 
        map.put("key1", 10000); 
        map.put("key2", 55000); 
        map.put("key3", 44300); 
        map.put("key4", 53200); 

        // print map details 
        System.out.println("HashMap:\n "
                           + map.toString()); 

        // provide value for new key which is absent 
        // using computeIfAbsent method 
        map.computeIfAbsent("key5", 
                            k -> 2000 + 33000); 
        map.computeIfAbsent("key6", 
                            k -> 2000 * 34); 

        // print new mapping 
        System.out.println("New HashMap:\n "
                           + map); 
    } 
} 

OutPut:

HashMap:
 {key1=10000, key2=55000, key3=44300, key4=53200}
New HashMap:
 {key1=10000, key2=55000, key5=35000, key6=68000, key3=44300, key4=53200}