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.

Below programs illustrate the computeIfAbsent(Key, Function) method:

Program 1:

// 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}

Program 2:

// 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<Integer, String> 
            map = new HashMap<>(); 
        map.put(10, "Aman"); 
        map.put(20, "Suraj"); 
        map.put(30, "Harsh"); 
  
        // print map details 
        System.out.println("HashMap:\n"
                           + map.toString()); 
  
        // provide value for new key which is absent 
        // using computeIfAbsent method 
        map.computeIfAbsent(40, k -> "Sanjeet"); 
  
        // this will not effect anything 
        // because key 1 is present 
        map.computeIfAbsent(10, k -> "Amarjit"); 
  
        // print new mapping 
        System.out.println("New HashMap:\n" + map); 
    } 
} 

Output:

HashMap:
{20=Suraj, 10=Aman, 30=Harsh}
New HashMap:
{20=Suraj, 40=Sanjeet, 10=Aman, 30=Harsh}

References: https://docs.oracle.com/javase/10/docs/api/java/util/HashMap.html#computeIfAbsent(K, java.util.function.Function)

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