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

// 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

// 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 10 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)