ConcurrentHashMap computeIfAbsent() method in Java with Examples

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

• If mapping function of this method returns null, then no value is recorded for new key in map.
• This method is used to atomically update a value for given key in ConcurrentHashMap.
• If the 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 because other threads can also uses this map.

Syntax:

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

Parameters: This method accepts two parameters: 

• key: key with which the value is to be associated.
• 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.
Exception: This method throws:

• NullPointerException: if the specified key or remappingFunction is null.
• llegalStateException: if the computation try to do a recursive update to this map that would otherwise never complete.
• RuntimeException: if the remappingFunction does so, in which case the mapping is unchanged.

Below programs illustrate the computeIfAbsent(Key, Function) method:
Program 1:

Java

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

import java.util.concurrent.*;

public class GFG {

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

        // create a ConcurrentHashMap and
        // add some values
        ConcurrentHashMap<String, Integer> map
            = new ConcurrentHashMap<>();
        map.put("Pencil", 1000);
        map.put("Laptop", 55000);
        map.put("Clothes", 4400);
        map.put("Mobile", 5300);

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

        // provide value for new key which is absent
        // using computeIfAbsent method
        map.computeIfAbsent("PC", k -> 60000);
        map.computeIfAbsent("Charger", k -> 800);

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

Output:

ConcurrentHashMap :
 {Laptop=55000, Clothes=4400, Pencil=1000, Mobile=5300}
new ConcurrentHashMap :
 {Laptop=55000, PC=60000, Clothes=4400, Pencil=1000, Charger=800, Mobile=5300}

Program 2:

Java

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

import java.util.concurrent.*;

public class GFG {

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

        // create a ConcurrentHashMap and
        // add some values
        ConcurrentHashMap<Integer, String> map
            = new ConcurrentHashMap<>();
        map.put(1, "1000RS");
        map.put(2, "5009RS");
        map.put(3, "1300RS");

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

        // provide value for new key which is absent
        // using computeIfAbsent method
        map.computeIfAbsent(4, k -> "6000RS");

        // this will not effect anything
        // because key 1 is present
        map.computeIfAbsent(1, k -> "8000RS");

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

Output:

ConcurrentHashMap : 
{1=1000RS, 2=5009RS, 3=1300RS}
new ConcurrentHashMap :
{1=1000RS, 2=5009RS, 3=1300RS, 4=6000RS}

Program 3: To show NullPointerException

Java

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

import java.util.concurrent.*;

public class GFG {

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

        // create a ConcurrentHashMap and
        // add some values
        ConcurrentHashMap<String, Integer> map
            = new ConcurrentHashMap<>();
        map.put("Pencil", 1000);
        map.put("Laptop", 55000);
        map.put("Clothes", 4400);
        map.put("Mobile", 5300);

        try {

            // provide value for new key which is absent
            // using computeIfAbsent method
            map.computeIfAbsent(null, k -> 60000);
        }
        catch (Exception e) {

            // print new mapping
            System.out.println("Exception: "
                               + e);
        }
    }
}

Output:

Exception: java.lang.NullPointerException

References: https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/ConcurrentHashMap.html#computeIfAbsent-K-java.util.function.Function-