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Showing content from https://www.geeksforgeeks.org/java/concurrenthashmap-compute-method-in-java-with-examples/ below:

ConcurrentHashMap compute() method in Java with Examples

Last Updated : 29 Jun, 2022

The compute(Key, BiFunction) method of ConcurrentHashMap class is used to compute a mapping for the specified key and its current mapped value (or null if there is no current mapping is found).

This method is used to atomically update a value for given key in ConcurrentHashMap.
If the remapping function throws an exception, the exception is re-thrown, and the current mapping is left unchanged.
During computation, update process on this map from other threads has been blocked so the computation must not attempt to update any other mappings of this Map
For example, This mapping append string value of mapping:

ConcurrentHashMap.compute(key, 
(key, value) -> (value == null) ? msg : value.concat(msg))

Syntax:

public V 
       compute(K key,
               BiFunction<? super K, ? super V, 
                  ? 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 new value associated with the specified key, or null if none. Exception: This method throws following exceptions:

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 compute(Key, BiFunction) method: Program 1:

Java


 // Java program to demonstrate
// compute(Key, BiFunction) 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("Book1", 10);
        map.put("Book2", 500);
        map.put("Book3", 400);

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

        // remap the values of ConcurrentHashMap
        // using compute method
        map.compute("Book2", (key, val)
                                 -> val + 100);
        map.compute("Book1", (key, val)
                                 -> val + 512);

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

Output:

ConcurrentHashMap: {Book3=400, Book1=10, Book2=500}
New ConcurrentHashMap: {Book3=400, Book1=522, Book2=600}

Program 2:

Java


 // Java program to demonstrate
// compute(Key, BiFunction) 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, "Kolkata");
        map.put(2, "Nadia");
        map.put(3, "Howrah");

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

        // remap the values of ConcurrentHashMap
        // using compute method
        map.compute(2, (key, val)
                           -> val.concat(" (West-Bengal)"));
        map.compute(3, (key, val)
                           -> val.concat(" (West-Bengal)"));

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

Output:

ConcurrentHashMap: {1=Kolkata, 2=Nadia, 3=Howrah}
New ConcurrentHashMap: {1=Kolkata, 2=Nadia (West-Bengal), 3=Howrah (West-Bengal)}

Program 3:To show NullPointerException

Java


 // Java program to demonstrate NullPointerException
// for compute(Key, BiFunction) 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, "Kolkata");
        map.put(2, "Nadia");
        map.put(3, "Howrah");

        try {

            // remap the values of ConcurrentHashMap
            // using compute method
            map.compute(null, (key, val)
                                  -> val.concat(" (West-Bengal)"));
        }
        catch (NullPointerException e) {

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

Output:

Exception: java.lang.NullPointerException

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

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