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:Â
Java
// Java program to demonstrate// compute(Key, BiFunction) method.Â
import java.util.concurrent.*;Â
public class GFG {Â
    // Main method    public static void main(String[] args)    {Â
        // 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);Â
        // print map details        System.out.println("ConcurrentHashMap: "                           + map.toString());Â
        // remap the values of ConcurrentHashMap        // using compute method        map.compute("Book2", (key, val)                                 -> val + 100);        map.compute("Book1", (key, val)                                 -> val + 512);Â
        // 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:Â
Java
// Java program to demonstrate// compute(Key, BiFunction) method.Â
import java.util.concurrent.*;Â
public class GFG {Â
    // Main method    public static void main(String[] args)    {Â
        // 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");Â
        // print map details        System.out.println("ConcurrentHashMap: "                           + map.toString());Â
        // 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)"));Â
        // 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 NullPointerExceptionÂ
Java
// Java program to demonstrate NullPointerException// for compute(Key, BiFunction) method.Â
import java.util.concurrent.*;Â
public class GFG {Â
    // Main method    public static void main(String[] args)    {Â
        // 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");Â
        try {Â
            // remap the values of ConcurrentHashMap            // using compute method            map.compute(null, (key, val)                                  -> val.concat(" (West-Bengal)"));        }        catch (NullPointerException e) {Â
            // print Exception            System.out.println("Exception: " + e);        }    }} |
Output:
Exception: java.lang.NullPointerException
