ConcurrentHashMap compute() method in Java with Examples

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: 

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

Program 2: 

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

Program 3:To show NullPointerException 

Exception: java.lang.NullPointerException

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