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: 
 

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: 
 

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

Program 3: To show NullPointerException
 

Exception: java.lang.NullPointerException

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