public default V compute (K key, BiFunction<? super K, ? super V, ? extends V> remappingFunction)

Attempts to compute a mapping for the specified key and its current mapped value (or null if there is no current mapping). For example, to either create or append a String msg to a value mapping:

 
 map.compute(key, (k, v) -> (v == null) ? msg : v.concat(msg))

(Method merge() is often simpler to use for such purposes.)

If the function returns null, the mapping is removed (or remains absent if initially absent). If the function itself throws an (unchecked) exception, the exception is rethrown, and the current mapping is left unchanged.

Parameters:

key		key with which the specified value is to be associated
remappingFunction		the function to compute a value

Returns:  the new value associated with the specified key, or null if none

Exceptions:

NullPointerException		if the specified key is null and this map does not support null keys, or the remappingFunction is null
UnsupportedOperationException		if the put operation is not supported by this map (optional)
ClassCastException		if the class of the specified key or value prevents it from being stored in this map (optional)

Since:  1.8

@implSpec The default implementation is equivalent to performing the following steps for this map, then returning the current value or null if absent:

 
 V oldValue = map.get(key);
 V newValue = remappingFunction.apply(key, oldValue);
 if (oldValue != null ) {
    if (newValue != null)
       map.put(key, newValue);
    else
       map.remove(key);
  else {
    if (newValue != null)
       map.put(key, newValue);
    else
       return null;
 }
 }

The default implementation makes no guarantees about synchronization or atomicity properties of this method. Any implementation providing atomicity guarantees must override this method and document its concurrency properties. In particular, all implementations of subinterface java.util.concurrent.ConcurrentMap must document whether the function is applied once atomically only if the value is not present.