public default V putIfAbsent (K key, V value)

If the specified key is not already associated with a value (or is mapped to null) associates it with the given value and returns null, else returns the current value.

Parameters:

key		key with which the specified value is to be associated
value		value to be associated with the specified key

Returns:  the previous value associated with the specified key, or null if there was no mapping for the key. (A null return can also indicate that the map previously associated null with the key, if the implementation supports null values.)

Exceptions:

UnsupportedOperationException		if the put operation is not supported by this map (optional)
ClassCastException		if the key or value is of an inappropriate type for this map (optional)
NullPointerException		if the specified key or value is null, and this map does not permit null keys or values (optional)
IllegalArgumentException		if some property 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, for this map:

 
 V v = map.get(key);
 if (v == null)
     v = map.put(key, value);

 return v;
 

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.