{
/**
* If the specified key is not already associated
* with a value, associate it with the given value.
* This is equivalent to
*
* if (!map.containsKey(key)) map.put(key, value);
* return get(key);
*
* Except that the action is performed atomically.
* @param key key with which the specified value is to be associated.
* @param value value to be associated with the specified key.
* @return previous value associated with specified key, or null
* if there was no mapping for key. A null return can
* also indicate that the map previously associated null
* with the specified key, if the implementation supports
* null values.
*
* @throws UnsupportedOperationException if the put operation is
* not supported by this map.
* @throws ClassCastException if the class of the specified key or value
* prevents it from being stored in this map.
* @throws IllegalArgumentException if some aspect of this key or value
* prevents it from being stored in this map.
* @throws NullPointerException this map does not permit null
* keys or values, and the specified key or value is
* null.
*
**/
public V putIfAbsent(K key, V value);
}