ConcurrentMap JDK 1.6)

public V putIfAbsent ( K key, V value)

If the specified key is not already associated with a value, associate it with the given value.

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))
       return map.put(key, value);
   else
       return map.get(key);

except that the action is performed atomically.
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.)
Parameters:
- key - key with which the specified value is to be associated
- value - value to be associated with the specified key
Throws:
- UnsupportedOperationException - if the put operation is not supported by this map
- ClassCastException - if the class of the specified key or value prevents it from being stored in this map
- NullPointerException - if the specified key or value is null, and this map does not permit null keys or values
- IllegalArgumentException - if some property of the specified key or value prevents it from being stored in this map

public boolean remove (Object key, Object value)

Removes the entry for a key only if currently mapped to a given value.

Removes the entry for a key only if currently mapped to a given value.
This is equivalent to


   if (map.containsKey(key) && map.get(key).equals(value)) {
       map.remove(key);
       return true;
   } else return false;

except that the action is performed atomically.
Returns: true if the value was removed
Parameters:
- key - key with which the specified value is associated
- value - value expected to be associated with the specified key
Throws:
- UnsupportedOperationException - if the remove operation is not supported by this map
- 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)

public V replace ( K key, V value)

Replaces the entry for a key only if currently mapped to some value.

Replaces the entry for a key only if currently mapped to some value.
This is equivalent to


   if (map.containsKey(key)) {
       return map.put(key, value);
   } else return null;

except that the action is performed atomically.
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.)
Parameters:
- key - key with which the specified value is associated
- value - value to be associated with the specified key
Throws:
- UnsupportedOperationException - if the put operation is not supported by this map
- ClassCastException - if the class of the specified key or value prevents it from being stored in this map
- NullPointerException - if the specified key or value is null, and this map does not permit null keys or values
- IllegalArgumentException - if some property of the specified key or value prevents it from being stored in this map

public boolean replace ( K key, V oldValue, V newValue)

Replaces the entry for a key only if currently mapped to a given value.

Replaces the entry for a key only if currently mapped to a given value.
This is equivalent to


   if (map.containsKey(key) && map.get(key).equals(oldValue)) {
       map.put(key, newValue);
       return true;
   } else return false;

except that the action is performed atomically.
Returns: true if the value was replaced
Parameters:
- key - key with which the specified value is associated
- oldValue - value expected to be associated with the specified key
- newValue - value to be associated with the specified key
Throws:
- UnsupportedOperationException - if the put operation is not supported by this map
- ClassCastException - if the class of a specified key or value prevents it from being stored in this map
- NullPointerException - if a specified key or value is null, and this map does not permit null keys or values
- IllegalArgumentException - if some property of a specified key or value prevents it from being stored in this map