Replaces the entry for a key only if currently mapped to some value.
This is equivalent to, for this {@code map}:
<pre> {@code
if (map.containsKey(key))
return map.put(key, value);
else
return null;}</pre>
except that the action is performed atomically.
@implNote This implementation intentionally re-abstracts the
inappropriate default provided in {@code Map}.
@param key key with which the specified value is associated
@param value value to be associated with the specified key
@return the previous value associated with the specified key, or
{@code null} if there was no mapping for the key.
(A {@code null} return can also indicate that the map
previously associated {@code null} with the key,
if the implementation supports null values.)
@throws UnsupportedOperationException if the {@code 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 NullPointerException if the specified key or value is null,
and this map does not permit null keys or values
@throws IllegalArgumentException if some property of the specified key
or value prevents it from being stored in this map
Replaces the entry for a key only if currently mapped to some value. This is equivalent to, for this {@code map}: <pre> {@code if (map.containsKey(key)) return map.put(key, value); else return null;}</pre>
except that the action is performed atomically.
@implNote This implementation intentionally re-abstracts the inappropriate default provided in {@code Map}.
@param key key with which the specified value is associated @param value value to be associated with the specified key @return the previous value associated with the specified key, or {@code null} if there was no mapping for the key. (A {@code null} return can also indicate that the map previously associated {@code null} with the key, if the implementation supports null values.) @throws UnsupportedOperationException if the {@code 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 NullPointerException if the specified key or value is null, and this map does not permit null keys or values @throws IllegalArgumentException if some property of the specified key or value prevents it from being stored in this map