Removes the mapping for the specified key from this map if present.
@param key key whose mapping is to be removed from the map
@return the previous value associated with <tt>key</tt>, or
<tt>null</tt> if there was no mapping for <tt>key</tt>.
(A <tt>null</tt> return can also indicate that the map
previously associated <tt>null</tt> with <tt>key</tt>.)
Removes the mapping for the specified key from this map if present.
@param key key whose mapping is to be removed from the map @return the previous value associated with <tt>key</tt>, or <tt>null</tt> if there was no mapping for <tt>key</tt>. (A <tt>null</tt> return can also indicate that the map previously associated <tt>null</tt> with <tt>key</tt>.)