Returns the value to which the specified key is mapped,
or {@code null} if this map contains no mapping for the key.
<p>More formally, if this map contains a mapping from a key
{@code k} to a value {@code v} such that {@code (key==null ? k==null :
key.equals(k))}, then this method returns {@code v}; otherwise
it returns {@code null}. (There can be at most one such mapping.)
<p>A return value of {@code null} does not <i>necessarily</i>
indicate that the map contains no mapping for the key; it's also
possible that the map explicitly maps the key to {@code null}.
The {@link #containsKey containsKey} operation may be used to
distinguish these two cases.
Returns the value to which the specified key is mapped, or {@code null} if this map contains no mapping for the key.
<p>More formally, if this map contains a mapping from a key {@code k} to a value {@code v} such that {@code (key==null ? k==null : key.equals(k))}, then this method returns {@code v}; otherwise it returns {@code null}. (There can be at most one such mapping.)
<p>A return value of {@code null} does not <i>necessarily</i> indicate that the map contains no mapping for the key; it's also possible that the map explicitly maps the key to {@code null}. The {@link #containsKey containsKey} operation may be used to distinguish these two cases.
@see #put(Object, Object)