see Map interface for descriptions and examples of all of HashMap's methods.
■ HashMap implements Map and extends AbstractMap. It is for everyday Map usage, where the order of the keys isn’t important.
■ It allows null values and (just one) null key.
■ HashMap doesn’t provide for sorting by keys – use TreeMap to get sorted keys. HashMap's physical order of keys will be unpredictable.
■ Methods: HashMap uses all the familiar Map methods and adds nothing. see Map interface for its method examples