• Permits multiple null values and one null key
• Equivalent
to Hashtable, except that it is unsynchronized( i.e. it’s methods are
unsynchronized) and permits nulls (Hashtable does not allows null keys
or values)
• Does not guarantee that the order of map will remain constant over time.
• Provides
constant-time performance for the basic operations (get and put),
assuming the hash function disperses the elements properly among the
buckets.
• Iteration time is proportional to the "capacity" of the HashMap instance plus its actual size
• In
multi-threaded environment, to prevent accidental unsynchronized access
to the map that can modify HashMap structurally, wrap HashMap using
Collections.synchronizedMap
Map m = Collections.synchronizedMap(new HashMap(...));
• Iterators of HashMaps are fail-fast
• Fail-fast
iterators throw ConcurrentModificationException on a best-effort basis.
But one must not write program that depends on this behaviour.
• get()
method returns the value to which the specified key is mapped, or null
if this map contains no mapping for the key. However, null does not
necessarily indicate that the map contains no mapping for the key; it's
also possible that the map explicitly maps the key to null.
• When
duplicate key put into HashMap, put() method overrides old value with
new value and returns the old value. put() method returns null if this
map contains no mapping for the key. However, a return value of null
does not necessarily indicate that the map contains no mapping for the
key; it's also possible that the map explicitly maps the key to null.
