Does "put" overwrite existing values?

Question

New to hashtables with a simple question. For some reason googling hasn't gotten me a straight answer. Say I've got an <int,String> hashtable set up:

myHashtable.put(1,"bird");
myHashtable.put(2,"iguana");

and I want to change "bird" to "fish" (and leave the index the same). Can I just do a simple put, or do I need to delete the entry, or what?

Answer 1

Yes.

If a mapping to the specified key already exists, the old value will be replaced (and returned). See Hashtable.put().

For multi-threaded environment, I'd recommend ConcurrentHashMap or another ConcurrentMap implementation. Though Hashtable is synchronized, there are more sophisticated implementations available now for concurrent mapping, such as Guava's MapMaker and CacheBuilder.

Also keep in mind the Map is going to have the type parameters <Integer, String> since primitive type parameters aren't supported.

Answer 2

hmmm ,just need add a line
myHashtable.put(1,"fish");
to see what's amazing happens

see this links:http://docs.oracle.com/javase/6/docs/api/java/util/Hashtable.html#put(K, V)

Returns:
the previous value of the specified key in this hashtable, or null if it did not have one