Does "put" overwrite existing values?
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.
hmmm ,just need add a linemyHashtable.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