How can I have a HashMap with unique keys in java?

How can I have a HashMap with unique keys in Java? Or even does this make any sense to have unique keys in HashMap or the keys are unique by default? I am a newbie. thx

Solution 1:

Hash map key is unique. Add duplicate key, then it will be overwritten.

 HashMap hm = new HashMap();
 hm.put("1", new Integer(1));
 hm.put("2", new Integer(2));
 hm.put("3", new Integer(3));
 hm.put("4", new Integer(4));
 hm.put("1", new Integer(5));// value integer 1 is overwritten by 5

By default Hashmap is not synchronized.

Solution 2:

The keys are unique in all maps. The difference between the various maps implementations concern the possibility of null keys, the order of iteration and concurrency issues.

Solution 3:

Try to look at the Java API for Map which is interface that HashMap implements. The first sentence is:

An object that maps keys to values. A map cannot contain duplicate keys; each key can map to at most one value.