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

Question

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

Answer 1

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.

Answer 2

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.

Answer 3

A generic hashmap is usually implemented as an associative array, so let's say your array has N elements, from 0 to N-1, when you want to add a new (key, value) pair, what it's done behind the scenes is (just conceptually):

1. index = hash(key) mod N
2. array[index] = value

So, by construction, a key is mapped to one and one only array entry.

Please note that it's actually a bit more complex than this: I am ignoring on purpose things like collision handling, rehashing, etc, you may have a good general idea here https://en.wikipedia.org/wiki/Hash_table

Answer 4

HashMap is a collection to store (key,value) pairs and According to the documentation of HashMap the keys are always unique.

If you add a key which already exists(collision) in the hashmap, the old value will be replaced.

Answer 5

HasMap has unique keys. as .keySet() returns Set which has unique members

Answer 6

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.