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

Keys _are_ unique by default. Note that `.keySet()` on a `Map` returns a `Set`, and elements of a `Set` are unique by default. If you attempt to push a value to an existing key, the old value will be overwritten. — fge, Jan 19 '12 at 08:54
note that "unique" in java means equal in terms of implementation of the methods equals and hashcode! — Hachi, Jan 19 '12 at 09:04

score 61 · Answer 1 · edited Jun 20 '16 at 18:05

61

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.

edited Jun 20 '16 at 18:05

Gaur93

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answered Jan 19 '12 at 08:54

Kushan

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so how do we make the key value relationship not unique,i.e,i want also the value 5 for the key 1 above ?Is there any method to serve that purpose – Vamsi Pavan Mahesh Dec 21 '13 at 04:51
@VamsiPavanMahesh, you will have to store some sort of `List` or `Map` for that corresponding `key`. `Map m = new HashMap<>();` – Emz Nov 22 '15 at 19:34

score 41 · Accepted Answer · answered Jan 19 '12 at 08:53

41

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.

answered Jan 19 '12 at 08:53

xpapad

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PrimosK · Answer 3 · 2018-01-27T09:02:15.917

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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.

edited Jan 27 '18 at 09:02

answered Jan 19 '12 at 09:02

PrimosK

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score 1 · Answer 4 · answered Jan 19 '12 at 08:56

1

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

answered Jan 19 '12 at 08:56

Anish Mohile

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score 1 · Answer 5 · answered Jan 19 '12 at 08:57

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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.

answered Jan 19 '12 at 08:57

Rajesh Pantula

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score 1 · Answer 6 · answered Jan 19 '12 at 08:59

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):

index = hash(key) mod N
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

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

6 Answers6

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