How to get the number of occurrences of a key in HashMap?

Question

I am have the Hashmap like this,

HashMap<String,String> epnSource = new HashMap<String, String>();

Now I have added the keys/values like this,

epnSource.put("10.3.2.227","EPN1");
epnSource.put("10.3.2.227","EPN2");
epnSource.put("10.3.2.166","EPN3");
epnSource.put("10.3.2.166","EPN4");
epnSource.put("10.3.2.161","EPN5");

I am trying to do every time before adding a value, I want to check number of occurrences of a key present in the HashMap. Suppose if key 10.3.2.227 has more than two occurrences I shouldn't added it and go for new one. Any suggestions will be helpful.

Answer 1

Suppose if value 10.3.2.227 has more than two occurrences ...

It won't. The way that you have implemented it, the "10.3.2.227" is a key of the Map, and a given key cannot appear more than once in a Map.

If you want a given key (eg "10.3.2.227") to map to multiple values (eg "EPN1" and "EPN1"), you need to use either a Map<String,Set<String>> or a MultiMap class from the Apache or Google/Guava collections libraries.

Answer 2

If the map previously contained a mapping for the key, the old value is replaced.

It is not possible duplicate key in HashMap.