MultiKey Map

MultiKey Map an Amazing Data Structure

Apache Commons API is just Amazing; it’s worth exploring and if you are not familiar with it try exploring “Google Guava” a much new API. It’s much better than Apache Commons Collections API.

It handles Generics and is thread safety.

Till now you will only know map as a key, value pair. You can get the value by map.get(key). 

Have you ever owned a Locker in a Bank, or have seen big Lockers in Jewellery shops to store diamonds. Those will have multiple keys, here I don’t mean a duplicate key but I want to emphasize that you can unlock them only via a sequence of Key.  In a typical bank locker I key remains with the Bank and the other is given to the Owner. Whenever you visit the bank to take out/keep your valuables the bank official will go along with you and use his key to unlock it. Here the locker is not open yet, after that you have to use your key to unlock the locker. Here this is just done to ensure security. Note that the sequence is very important here.

Multikey Locker

The multi key map also work on the same lines. You can have a set of keys to map a value. Let’s us understand this with below Figure. People who are already aware of Hashed data structures will be able to enjoy this example in a better way.

Syntax of a Normal Java Map

Map map = new HashMap ()// Generics supported

Syntax of a MultikeyMap

MultikeyMap multikeyMap = new MultiKeyMap()// It does not support generics

To understand it you can assume

MultikeyMap multikeyMap  ;

We will be learning more about multyikey map with the below example.

Let us discuss the Example:

We have to fetch the name of the person based on the city and company;

Lets consider the below data

MultiKeyMap multiKeyMap = new MultiKeyMap();

             multiKeyMap.put("New York","IBM","Sam");

             multiKeyMap.put("Sydney","Infosys","Honey");

             multiKeyMap.put("Prague","JP Morgan","Peter");

             multiKeyMap.put("Scotland","RBS","Deny");

             multiKeyMap.put("Paris","Nomura","Lily");

             multiKeyMap.put("Melbourne","Citi Bank","Sandy");

             multiKeyMap.put("Aukland","Bank of America","Tommy");

Values stored in the memory:

Hascode	Multi Keys	Value
11111111	New York,IBM	Sam
22222222	Sydney,Infosys	Honey
33333333	Prague,JP Morgan	Peter
44444444	Scotland,RBS	Deny
55555555	Paris,Nomura	Lily
66666666	Melbourne,Citi Bank	Sandy
77777777	Aukland,Bank of America	Tommy

Now if we say

1)    System.out.println(multiKeyMap.get("Sydney","Infosys"));   

2)    System.out.println(multiKeyMap.get("Paris","Nomura"));

3)    System.out.println(multiKeyMap.get("Nomura","Paris"));

4)    System.out.println(multiKeyMap.get("Sydney","IBM"));

We get the below outputs:

   1)  Honey // match was found

   2)  Lily  // match was found

   3)  null // Because here we reversed the Order of the keys and no hashcode was       found for that multikey mapped

   4)  null // here we gave two different keys and it was not mapped.

Note that MultiKeyMap is not synchronized and is not thread-safe. If you wish to use this map from multiple threads concurrently, you must use appropriate synchronization.

Refer the Apache Commons API for more information