Implementation of Mapping a single key to multiple values using hashmap

Question

I have a list of tag_ids configured with the test_id_val in a table.I retrieve the Data using the following query

Select test_id_val , tag_id from p_details

For a straightforward case i would rather use a hash map to store key value pair and retrieve the data,But in my case the test_id_val can accept null values

For ex consider the following structure :

 1. APP TR202 
 2. APP TR204 
 3. APP TR205 
 4.NULL TR206 
 5.NULL TR207 
 6.NULL SM504 
 7.NULL SM505

So i will be having a list of tag_ids values to map to a test_id key which can either be null or contain values.My requirement is to map the test_id key to the list of tag_ids

Is there a specific way to do this?

Thanks for the help and time..

Simon Stastny · Accepted Answer · 2013-09-09 05:12:22Z

1

You might consider using some implementation of com.google.common.collect.Multimap.

You can put multiple values associated with one key and then retrieve them as a collection with the map.get(key) method. In your case:

Multimap<String, String> mmap = HashMultimap.create();

mmap.put("APP", "TR202");
mmap.put("APP", "TR204");
mmap.put("APP", "TR205");
mmap.put(null, "TR206");
mmap.put(null, "TR207");
mmap.put(null, "SM504");
mmap.put(null, "SM505");

for (String key : mmap.keySet()) {
  for (String value : mmap.get(key)) {
    System.out.println(key + ": " + value);
  }
}

Prints:

null: SM505
null: SM504
null: TR207
null: TR206
APP: TR202
APP: TR205
APP: TR204

edited Sep 9, 2013 at 5:12

answered Sep 9, 2013 at 5:02

Simon Stastny

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Vimal Bera · Accepted Answer · 2013-09-09 04:52:41Z

0

You can use ArrayList to store the test_id_val. Then in HashMap, store tag_id as a key and index of ArrayList to test_id_val as a value of HashMap. ArrayList allows a null value so you can easily store the null value of specific tag_id. Like this..

KEY     VALUE               ArrayList
TR202     1                 1-->  APP 
TR204     2                 2-->  APP 
TR205     3                 3-->  APP 
TR206     4                 4-->  NULL 
TR207     5                 5-->  NULL

answered Sep 9, 2013 at 4:52

Vimal Bera

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musical_coder · Accepted Answer · 2013-09-09 04:55:50Z

0

You might want to reconsider if this really is the best database schema design you have, but if you choose to stick with it, create a wrapper class for test_id_val in which you specify if you really mean null via a boolean:

private class TestIdVal {
    String test_id_val;
    boolean isNull;
}

You can then instantiate new TestIdVal() objects to use as keys for the map. I don't recommend using a list of tag_id as the key, as that doesn't seem to mirror your schema correctly. It seems that you're trying to do test_id_val => List of tag_id.

answered Sep 9, 2013 at 4:55

musical_coder

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