HashMap with ArrayList

Question

I have a java question about a HashMap with an ArrayList in it. The key is a string and the value is the ArrayList (HashMap<String, ArrayList<Object>>)

I am given a list of multiple keys and I need to put the corresponding values into the ArrayList, and I'm not sure how to do it.

Here's an example:

(What I am given)

ABC
ABC
ABC
123
123

The keys are actually file names so the file is the value for the ArrayList. Now I have to separate those filenames so i can add the files to the ArrayList. Does this make sense?

This is the only code I have:

Set<String> tempModels = Utils.getSettingSet("module_names", new HashSet<String>(), getActivity());
        for(String s : tempModels) {
            Log.d("Loading from shared preferences: ", s);
            String key = s.substring(0, 17);

        }

if you get null value then create an array list and add value to it and then use put() — Jigar Joshi
– Jigar Joshi, Commented Jun 13, 2014 at 19:40
the hashmap is empty to begin with though can't see it, where is it though? — Bhesh Gurung
– Bhesh Gurung, Commented Jun 13, 2014 at 19:46

Elliott Frisch · Accepted Answer · 2014-06-13 19:42:22Z

4

I believe you're looking for something like this,

List<Object> al = map.get(key);
if (al == null) {
  al = new ArrayList<Object>();
  map.put(key, al);
}
al.add(value);

Please note that the above uses raw Object Lists. You should (if possible) pick a proper type, and use it.

Edit

Some prefer the (slightly shorter) form,

if (!map.containsKey(key)) {
  map.put(key, new ArrayList<Object>());
}
map.get(key).add(value);

answered Jun 13, 2014 at 19:42

Elliott Frisch

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6 Comments

Bhesh Gurung Over a year ago

There is no way key could be null, actually the whole validation on the key seems unnecessary. Also, HashMap does allow null key.

Braj Over a year ago

use containsKey() that is better than checking value as null.

Elliott Frisch Over a year ago

@BheshGurung Agreed. Edited.

Braj Over a year ago

@ElliottFrisch I you like any comment then it's a good practice to up-vote it. Isn't it?

Nelson.b.austin Over a year ago

Thank you very much! I implemented it and it worked!

|

Brett Okken · Accepted Answer · 2014-06-13 21:08:28Z

2

This is a good use for a guava Multimap, which manages the existence/creation of the inner List.

http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Multimap.html

http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/ArrayListMultimap.html

answered Jun 13, 2014 at 21:08

Brett Okken

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