Creating Global Map Java

Question

I would like to have a global HashMap <String, String> available to my whole project.

The hashmap will remain the same throughout the whole project, so it only needs to be instatiated once. My idea was to have it in a Utils class and have a static public class like so:

public static String  getValue(String key){
    return map.get(key);
}

The problem is, I don't want to have to run code to fill the hashmap with the Strings everytime I call getValue. So, where do I instantiate the code?

If I could fill the hashmap similar to:

HashMap hm = {{Key, Value}, {Key, Value}......}

It could then be global to the utils class and would possibly work.

Does anything speak against a singleton class holding the Map? — Roman Vottner, Dec 30 '13 at 16:24
My first thought is not to have a global map in the first place. My second is that you haven't told us anything about where you *do* want to populate the map... — Jon Skeet, Dec 30 '13 at 16:24
Please provide more information as people above have specified. Additionally - if your hash map should be the same throughout the whole project you need to make it 'final'. — e.doroskevic, Dec 30 '13 at 16:26
I would like the global Map because many classes are required to use it — Allan Macmillan, Dec 30 '13 at 16:26
[first link in google on searching for 'java singleton enum pattern'](http://javarevisited.blogspot.co.at/2012/07/why-enum-singleton-are-better-in-java.html) — Roman Vottner, Dec 30 '13 at 16:27

score 5 · Accepted Answer · answered Dec 30 '13 at 16:24

You can instantiate it in a static block in the same Utils class.

static {
   // init code
}

Also you may want to look at the Singleton design pattern as an alternative to keeping static things (class, fields) as suggested here.

I assume your whole app/project is loaded by a single ClassLoader so you don't need to worry about static block called multiple times or Singleton objects instantiated multiple times (you want it instantiated just once).

score 0 · Answer 2 · edited May 23 '17 at 11:46

0

You should use Joshua Bloch's "single element enum type" method, from this post. If your map should be immutable, consider an ImmutableMap from the guava library.

edited May 23 '17 at 11:46

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answered Dec 30 '13 at 16:29

Paul Sanwald

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Brandon · Answer 3 · 2013-12-30T16:55:42.913

package pairtest;

import java.util.HashMap;

public class Util {

    private static final HashMap<String, String> map = new HashMap<>();
    private static Util instance = new Util();

    private Util() {
    }

    public static Util getInstance() {
        return instance;
    }

    public static String getValue(String key) {
        return map.get(key);
    }

    public static void add(String[][] pairs) {
        for(String[] pair : pairs) {
            map.put(pair[0], pair[1]);
        }
    }

    public static void add(String[] keys, String[] values) {
        for (int i = 0; i < keys.length; ++i) {
            map.put(keys[i], values[i]);
        }
    }
}


package pairtest;

public class PairTest {

    public static void main(String[] args) {
        String[][] arg = {{"key", "value"}, {"key", "value"}, {"key", "value"}};
        Util.add(arg);
    }
}

Creating Global Map Java

3 Answers3