java update hashmap

Question

I have a problem with changing values in an HashMap>. First I do this:

HashMap<String, ArrayList<Integer>> countries = new HashMap<>();
ArrayList<Integer> medals = new ArrayList<>();
medals.add(0); medals.add(0); medals.add(0);
for(int i = 0; i < COUNTRY.length; i++){
    countries.put(COUNTRY[i], medals);
}

I fill the HashMap with keys from a static array and add a default value of an ArrayList that is filled with 0, 0, 0. Then I do this:

Integer plus = new Integer(1);
for(int j = 0; j < athletes.size(); j++){
    if(j == 3)
        break;
    Athlete a = athletes.get(j);
    ArrayList<Integer> medals = countries.get(a.getCountry());
    Integer medal = medals.get(j) + plus;
    medals.set(j, medal);
    countries.put(a.getCountry(), medals);
}

I have an ArrayList filled with athletes that is already sorted by the athlete's results. What I'm trying to do is take the top 3 placed athletes, and update my HashMap value so the ArrayList will have 3 numbers that represent how many gold, silver and bronze medals every country has won. The problem is that when I try to replace the old value with a new ArrayList it replaces all the values in the HashMap with new ArrayList's instead of just the one with the matching key.

I have no idea what the problem is, any suggestions??

jlordo · Accepted Answer · 2012-11-28 08:48:31Z

8

The problem is in this piece of code:

HashMap<String, ArrayList<Integer>> countries = new HashMap<>();
ArrayList<Integer> medals = new ArrayList<>();
medals.add(0); medals.add(0); medals.add(0);
for(int i = 0; i < COUNTRY.length; i++){
    countries.put(COUNTRY[i], medals);
}

You only create a single instance of your List, and associate it with every country.

A possible fix would be to initialize a new List in each loop iteration.

HashMap<String, List<Integer>> countries = new HashMap<>();
for(int i = 0; i < COUNTRY.length; i++){
    List<Integer> medals = Arrays.asList(0, 0, 0);
    countries.put(COUNTRY[i], medals);
}

If your List has the fixed size of 3, why don't you use a simple int[] instead?

This is how the code would look using an int[]

Map<String, int[]> countries = new HashMap<>();
for(int i = 0; i < COUNTRY.length; i++){
    countries.put(COUNTRY[i], new int[3]);
}
//
for(int j = 0; j < athletes.size(); j++){
    if(j == 3)
        break;
    Athlete a = athletes.get(j);
    int[] medals = countries.get(a.getCountry());
    medals[j]++;
    // last to lines could even be
    // countries.get(a.getCountry())[j]++;
}

edited Nov 28, 2012 at 8:48

answered Nov 28, 2012 at 8:36

jlordo

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2 Comments

zkranc Over a year ago

thank you, I should've known I made a stupid mistake :P and yes I guess int[] is simpler :)

maia Over a year ago

I struggled with this issue for SO LONG, and this was the simple fix. Thank you so much!

Priyank Doshi · Accepted Answer · 2012-11-28 08:38:01Z

0

You should use google-guava's MultiMap . It is design for exactly this situation where you need Map<K,List<V>> type of structure. i.e. multiple value for same key.

answered Nov 28, 2012 at 8:38

Priyank Doshi

13.2k18 gold badges61 silver badges82 bronze badges

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