java - ArrayList as key in HashMap

Question

Would it be possible to add an ArrayList as the key of HashMap. I would like to keep the frequency count of bigrams. The bigram is the key and the value is its frequency.

For each of the bigrams like "he is", I create an ArrayList for it and insert it into the HashMap. But I am not getting the correct output.

public HashMap<ArrayList<String>, Integer> getBigramMap(String word1, String word2) {
    HashMap<ArrayList<String>, Integer> hm = new HashMap<ArrayList<String>, Integer>();
    ArrayList<String> arrList1 = new ArrayList<String>();
    arrList1 = getBigram(word1, word2);
    if (hm.get(arrList1) != null) {
        hm.put(arrList1, hm.get(arrList1) + 1);
    } else {
        hm.put(arrList1, 1);
    }
    System.out.println(hm.get(arrList1));
    return hm;
}


public ArrayList<String> getBigram(String word1, String word2) {
    ArrayList<String> arrList2 = new ArrayList<String>();
    arrList2.add(word1);
    arrList2.add(word2);
    return arrList2;
}

Answer 1

Yes you can have ArrayLists as a keys in a hash map, but it is a very bad idea since they are mutable.

If you change the ArrayList in any way (or any of its elements), the mapping will basically be lost, since the key won't have the same hashCode as it had when it was inserted.

The rule of thumb is to use only immutable data types as keys in a hash map. As suggested by Alex Stybaev, you probably want to create a Bigram class like this:

final class Bigram {

    private final String word1, word2;

    public Bigram(String word1, String word2) {
        this.word1 = word1;
        this.word2 = word2;
    }

    public String getWord1() {
        return word1;
    }

    public String getWord2() {
        return word2;
    }

    @Override
    public int hashCode() {
        return word1.hashCode() ^ word2.hashCode();
    }

    @Override
    public boolean equals(Object obj) {
        return (obj instanceof Bigram) && ((Bigram) obj).word1.equals(word1)
                                       && ((Bigram) obj).word2.equals(word2);
    }
}