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

Try this ,this will work.

 public Map<List, Integer> getBigramMap (String word1,String word2){
    Map<List,Integer> hm = new HashMap<List, Integer>();
    List<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;
}

Answer 2

From the documentation:

Note: great care must be exercised if mutable objects are used as map keys. The behavior of a map is not specified if the value of an object is changed in a manner that affects equals comparisons while the object is a key in the map. A special case of this prohibition is that it is not permissible for a map to contain itself as a key. While it is permissible for a map to contain itself as a value, extreme caution is advised: the equals and hashCode methods are no longer well defined on such a map.

You have to take care when you are using mutable objects as keys for the sake of hashCode and equals.

The bottom line is that it is better to use immutable objects as keys.