Unique Word Abbreviation

An abbreviation of a word follows the form <first letter><number><last letter>. Below are some examples of word abbreviations:

a) it                      --> it    (no abbreviation)

     1
b) d|o|g                   --> d1g

              1    1  1
     1---5----0----5--8
c) i|nternationalizatio|n  --> i18n

              1
     1---5----0
d) l|ocalizatio|n          --> l10n

Assume you have a dictionary and given a word, find whether its abbreviation is unique in the dictionary. A word's abbreviation is unique if no other word from the dictionary has the same abbreviation.

Example: 

Given dictionary = [ "deer", "door", "cake", "card" ]

isUnique("dear") -> false
isUnique("cart") -> true
isUnique("cane") -> false
isUnique("make") -> true

Solution: run time complexity is O(n), space complexity is O(n).

public class ValidWordAbbr {
    
    HashMap> map = new HashMap>();
    
    public ValidWordAbbr(String[] dictionary) {
        for (String s : dictionary) {
            if (s == null || s.length() == 0) {
                continue;
            }
            
            if (!map.containsKey(getAbbr(s))) {
                ArrayList list = new ArrayList();
                list.add(s);
                map.put(getAbbr(s), list);
            } else {
                if (!map.get(getAbbr(s)).contains(s)) {
                    map.get(getAbbr(s)).add(s);
                }
            }
        }
    }

    public boolean isUnique(String word) {
        if (word == null || word.length() == 0) {
            return true;
        }
        
        String key = getAbbr(word);
        if(!map.containsKey(key)) {
            return true;
        } else {
            if (map.get(key).size() == 1 && map.get(key).get(0).equals(word)) {  // dictionary[hello], word="hello" -> unique
                return true;
            }
        }
        return false;
    }
    
    public String getAbbr(String str) {
        if (str.length() <= 2) {
            return str;
        } 
        
        return str.charAt(0) + String.valueOf(str.length() - 2) + str.charAt(str.length() - 1);
    }
}


// Your ValidWordAbbr object will be instantiated and called as such:
// ValidWordAbbr vwa = new ValidWordAbbr(dictionary);
// vwa.isUnique("Word");
// vwa.isUnique("anotherWord");