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)
b) d|o|g                   --> d1g
   1  1
     1---5----0----5--8
c) i|nternationalizatio|n  --> i18n
     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

要注意一种情况：isUnique("cake"), 应该是true. 虽然他的缩写在afterabbr出现，但是那就是他自己，没有别的跟他一样

public class ValidWordAbbr {
    String[] dict;
    HashMap<String, Set<String>> afterAbbr;

    public ValidWordAbbr(String[] dictionary) {
        this.dict = dictionary;
        this.afterAbbr = new HashMap<String, Set<String>>();
        for (String item : dictionary) {
            String str = abbr(item);
            if (afterAbbr.containsKey(str)) {
                afterAbbr.get(str).add(item);
            }
            else {
                HashSet<String> set = new HashSet<String>();
                set.add(item);
                afterAbbr.put(str, set);
            }
        }
    }

    public boolean isUnique(String word) {
        String str = abbr(word);
        if (!afterAbbr.containsKey(str)) return true;
        else if (afterAbbr.containsKey(str) && afterAbbr.get(str).contains(word) && afterAbbr.get(str).size()==1) return true;
        return false;
    }
    
    public String abbr(String item) {
        if (item == null) return null;
        if (item.length() <= 2) return item;
        StringBuffer res = new StringBuffer();
        res.append(item.charAt(0));
        res.append(item.length()-2);
        res.append(item.charAt(item.length()-1));
        return res.toString();
    }
}


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