group anagram

Given an array of strings, group anagrams together.

For example, given: ["eat", "tea", "tan", "ate", "nat", "bat"], Return:

[ ["ate", "eat","tea"], ["nat","tan"], ["bat"] ]

all input lowercase

public class Solution {
    public List<List<String>> groupAnagrams(String[] strs) {
        List<List<String>> list = new ArrayList<List<String>>();
        if(strs == null) return list;
        HashMap<String, List<String>> map = new HashMap<String, List<String>>();
        for(int i = 0; i < strs.length; i++ ){
            String code = hashcode(strs[i]);
            if(map.containsKey(code)){
                map.get(code).add(strs[i]);
            }else{
                List<String> cur = new ArrayList<String>();
                cur.add(strs[i]);
                map.put(code, cur);
            }
        }
        for (Map.Entry<String, List<String>> entry : map.entrySet())
        { 
            //String key = entry.getKey();
           List<String> cur = entry.getValue();
           list.add(cur);
        }
        return list;       
    }

    public String hashcode(String s){
        if( s == null ) return s;
        char[] arr = s.toCharArray();
        Arrays.sort(arr);
        return new String(arr);
    }
}