public List<String> prefixSearch(String key) {
        List<String> value = new ArrayList<String>();
        if (StringUtil.isEmpty(key)) {
            return value;
        }

        char k = key.charAt(0);
        int index;
        if (Character.isUpperCase(k)) {
            index = k - UPPERCASE_STAR;
        } else {
            index = k - LOWERCASE_STAR;

        }
        if (root.children != null && root.children[index] != null) {
            return query(root.children[index], value,
                    key.substring(1), String.valueOf(k));
        }
        return value;
    }
@Test
    public void prefixSearch8() throws Exception {
        TrieTree trieTree = new TrieTree();

        List<String> ab = trieTree.prefixSearch("");
        String result = "";
        for (String s : ab) {
            result += s + ",";
            System.out.println(s);
        }
        Assert.assertTrue(result.equals(""));
    }