问Java StringBuilder字符追加返回不需要的数字

EN

 String winningCard(String trick, char trump) {
    StringBuilder sb = new StringBuilder();
    char suit;
    char rank;
    for(int i = 1;   i < trick.length(); i+=2) {
        if(trick.charAt(i) == trump) {
            suit = trick.charAt(i);
            rank = trick.charAt(i-1);
            sb.append(rank + suit); //issue here, returns a weird number
            break;
        }
    }
    String result = sb.toString();
    return result;
}