获取4位包括字母+数字的随机码

/**
     * 获取4位包括字母+数字的随机码
     * @return 
     */
    public static  String getRandomValidStr(){
        String letters[] = {"A","B","C","D","E","F","G","H","I","J","K","L","M","N","O","P","Q","R","S","T","U","V","W","X","Y","Z",
                     "a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z"};
        String numbers[] = {"0","1","2","3","4","5","6","7","8","9"};
        
        int r1 = (int) Math.round(Math.random()*(letters.length-1));
        int r2 = (int) Math.round(Math.random()*(letters.length-1));
        int r3 = (int) Math.round(Math.random()*(numbers.length-1));
        int r4 = (int) Math.round(Math.random()*(numbers.length-1));
        
        String str[] = {letters[r1],letters[r2],numbers[r3],numbers[r4]};
        
        List<String> list = Arrays.asList(str);
        
        Collections.shuffle(list);
        
        return list.get(0)+list.get(1)+list.get(2)+list.get(3);
    }