如何将带有数字和字母的字符串分隔为仅数字？

public static String carRentalCode(String licensePlate) {
    // precondition:    licensePlate is a valid plate as described in pdf
    // postcondition:   return the car rental code for the licensePlate as described
    int values = 0;
    int ascii = 0;
    int sum = 0;
    int convert = 0;
    int count = 0;
    int a = 0;
    char letter;
    String license;
    char[] ch = new char[licensePlate.length()];
    for (int i = 0; i < licensePlate.length(); i++) { 
        ch[i] = licensePlate.charAt(i); 
    }
    for(int i = 0; i< ch.length; i++)
    {
        if(Character.isDigit(ch[i]))
        {
            values += Character.getNumericValue(ch[i]);
        }
        if(Character.isAlphabetic(ch[i]))
        {
            ascii += (int) ch[i];
            count++;
        }
    }
    sum = values + ascii;
    convert = sum%26 + 65;
    letter = (char)convert;
    String[] letters = licensePlate.split("\\d+");
    String lowercase = Arrays.toString(letters).toLowerCase();
    String code = Integer.toString(sum);
    return letter + code + lowercase;

在这里，我应该根据给定的车牌（测试仪车牌为"123ABC456"）返回租车代码。我剩下的唯一问题是我的小写字母返回，[[,abc]]而不是正确的答案[abc]。

您如何固定前面的空白区域，并且有两对而不是一对？还是有其他方法仅获取字符串中(ABC)与数字（123和456）分开的字母并返回它？

预期：```js L219[abc]

实际：  ```js
L219[[, abc]]