Identifying numeric substrings of an alphanumeric string, and summing them

I am going through the CodingBat exercises for Java. Here is the task I have just completed:

Given a string, return the sum of the numbers appearing in the string, ignoring all other characters. A number is a series of 1 or more digit chars in a row.

And here is my code:

public int sumNumbers(String str){
    int total = 0;

    for (int i = 0; i < str.length(); i++) {
        if (Character.isDigit(str.charAt(i))) {
            StringBuilder appendNums = new StringBuilder();
            appendNums.append(str.charAt(i));
            for (int j = i+1; j < str.length(); j++) {
                if (Character.isDigit(str.charAt(j))) {
                    appendNums.append(str.charAt(j));
                } else {
                    break;
                }
            }
            String appendNums2String = appendNums.toString();
            total += Integer.parseInt(appendNums2String);
            i += appendNums2String.length()-1;
        }
    }
    return total;
}

The questions I have are:

1. Are there any parts of this code you find to be poor practise or poor quality?
2. Is it acceptable to append one digit before the (conditional) digits that proceed it, or should I be identifying the entire block first, and then append?
3. What is a more concise way of solving this?
4. For this question I relied heavily on Eclipse's debugger until I got it right. Is this good or bad practise?
5. Is it a better idea to use a while loop instead of the second for loop, because the characters tested are relative to i rather than the string's length? (I played around with this but couldn't figure it out.)