NumberParser.java

import java.util.Scanner;

/**
 * Input: One String with numbers. numbers are separated by a single space
 * Output: numbers recognized and their sum Allowed: isDigit() substring()
 * charAt() Integer.parseInt() length() Forbidden: split(), arrays or
 * collections
 */
public class NumberParser {

    public static void main(String[] args) {
        // print out the welcome header that gives the user instructions:
        System.out.println("Welcome to the Number Parser! Please enter one line"
                + " of numbers, separated by a single space between each,"
                + " followed by a return.");

        // get the user input
        Scanner stdin = new Scanner(System.in);
        String line = stdin.nextLine();

        // make sure the user input isn't empty or null
        if (line == null) {
            System.out.println("Invalid input. Input was null");
            System.exit(0);
        }

        /*
         * Now we have a valid input, and can parse out the numbers
         */

        // set up variables that will keep track of things as we go
        int sum = 0;
        int beginIndex = 0;

        // step through the input String one character at a time.
        for (int currentIndex = 0; currentIndex < line.length(); ++currentIndex) {
            // get the character at the current index
            char currentChar = line.charAt(currentIndex);

            // if the character is NOT a digit, then we have reached the end of a number.
            if (!Character.isDigit(currentChar)) {

                // make sure the number substring has at least one character in it
                if (currentIndex - beginIndex > 0) {

                    // pull out the number substring.
                    String numberSubstring = line.substring(beginIndex, currentIndex);

                    // convert that number substring into an integer
                    int currentNumber = Integer.parseInt(numberSubstring);

                    // print out the number
                    System.out.print(currentNumber + ", ");

                    // add the number to the running total
                    sum = sum + currentNumber;
                }

                // we're on a non-digit character, so a new number substring might begin next character
                beginIndex = currentIndex + 1;
            }
        }

        // print out the sum
        if (sum > 0) {
            System.out.println("sum: " + sum);
        } else {
            System.out.println("No numbers present.");
        }
    }
}