Java-Code

	/**
	 * Calculates the checksum for the number given as {@link String}.
	 * 
	 * @param value
	 *            String representation of a number for which you want the
	 *            checksum to be calculated
	 * @return the checksum of the specified number
	 * @throws IllegalArgumentException
	 *             if you pass an empty {@link String}, null or a "dirty" String
	 *             (only numbers and a leading minus are allowed)
	 */
	static int calculateChecksum(String value) {
		if (value == null || value.isEmpty()) {
			throw new IllegalArgumentException("No empty input or null as argument allowed");
		}
		// Removing potentially leading minus sign from input string
		final boolean isNegative = value.startsWith("-") ? true : false;
		if (isNegative) {
			value = value.substring(1, value.length());
		}
		// Validating and splitting input string into single digits
		int[] digits = new int[value.length()];
		for (int i = 0; i < digits.length; i++) {
			if (!Character.isDigit(value.charAt(i))) {
				throw new IllegalArgumentException("Input value must not contain any characters but digits");
			}
			digits[i] = Character.getNumericValue(value.charAt(i));
		}
		int result = 0;
		// Actual checksum calculation
		for (int i = 0; i < digits.length; i++) {
			result += digits[i];
		}
		// Bringing back the minus according as input string was negative
		return isNegative ? -1 * result : result;
	}

	/**
	 * This is just glue code to fit the task's requirement.
	 * 
	 * @param input
	 */
	private static void evaluateInput(String input) {
		int valueOfInput = 0;
		try {
			valueOfInput = Integer.valueOf(input);
		} catch (NumberFormatException e) {
			// tolerating wrong input here yet, checksum method will handle it
			System.out.println("Your input was not a number, this will not end well...");
		}
		if (valueOfInput < 0 || valueOfInput > 10000) {
			System.out.println("Your input was outside the allowed range, but I am smart enough anyway. Result is being calculated.");
		}
		StringBuilder builder = new StringBuilder();
		builder.append("Checksum: ");
		final boolean isNegative = input.charAt(0) == '-';
		final char[] inputAsCharArray = input.toCharArray();
		for (int i = isNegative ? 1 : 0; i < inputAsCharArray.length; i++) {
			if (isNegative) {
				builder.append("-");
			}
			builder.append(inputAsCharArray[i]);
			builder.append(" + ");
		}
		builder.replace(builder.length() - 2, builder.length(), "= ");
		builder.append(Checksum.calculateChecksum(input));
		System.out.println(builder.toString());
	}

	public static void main(String[] args) {
		System.out.print("Enter a number (0-10000):");
		try (Scanner input = new Scanner(System.in)) {
			Checksum.evaluateInput(input.nextLine().trim());
		}
	}