Number.java

public class Number {

    private final int number;

    public Number(int number) {
        this.number = number;
    }

    public Number(String parseableString) {

        if (!parseableString.matches("-?[0-9]+")) {
            this.number = 0;
        } else {
            this.number = Integer.parseInt(parseableString);
        }
    }

    public int getValue() {
        return number;
    }

    public int add(int addend) {
        return number + addend;
    }

    public int subtract(int subtrahend) {
        return number - subtrahend;
    }

    public int multiplyBy(int multiplier) {
        return number * multiplier;
    }

    public double divideBy(int divisor) {
        if (divisor == 0) {
            throw new ArithmeticException("Division by zero is not allowed.");
        }
        return (double) number / divisor;
    }

    public String toString() {
        return String.valueOf(number);
    }
}