Default constructor generate two random equations

Need to create a default constructor in a programmer defined class that generates a random question, addition or subtraction. And when adding the numbers must be random from 0-12 and when subtracting the first number must be from 6-12, while the second is less than the first number. Here's my progress as of now:

package project4;

public class Question {
    private int num1;
    private int num2;
    private char operand;

    public Question()
    {
        operand = '+';
        num1 = (int)(Math.random())*12;
        num2 = (int)(Math.random())*12;




        operand = '-';
        num1 = ((int)(Math.random())*12+6);
        num2 = (int)(Math.random()) << num1;

    }


    public String toString()
    {
        String str = new String(num1 + " " + operand + " " + num2);
        return str;
    }

}

However when I call it in the application class like I did underneath, I get 6-0 please help.

package project4;

    public class Project4App 
{
        public static void main(String[] args)
{
        Question question1 = new Question();
        System.out.println(question1.toString());



    }
          }