Question

I am trying to make a simple calculator program where a user can opt to do an operation and enter the numbers as long as he/she wishes to.

I just have problem because whenever I would reach inside my loop and ask the user if he/she want to continue and whenever I would run the program, I would have an “Exception in thread “main” java.lang.NullPointerException”

Scanner myInput=new Scanner(System.in);

System.out.print("Do you have numbers to compute?");
ans=myInput.findInLine(".").charAt(0);

while ((ans=='Y')||(ans=='y'))
{
    //get the numbers
    //provide the menu
    //get the user's choice

    switch (calc)
    {
        case 1: out.println("Sum is: " +(num1+num2)); break;
        case 2: out.println("Difference is: " +(num1-num2)); break;
        case 3: out.println("Product is: " +(num1*num2)); break;
        case 4: out.println("Quotient is: " +(num1/num2)); break;
        case 5: out.println("Modulo is is: " +(num1%num2)); break;
        case 6: out.println("Sum is: " +(num1+num2));
                    out.println("Difference is: " +(num1-num2));
                    out.println("Product is: " +(num1*num2));
                    out.println("Quotient is: " +(num1/num2));
                    out.println("Modulo is is: " +(num1%num2)); break;
                    default: out.println("Invalid."); break;
    }
       out.println("Compute another?");
       ans=myInput.findInLine(".").charAt(0);
}

May I humbly ask what can I do with this program so that it will ask again for the user’s input whether to continue or not? Thanks in advance for your help.

Answer 1

The NullPointerException is being thrown as Scanner.findInLine() can return null if it does not find the requested String/Pattern. The code invokes the method charAt(0) on the null String. Changing the code to the following will resolve this:

Scanner myInput=new Scanner(System.in);
char ans;

System.out.print("Do you have numbers to compute?");
ans=myInput.findInLine(".").charAt(0);

while ((ans=='Y')||(ans=='y'))
{
    // Code omitted

    System.out.print("Compute another?");
    myInput.nextLine();
    String s = myInput.findInLine(".");
    ans = (null == s) ? 'n' : s.charAt(0);
}