[Jposemato]

Okay, start off by saying, i'm new to java. A lot of it confuses. Please bare with me as I make mistakes and won't understand things at first lol. For the following solutions please try to explain why what you did worked though as I am trying to learn with this, not actually make a useful program.

Heres the the current program, a basic calculator built to work through JOptionPane panels, I'll explain my problem at the bottom:

import java.util.*;
import javax.swing.*;
import java.io.*;
import java.util.regex.*;


public class Calculator {

    static Scanner console = new Scanner(System.in);
    
	public static void main(String[] args) throws java.lang.NumberFormatException
	{
		double num1;
		double num2;
		double sum;
		String sign;
		String num1a;
		String num2a;
		String inStr1;
		String inStr2;
		String outStr;
		int repeats;
		num1a = "a";
		num2a = "b";
		sum = 1;
		
        inStr1 = JOptionPane.showInputDialog("Please Input an Equation\n" +
				"* = multiply, / = Divide,\n" +
				"Include spaces and Decimals (Using Doubles!)\n" +
				"For example: 2.0 + 4.2, Then ENTER!");
		String data = inStr1;
		String[] values = data.split(" ");
		num1a = values[0];
		sign = values[1];
		num2a = values[2];
		
 		num1 = Double.parseDouble(num1a);
		num2 = Double.parseDouble(num2a);
		
		if (sign.equals("+")){
			sum = num1 + num2;
		}
		else if (sign.equals("-")){
			sum = num1 - num2;
		}
		else if (sign.equals("*")){
			sum = num1 * num2;
		}
		else if (sign.equals("/")){
			sum = num1 / num2;
		} 
		else {
			JOptionPane.showMessageDialog(null, "User error. Please follow instructions.");
			
		}
		outStr = (num1 + " " + sign + " " + num2 + " = " + sum);
		JOptionPane.showMessageDialog(null, ("" + outStr));
		JOptionPane.showConfirmDialog(null, "Do More Calculations?", "Calculator", JOptionPane.YES_NO_OPTION);
							
	}
}

Okay so right now, that all works fine. The problem is... When you have that guy who really just wants to ruin my work and enter "eb a + asf sakasf" And it throws back a big error. Instead of crashing and returning my java lang exception, how can I make it say something along the lines of "User Error" as I have for the sign (If anything but +, -, /, * entered it returns that.). and then after giving the message go to the Do More Calculations? part.

The second question is... and I know i'm going to be hated for this... How do I get around java not having goto? I'm rather used to it and I don't know how to use java to return to the top of the program, perhaps some kind of loop method? I want to make it so if the user presses Yes on "JOptionPane.showConfirmDialog(null, "Do More Calculations?", "Calculator", JOptionPane.YES_NO_OPTION);" it returns to the top.

Thanks ahead of time!