How can I keep my value out of an if-statement?

I tried making a very basic calculator, but if I try to output the calculated result I get the error that my variable "result" has not been initialised, even though I did initialise it inside the if-statement at the bottom. When I put the "System.out" line in my if-statement it works, so I have been wondering how can initialise a variable in my statement and keep the value.

Sorry if this was asked before, but all I found were threads that talked about using "return" to give out a value, but not a variable.

import javax.swing.JOptionPane;
class Calculator{
public static void main(String[] args)
{
    double z1, z2, result;
    String input, s;

    input = JOptionPane.showInputDialog("Input a number:");

  //check whether or not the input is a number
        if(isNumber(input) == false)
        {
            JOptionPane.showMessageDialog(null, "Input a real Number", "Error",JOptionPane.ERROR_MESSAGE);
            System.exit(0);
        }

    z1 = Double.parseDouble(input);

    input = JOptionPane.showInputDialog("Input a second number:");

  //check whether or not the input is a number
        if(isNumber(input) == false)
        {
            JOptionPane.showMessageDialog(null, "Input a real number", "Error",JOptionPane.ERROR_MESSAGE);
            System.exit(0);
        }
    z2 = Double.parseDouble(input);

    s = JOptionPane.showInputDialog("Input an operation(+,-,*,/):");        

    if(s.equals("+"))
    { result = z1 + z2; }
    if(s.equals("-"))
    { result = z1 - z2; }
    if(s.equals("*"))
    { result = z1 * z2; }
    if(s.equals("/"))
    { result = z1 / z2; }

    System.out.println(result);