我怎样才能让我的价值观不存在if陈述中？

Question

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.

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);

Answer 1

Since this code:

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

does not have a final else statement it is not guaranteed that result will be initialized.
One way to overcome this problem:

    if (s.equals("+")) {
        result = z1 + z2;
    } else if (s.equals("-")) {
        result = z1 - z2;
    } else if (s.equals("*")) {
        result = z1 * z2;
    } else if (s.equals("/")) {
        result = z1 / z2;
    } else {
        result = 0.0;
    }

or in the definition:

double result = 0.0;

Answer 2

The problem is that the environment checks whether everything is okay and sees that you have a few ifs where the variable might be initialized and finds that result is not guaranteed to be initialized. As a result you get this error. Try to initialize it at the start:

double z1, z2, result = 0;