Basic Java calculator

Question

Just started learning java and I'm trying to make a calculator with JPanes in the easiest way possible. I can manage to get it to do simple concatenation, but I can't get it to add the entered numbers together. Any help is appreciated.

import javax.swing.*;

public class fiveInputs{
    public static void main (String [] args) {

        java.lang.String num = "";
        java.lang.String num2 = "";
        java.lang.String num3 = "";
        java.lang.String num4 = "";
        java.lang.String num5 = "";

        num = JOptionPane.showInputDialog("First number: ");
        num2 = JOptionPane.showInputDialog("Second number: ");
        num3 = JOptionPane.showInputDialog("Third number: ");
        num4 = JOptionPane.showInputDialog("Fourth number: ");
        num5 = JOptionPane.showInputDialog("Fifth number: ");

        java.lang.String sum = "";

        if (sum.equals(""));
           JOptionPane.showInputDialog (num - num2 - num3 - num4 - num5);

        System.out.println(sum);

    }
}

What happens when you try to subtract a String from a String? Also, this if statement `if (sum.equals(""));` does nothing whether or not it is true. — Compass, Feb 10 '15 at 21:19
See also this [calculator example](http://stackoverflow.com/a/7441804/418556). It uses `ScriptEngine` to evaluate the expression in the text field. — Andrew Thompson, Feb 10 '15 at 21:44

Adam · Answer 1 · 2015-02-10T21:23:54.803

You cannot add and subtract strings as if they were numbers. You need to parse them into a numeric type, for example integer, double, etc.

int num = Integer.parseInt(JOptionPane.showInputDialog("First number: "));
int num2 = Integer.parseInt(JOptionPane.showInputDialog("Second number: "));
int num3 = Integer.parseInt(JOptionPane.showInputDialog("Third number: "));
int num4 = Integer.parseInt(JOptionPane.showInputDialog("Fourth number: "));
int num5 = Integer.parseInt(JOptionPane.showInputDialog("Fifth number: "));

Integer.parseInt can throw a NumberFormat exception if you give it something it doesn't regard as a number e.g. abc, an empty string etc. Use this as a starting point for any validation.

Couple of asides

java.lang.String, all classes in java.lang are implicity imported, you don't need the package
you don't need to declare all your variables at the top of a method like some languages, do it wherever limits their scope the best.

Basic Java calculator

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