Question

I am totally new to programming. I´m reading some tutorials on the internet, and also Barry Burds “Java for dummies” while trying to learn programming Java. I have tried all variations I could think of without success.
In one exercise I am supposed to make the following program to print a message in a “messageDialogBox” which should contain the numbers wrote in by the user of the program. Unfortunately I get thhe following error message when trying to compile: Can somebody please help me get the code to work? What is wrong with the code?

5 errors
Addition2.java:24: ')' expected
      JOptionPane.showMessageDialog( null, firstnumber "+" secondnumber
                                                      ^
Addition2.java:25: not a statement
      + sum, "Results", JOptionPane.PLAIN_MESSAGE );
      ^
Addition2.java:25: ';' expected
      + sum, "Results", JOptionPane.PLAIN_MESSAGE );
           ^
Addition2.java:25: not a statement
      + sum, "Results", JOptionPane.PLAIN_MESSAGE );
                                   ^
Addition2.java:25: ';' expected
      + sum, "Results", JOptionPane.PLAIN_MESSAGE );

The code is the following:

import javax.swing.JOptionPane;

public class Addition2
{
   public static void main( String args[] )
   {
      String firstnumberstring;
      String secondnumberstring;

      int firstnumber;
      int secondnumber;
      int sum;

      firstnumberstring = JOptionPane.showInputDialog( 
            "Write first number" );
      secondnumberstring =
         JOptionPane.showInputDialog( "Write second number" );

      firstnumber = Integer.parseInt( firstnumberstring );
      secondnumber = Integer.parseInt( secondnumberstring );

      sum = firstnumber + secondnumber;

      JOptionPane.showMessageDialog( null, firstnumber "+" secondnumber
      + sum, "Results", JOptionPane.PLAIN_MESSAGE );
   } 
}

Answer 1

You need to concatenate strings using the string cat operator: +

JOptionPane.showMessageDialog( null, firstnumber  + "+" + secondnumber + " = " + sum, "Results", JOptionPane.PLAIN_MESSAGE );

Btw; it’s considered a good thing to declare variables where they are assigned for the first time – and not sticking to the old c- requirement (with ancient roots) that all variables must be declared before the actual code.

String firstnumberstring = JOptionPane.showInputDialog( 
        "Write first number" );

It helps a little when trying to figure out where a variable is used.
It’s particulary good when a variable is used only in a specific scope; ( inside curly brackets)