Java Swing Objects Unreachable

[Lokesh Poovaragan]

I am a beginner in Java Swing: and I want to know how to code GUI. My program has a JMenuItem so I had to make an actionListener for it, hence I follow the object oriented way of writing my program. Basically what I am trying to say is that I did not write all the GUI start up code in my main method, I wrote a seperate class and called the code from there
let me show you a glimpse of how I have written my program:

class Main{
    public static void main(String[] args)throws Exception{
       GUICode gui = new GUICode();
       gui.startGUI();
    }
}


class GUICode{
    public void startGUI() throws Exception{
       JFrame jframe = new JFrame();
       JTextArea jtext = new jTextArea();
       //Assume that I have connected the jtextarea with the jframe, all that is working perfectly 
       //and all the other GUI startup code over here: including the JMenuButtons.  Also I set up
       // parameters such as jframe.setVisible(true) everything like parameters I set over here.
      Font matrix = Font.createFont(Font.TRUETYPE_FONT, is).deriveFont(20f);
      //This font code, just saying: throws an exception, called FontFormatException, and also a
      // IOException, just saying, this doesnt bother me untill later.

    }

    public void show(){
       //with this method, I want to access the objects I made for the GUI such as:
       jtext.setText("this text")
       //but the problem comes here: the compiler tells me that the "jtext" variable does not exist!
       // I can clearly see why it would not exist! its because: jtext is started by another method
       // and I want to access that method's Objects, which is not possible.
      
    }
}

I soon solved this problem by myself, not knowing what to do, I thought that if I moved the JFrame and all the gui Object declarations outside, it would benefit me, hence I moved it outside, like this:

class Main{
    public static void main(String[] args){
       GUICode gui = new GUICode();
       gui.startGUI();
    }
}


class GUICode{



       JFrame jframe = new JFrame();
       JTextArea jtext = new jTextArea();
       //Assume that I have connected the jtextarea with the jframe, all that is working perfectly 
       //and all the other GUI startup code over here: including the JMenuButtons
       Font matrix = Font.createFont(Font.TRUETYPE_FONT, is).deriveFont(20f);
       //Here is my next hurdle: you see Font throws an exception, and since I am not exactly inside
       // any method, and since I am actually outside of any method declaration, I CANNOT throw
       // the exception :(


    public void startGUI(){
      //but still, the parameters are still set from here, such as updating the setVisible method and
      // all the other parameters are still set from here.
    }

    public void show(){
       //with this method, I want to access the objects I made for the GUI such as:
       jtext.setText("this text")
       //with this method of putting the object declarations on the outside, I think the compiler was
       // ok with me, but the compiler still complained that "Font" throws an exception which I have
       // not caught, but I Could not catch the expression mainly becuase I its not inside any method.
      
    }
}

with this I (think) I solved one problem, but I started another, as Font threw an exception, I cannot get it to compile, hence after further head banging, I came accross something called as a Constructor, I thought it was worth a shot, and I tried that method too: like so:

class Main{
    public static void main(String[] args){
       GUICode gui = new GUICode();
       gui.startGUI();
    }
}


class GUICode{

   public GUICode throws Exception{

       JFrame jframe = new JFrame();
       JTextArea jtext = new jTextArea();
       //Assume that I have connected the jtextarea with the jframe, all that is working perfectly 
       //and all the other GUI startup code over here: including the JMenuButtons
       Font matrix = Font.createFont(Font.TRUETYPE_FONT, is).deriveFont(20f);
       //Since I am inside SOME type of method declaration, I figured that the throws Exception
       // would work, and I (think) it sort of did.
     }

    public void startGUI(){
      //Now both the methods, startGUI() and show() both of them, CANNOT, find any thing that I have
      // declared, Its almost as though the Object declarations wernt even declared anywhere!
      //So I couldnt even set any parameters now...

    }

    public void show(){
       //with this method, I want to access the objects I made for the GUI such as:
       jtext.setText("this text")
       //this method and the previous method, both could not even FIND the Object Declarations.

      
    }
}

I wish that all methods can easily access the object declarations

Hence I am stuck at this, I cannot come up with any other way to twist and turn this, so I wanted to ask here,
I really hope I have typed all this out clearly...

for now, can you please tell me, which method was correct and where I went wrong? or if all three of my methods were wrong and I have to declare the objects somewhere else where it can be accessed by everything?

-Lokesh

[Michael Dunn]

all 3 methods would work, except the JTextArea needs to be a class field (not a local variable),
as it needs to be referenced by show()

something like this

class GUICode{
   JTextArea jtext;//needs to be 'seen' by show()
   public GUICode(){
       JFrame jframe = new JFrame();
       //JTextArea jtext = new jTextArea();
       jtext = new JTextArea();
     }
    public void startGUI(){
    }
    public void show(){
       jtext.setText("this text");
    }
    public static void main(String[] args){
       SwingUtilities.invokeLater(new Runnable(){//always start your gui on the Swing thread = EDT
        public void run(){
          new GUICode();
          //new GUICode().startGUI();//if the code in the constructor is in startGUI instead
        }
       });
    }
}

[Lokesh Poovaragan]

would it be ok, if I declared only the "Font f" outside as a class field, and the rest of it "f = createFont" indside the method? would that be good coding structure? also how is it done professionally? I may have 3 methods of doing it, but which one is most widely and/or the recommended way of doing it?

[Michael Dunn]

> would it be ok, if I declared only the "Font f" outside as a class field, and the rest of it "f = createFont" indside the method?

yes, that's a common way.

> would that be good coding structure? also how is it done professionally?

the professional way is what the boss wants :-)

> I may have 3 methods of doing it, but which one is most widely and/or the recommended way of doing it?

all the sun/oracle tutorials were coded with a createAndShowGUI() (they used to, haven't read them for a while).

possibly one of the main considerations is code re-usability e.g. if you have everything, including a JFrame,
tied up in a constructor, you can't add that class to, say, an applet, or anything else, it's just a stand-alone.

here's an example of the createAndShowGUI() where the panel will display in a JFrame

import java.awt.*;
import javax.swing.*;

public class MyPanel extends JPanel
{
  public MyPanel()
  {
    setLayout(new GridBagLayout());
    setPreferredSize(new Dimension(400,300));
    setBackground(Color.RED);
    JLabel lbl = new JLabel("Hello World");
    add(lbl,new GridBagConstraints());
  }
  public void createAndShowGUI()
  {
    JFrame f = new JFrame();
    f.getContentPane().add(this);
    f.pack();
    f.setLocationRelativeTo(null);
    f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    f.setVisible(true);
  }
  public static void main(String args[])
  {
    SwingUtilities.invokeLater(new Runnable(){
      public void run(){
        new MyPanel().createAndShowGUI();
      }
    });
  }
}

now create another .java file (keep all this in the same folder)

import java.awt.*;
import javax.swing.*;

public class MyApplet extends JApplet
{
  public void init()
  {
    add(new MyPanel());
  }
}

finally, copy this into Notepad and save as [anyNameYouWant].html

<html>
  <body>
    <center>      
    <APPLET CODE = "MyApplet.class" WIDTH = 400 HEIGHT = 300>
    Your browser does Not support Java Applets.</applet>     
    </center>
  </body>
</html>

double-click the html, and your browser will open, now run MyPanel.java,
and a Jframe will display the same thing

[Lokesh Poovaragan]

Okay Michael Dunn, I think I get it completely! I went with Declaring all my Object Declarations outside and then inside the block, I entered "f = createFont()" so that did the trick, I also managed to get the other function to run properly this way! and so to speak, all my problems have been solved, hence a BIG THANKS to you for helping me out, and clearly making me understand step by step!

also to set up and run the GUI components inside a Swing Thread was a bit of an overkill right now, I can say I have not understood Java in that much depth to understand how to use Threads yet, but I will soon get there, till then a single main Thread should suffice for whatever simple thing I am trying to do.

so at last a very big thanks for helping me step by step!

[Michael Dunn]

> also to set up and run the GUI components inside a Swing Thread was a bit of an overkill right now

read this for examples of what can happen if not started on the EDT

http://bitguru.wordpress.com/2007/03/21/will-the-real-swing-single-threading-rule-please-stand-up/