First GUI Problem

[shodai]

I am trying to teach myself some java but I have run into some difficulty in trying to create a GUI. I have made a couple of simple ones but just by copying code and trying to figure out what is going on. I found this code and have been trying to figure things out. I have made a couple of changes already and am down to one error on line 14 "c.add(e);" I am stumped and was wondering if anyone can see what I might be missing.

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
 
public class simpleForm extends JFrame {
  private JPanel contentPanel;
  private JLabel jLabel1;
  private JLabel jLabel2;
  private JTextField jTextField1;
  private JButton jButton1;
 
  private void addElement (Container c, Component w, int x, int y, int h, int j) {
    w.setBounds(x, y, h, j); 
    c.add(e);
  }
 
  simpleForm() { 
    contentPanel = (JPanel)getContentPane();
    contentPanel.setLayout(null); // a blank form
    jLabel1 = new JLabel("Action:");
    addElement(contentPanel, jLabel1,0,0,100,20); 
    jTextField1 = new JTextField(); 
    addElement(contentPanel, jTextField1,0,20,100,20); 
    jButton1 = new JButton("Click Me");
    addElement(contentPanel, jButton1,0,100,100,20); 
 
    jButton1.addActionListener(new ActionListener() {
      public void actionPerformed(ActionEvent e) { 
        jTextField1.setText("Button clicked"); 
      } 
    }); 
 
    setTitle("A Simple Form"); 
    setSize(200, 200); // The GUI dimensions 
    setLocation(new Point(150, 150)); //The GUI position
    setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
    setVisible(true); 
  }
 
  public static void main(String args[]) {
    new simpleForm(); 
  }
}

[pbrockway2]

c.add(e);

What is e supposed to represent?

The reason for the compiler message is that you are using the variable e, but it not declared anywhere.

What do you intend your addElement() method to do?

[shodai]

e is connected to the ActionEvent in line 28. I tried declaring it in the addElement parameters, but it did not like that.

As I said, this is not my original code. I am trying to learn by recreating and analysing existing code. So ultimately I am not really sure what the point of c.add(e) is even for. I can remove that line of code and the program runs but I only get a blank frame with nothing inside. So......

What I am trying to do is create a frame and add some basic elements within the frame.

[pbrockway2]

Honestly, if you can't say what c.add(e) is supposed to do, I can't really say how it could be changed to do that.

---

In my opinion copying other people's code isn't such a great way to learn a technology - not unless the code is full of comments and works step by step elaborating the technology it is trying to illustrate. And in that case it becomes a tutorial.

But that's just my opinion... and if you want to copy that code, make sure you have copied it correctly. There's a good chance that you have made a typo in the line that says "c.add(e);"

Unless you copied the code from here in which case the error is in the original. But that page *does* explain what the addElement() method is supposed to do, and what the w parameter is.

---

I feel a bit mean not saying what the error is. So try this:

private void addElement (Container c, Component w, int x, int y, int h, int j) {
    w.setBounds(x, y, h, j); // set the size and bounds of the widget 
    c.add(w); // add the widget to the container.
}

Note that the line is "c.add(w)".

That said the code is a bit cr@ppy. Not just the typo (although that's symptomatic), but it's not Java style to start the class name with a lower case letter, and not generally good Swing practice to use the setBounds() way of laying out widgets within their container.

If I knew a really good Swing introductory tutorial I'd recommend it. Perhaps others here do.

[shodai]

Thank you so much for taking the time to find the error. You are totally correct that trying to reverse engineer a persons code is not a great approach, but when I look up tutorials I seem to find that there is no consistency in how people go about doing things. If anyone has a resource or two that they think would be helpful I would be extremely grateful.

Thanks again pbrockway2.

[pbrockway2]

You're welcome.

Oracle's Tutorial is very good - arguably the best - but the Swing section assumes some familiarity with Java and it attempts to be rather comprehensive so it's long. Also it presents things a piece at a time rather disconnectedly rather than building up a modest application which is probably a better way to proceed if you're new to programming and/or Swing. Hence my reluctance to simply point you there and say grumpily "don't copy code".

Whatever you do, understand every line. And any surrounding textual comment. In this case the author did discuss "widgets" enough to give a clue about what was going on (provided you knew enough about Swing to already have a fair idea what was going on!. You can find every method documented in the API docs, so bookmark that resource and consult it so that no method call is a mystery.

If you find things on the web that don't wrok or are unclear, ask about them on forums like this. And don't be afraid to give the url: the more information, the better.

When I started to write code, I commented things. A lot. Probably I over commented them, but that's by far a lesser evil and easily fixed later with the delete key. But the act of commenting on discoveries as I made them helped to fix them in my mind. Java comments and meaningful variable names are essential for anything more complex than "Hello World" and the compiler isn't going to help you here: you have to be disciplined and do this for yourself. The easiest thing is if the discipline becomes a habit. The method in question could be rewritten:

    /**
     * Sets the size and position of a component and adds it to a container.
     *
     * @param  con     the container to which the component will be added
     * @param  widget  the component to be added
     * @param  left    the x- coordinate of the component's position
     * @param  top     the y- coordinate of the component's position
     * @param  width   the width to make the component
     * @param  height  the height to make the component
    */
private void addElement (Container con, Component widget, int left, int top, int width, int height) {
    widget.setBounds(left, top, width, height); 
    con.add(widget);
}

(Notice the order of the parameters in setBounds(). The author had called the third one "h" suggesting he thought it was height. This is the sort of mistake I make and it's corrected by using proper variable names and looking up the setBounds() documentation.)

Good luck, and happy coding!