static reference variable in GUI

[Leandro Coutinho]

Hi!

I think that all fields (variables) related with the GUI must be static, am I right?
Because there is no reason to have more than one instance of the same object, like label, button, and the frame.

Here is an example:

import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;
import java.awt.Insets;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JTextField;

public class GridBag2 {
	private static JFrame frame;
	private static JLabel lblName;
	private static JTextField txtName;
	private static JButton btnSave;
	
	public static void addComponentsToPane() {
		frame.setLayout(new GridBagLayout());
		
		GridBagConstraints c = new GridBagConstraints();
		c.fill = GridBagConstraints.NONE;
		c.insets = new Insets(5, 5, 5, 5);
				
		lblName = new JLabel("Name:");
		c.anchor= GridBagConstraints.NORTHEAST;
		c.gridx = 0;
		c.gridy = 0;
		frame.add(lblName, c);
		
		txtName = new JTextField(20);
		c.anchor= GridBagConstraints.NORTHWEST;
		c.gridx = 1;
		frame.add(txtName, c);
		
		btnSave = new JButton("Save");
		c.anchor= GridBagConstraints.NORTHWEST;
		c.gridx = 1;
		c.gridy = 1;
		frame.add(btnSave, c);
	}
	
	public static void createAndShowGUI() {
		// Create and set up the window.
		frame = new JFrame("My App");
		frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		
		// Set up the content pane.
		addComponentsToPane();
		
		// Display the window.
		frame.pack();
		frame.setVisible(true);
	}
	
	public static void main(String[] args) {
		javax.swing.SwingUtilities.invokeLater(new Runnable() {
			public void run() {
				createAndShowGUI();
			}
		});
	}
}

What do you think?

[John Sutt]

Hi Leandro,

I tend to disagree with you.

Making the variables static will only make sure that the program does not work correctly if the
GridBag2 class is instantiated multiple times.
For example, the txtName textfield probably is editable by the user. You don't want this to be
static!

(If you really want to enforce there is a single instance at most, you could use the Singleton pattern.)

I would make none of those variables static.

[Leandro Coutinho]

Thanks for your reply.

John Sutt wrote:
For example, the txtName textfield probably is editable by the user. You don't want this to be
static!

Why not?

[John Sutt]

I tested it now with the below program.
In case the JTextField tf is static, it did not show up in the second JFrame.
If you remove the "static", it works fine.

import java.awt.BorderLayout;

import javax.swing.JFrame;
import javax.swing.JTextField;


public class Leandro1 extends JFrame {

	private static JTextField tf = new JTextField();

	public Leandro1(String title) {
		super(title);
		setLayout(new BorderLayout());
		add(tf, BorderLayout.CENTER);
	}
	
	public static void createGUI() {
		Leandro1 l = new Leandro1("Leandro Test");
		l.pack();
		l.setVisible(true);
	}
	
	public static void main(String[] args) {
		createGUI();
		createGUI();
	}
}

    

[K. Tsang]

Hi John, from what I understand the static variable and where to instantiate the component is 2 seperate unrelated issues. If you declare and instantiate the Swing JTextField as instance variable for a dialog box, the 2nd time you load it this JTextField component will not re-instantiate meaning you can see the original text entered before. Therefore declare it but don't instantiate it until inside the methods - then you won't get this problem.