Problem with GUI.

[djokovic]

Hello, I wanted to create program with JFrame window. In that window I want to add Button, and when people press the button, it will paint the shape on the screen.

First I made abstract superclass of all shapes.

import javax.swing.JPanel;
import javax.swing.JButton;

abstract public class Draw extends JPanel{
	 JButton button;
	 String buttonTitle;
	 
	abstract public JButton getButton();
}

Then I created one subclass, DrawRect

import javax.swing.JButton;
import java.awt.Graphics;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

public class DrawRect extends Draw{
	
	public JButton getButton() {
		buttonTitle = "Draw Rect";
		button = new JButton(buttonTitle);
		return button;
	}
	
	public void paintComponent(Graphics g){
		g.drawRect(50, 50, 120, 120);
	}

}

And this is my main class

import java.awt.BorderLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JFrame;


public class test extends JFrame implements ActionListener{
	
   Draw d = new DrawRect();
   
   public static void main(String[] args){
	   test t = new test();
	   t.run(); 
   }
   
   public void run(){
	   
	   setSize(300,300);
	   setVisible(true);
	   getContentPane().add(BorderLayout.NORTH,d.getButton());
	   
	   d.getButton().addActionListener(this);
   }


   public void actionPerformed(ActionEvent e) {
	   getContentPane().add(BorderLayout.CENTER,d);
	   
	   
   }
        
}

So, the problem is, that it doesn't work, when I press the button, but If I would copy this line:

getContentPane().add(BorderLayout.CENTER,d);

and add in run() method this pop up when I start program:


But as I said before, if I would remove this line from run, and add same line in actionPerformed method, it doesn't work, even if i click the button. Can you tell me what's wrong here?

[Fubarable]

After adding the new component to the contentPane, you may want to next call revalidate() and then repaint() on the contentPane. The revalidate tells the contentPane to re-lay-out the components it holds, and the repaint tells the contentPane to repaint itself and its children. Now since contentPane is declared as a Container, the compiler won't like you calling revalidate() since this is a method of JComponent, but since we all know that the contentPane is really a JPanel, we can cast it as such and then be able to call the method:

JPanel myContentPane = (JPanel) getContentPane();
myContentPane.add(someComponent, BorderLayout.CENTER);
myContentPane.revalidate();
myContentPane.repaint(); // not always needed

[djokovic]

After adding the new component to the contentPane, you may want to next call revalidate() and then repaint() on the contentPane. The revalidate tells the contentPane to re-lay-out the components it holds, and the repaint tells the contentPane to repaint itself and its children. Now since contentPane is declared as a Container, the compiler won't like you calling revalidate() since this is a method of JComponent, but since we all know that the contentPane is really a JPanel, we can cast it as such and then be able to call the method:

JPanel myContentPane = (JPanel) getContentPane();
myContentPane.add(someComponent, BorderLayout.CENTER);
myContentPane.revalidate();
myContentPane.repaint(); // not always needed

So I need to add your code to actionPerformed method and change someComponent to "d"? I'm asking because it doesn't work also.

[Fubarable]

You are creating a new and different JButton each time you call getButton(). Thus the JButton that is being displayed is not the same as the one with the ActionListener (which you'd discover if you placed System.out.println(...) debug statements in the actionPerformed method). If you're going to do lazy initialization of the button, check first if button is null before creating a new one.

[djokovic]

You are creating a new and different JButton each time you call getButton(). Thus the JButton that is being displayed is not the same as the one with the ActionListener (which you'd discover if you placed System.out.println(...) debug statements in the actionPerformed method). If you're going to do lazy initialization of the button, check first if button is null before creating a new one.

So, I need to add actionlistener in the same class where button was created? Thank you, but if I would have ten buttons, it is ok to do everything in one class? I don't remember now, but in one Java book, I read, that I need to split work, and don't do everything in one place.

[Fubarable]

So, I need to add actionlistener in the same class where button was created?

I did not say anything like that, and if you thought I did, please show me where. I said if you are going to have lazy initialization or creation of your JButton, you'd better first check if its null. If it's not null, then return the actual JButton instance held by the button variable and don't create a new one. You don't want to create a bunch of buttons when all you need is one.