plz help me solving this small but annoying problem!! plz.. i need help urgently.

[Y. Progammer]

public void paintComponent(final Graphics g) {
super.paintComponent(g);
g.setColor(Color.BLACK);

jButton1.addActionListener(
new ActionListener() {

public void actionPerformed(ActionEvent event) {
if (event.getSource() == jButton1) {
g.drawLine(100,100,100,100);
}
}
});
}



I made a panel and added to it a button. What I want exactly is that when I press this button a line should be drawn..


The problem is the object 'g' isn't responding i.e., I am unable to use it to plot the line. Plz help:(

[JosAH]

public void paintComponent(final Graphics g) {
super.paintComponent(g);
g.setColor(Color.BLACK);

jButton1.addActionListener(
new ActionListener() {

public void actionPerformed(ActionEvent event) {
if (event.getSource() == jButton1) {
g.drawLine(100,100,100,100);
}
}
});
}



I made a panel and added to it a button. What I want exactly is that when I press this button a line should be drawn..


The problem is the object 'g' isn't responding i.e., I am unable to use it to plot the line. Plz help:(

Don't add that action listener to the button in the paintComponent( ... ) method; that method is called many many times so you end up with as many action listeners all connected to that single button.

An alternative scenario would be something like this: first define an interface for something that can draw:

public interface Drawer {
   void draw(Graphics g);
}

Next, define a List<Drawer> drawerList in your components and methods to add and remove Drawers to/from it. Finally add the following code to your paintComponent( ... ) method:

for (Drawer d : drawerList)
   d.draw(g);

in words: when the component needs painting offer all registered Drawers an opportunity to draw on the Graphics.

Your line drawing Drawer would look something like this:

new Drawer() {
   public void draw(Graphics g) {
      g.drawLine(100,100,200,200); // not to 100,100; read the API
   }
}

... and add it to that list once. Writing that last piece of code I realized that your original drawer draws a line from point 100,100 to point 100,100; i.e. it is (almost) invisible. Nevertheless, the scenario I outlined above is better than registering listeners in the paintComponent( ... ) method over and over again.

kind regards,

Jos

[Y. Progammer]

Why it is that adding action listener to the button in the paintComponent( ... ) method; that method is called many many times..??? plz explain.

[collin389]

paintComponent is called every time that it needs to draw the window. That is, whenever you move, or resize it. and if you make a new oject, action listener, and keep adding it to the button, your button will be calling every single action listener. Ex. you drag your window 500 pixels, about half a screen. Depending on how slow you drag it, paintComponent will be called around 500 times. That means that you have created 501 action listeners which is horrible for memory, when you click the button, it will draw the line, and then call paintComponent() to redraw it. that means that for every action listener, drawLine() and therefore paintComponent() will be called, resulting in more action listeners, 1002. this is obviously even worse for memory. and a horrible way to try to paint something.

[JosAH]

Why it is that adding action listener to the button in the paintComponent( ... ) method; that method is called many many times..??? plz explain.

Put the following line in your paintComponent( ... ) method and see for yourself:

System.out.println("adding another listener ...");

kind regards,

Jos

[Y. Progammer]

jButton.addActionListener(
                new ActionListener() {

                    public void actionPerformed(ActionEvent event) {
                        if (event.getSource() == jButton) {
                            System.out.println("sd");
                        }
                    }
                });

Thnx for reply but I do have few more questions !

PROBLEM 1:
My questions are based on following two observations::

OBSERVATION 1:

run the file..
press button
[output]
sd
sd
minimized window
maximized window
press button
[output]
sd
sd
sd
minimized window
maximized window
press button
[output]
sd
sd
sd
sd
minimized window
maximized window
press button
[output]
sd
sd
sd
sd
sd

In this observation it is clear that whenever the window appears again and again.. the paintComponent() method is called and consequently yet another actionListener is added to the button apart existing actionListeners and when the button is pressed all the current actionListeners respond. ok. thats fine.

[COLOR="rgb(46, 139, 87)"]OBSERVATION 2[/COLOR]:

run the file..
press button
[output]
sd
sd
press button
[output]
sd
sd
press button
[output]
sd
sd
press button
[output]
sd
sd

In this observation the window appears only once that is it never minimized and maximized nor it is ever resized and so the paintComponent() method is never called again and again and so the number of initially existing actionListeners respond. ok. thats f9.

But the big questions is that ... why the "number of actionListeners = 2 " for the very first time when the file is run.. (as observable from the output above sd sd).
plz explain.. how many times the method paintComponent() method is called initailly and from where and why it is called twice making the number of actionListeners = 2.

PROBLEM 2:

jButton.addActionListener(
                new ActionListener() {

                    public void actionPerformed(ActionEvent event) {
                        if (event.getSource() == jButton) {
                            g.drawLine(100,100,600,600);
                        }
                    }
                });

ok. I understood that this method of adding actionListner from inside that paintComponent() method is wrong way to draw anything.. but my question is why it is that the line is never drawn when the button is pressed.{It is never drawn.. I checked myself.} Why it happened.. plz explain.:confused:

[Fubarable]

In all likelihood the Graphics object you are drawing with is no longer the current active graphics object. The bottom line is that this approach is completely wrong.

I don't know why paintComponent is called initially twice, but I do know that we have only limited control as to when and how often it gets called. We can suggest that it be called via repaint(), but even this is sometimes ignored.

Also, regarding your "urgency", please understand that this can sometimes have the opposite effect that you intend, that sometimes stating this in a thread title will induce others not to reply. To learn why, please have a look at this link from JavaRanch: Ease Up

Much luck!

[Y. Progammer]

ok. I would keep in mind for the new threads.. and thanx for the reply.

But what about PROBLEM 2:

[Fubarable]

ok. I would keep in mind for the new threads.. and thanx for the reply.

cool.

But what about PROBLEM 2:

I answered that in the first statement of my reply above.

[Y. Progammer]

sO which object is the currently active object??

[Fubarable]

sO which object is the currently active object??

The currently active Graphics object is the one that is passed to the paintComponent method at the time that this method is called. Please read all you can at the Sun/Oracle tutorials on Swing graphics as it will explain all and will really help you immensely. Honest.

[Y. Progammer]

can you plz give the exact link.. thanx.

[Fubarable]

can you plz give the exact link.. thanx.

Sure:
Lesson: Performing Custom Painting (The Java™ Tutorials > Creating a GUI With JFC/Swing)

And always, GIYF. Much luck!

[Fubarable]

Some example code that shows lines being drawn on the press of a button that might or might not help you:

import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.geom.Line2D;
import java.util.ArrayList;
import java.util.List;
import java.util.Random;

import javax.swing.*;

public class DrawLineEg extends JPanel {
  private boolean showRedLine = false;
  private Random random = new Random();
  private List<Line2D> randomLineList = new ArrayList<Line2D>();

  public DrawLineEg() {
    setPreferredSize(new Dimension(700, 500));
 
    // button to show or hide red line.  The button simply toggles
    // the value of a boolean variable, showRedLine, calls repaint 
    // on the main JPanel and the paintComponent method decides whether or
    // not to draw the line depending on the value of showRedLine
    JButton toggleLineBtn = new JButton("Toggle Red Line");
    toggleLineBtn.addActionListener(new ActionListener() {
      public void actionPerformed(ActionEvent e) {
        showRedLine = !showRedLine;
        DrawLineEg.this.repaint();
      }
    });
    
    // this button will create a random Line2D object and add it to a 
    // List<Line2D> called randomLineList and then calls repaint()
    // the paintComponent method will iterate through this list, painting
    // all Line2D objects in the list.  Because this requires a Graphics2D object, 
    // the paintComponent's Graphics object must be explicitely cast to a 
    // Graphics2D object before this will work
    JButton addRandomLineBtn = new JButton("Add Random Blue Lines");
    addRandomLineBtn.addActionListener(new ActionListener() {
      public void actionPerformed(ActionEvent e) {
        Dimension size = DrawLineEg.this.getSize();
        
        int x1 = random.nextInt(size.width);
        int y1 = random.nextInt(size.height);
        int x2 = random.nextInt(size.width);
        int y2 = random.nextInt(size.height);
        
        Line2D line = new Line2D.Double(x1, y1, x2, y2);
        randomLineList.add(line);
        DrawLineEg.this.repaint();
      }
    });
    
    add(toggleLineBtn);
    add(addRandomLineBtn);
  }
  
  @Override
  protected void paintComponent(Graphics g) {
    super.paintComponent(g);
    
    // draw red line if boolean true
    if (showRedLine) {
      g.setColor(Color.red);
      g.drawLine(100, 100, 600, 300);
    }
    
    g.setColor(Color.blue);
    // draw all random lines held in randomLineList
    Graphics2D g2 = (Graphics2D) g;
    for (Line2D line : randomLineList) {
      g2.draw(line);
    }
  }
  
  private static void createAndShowUI() {
    JFrame frame = new JFrame("DrawLineEg");
    frame.getContentPane().add(new DrawLineEg());
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.pack();
    frame.setLocationRelativeTo(null);
    frame.setVisible(true);
  }

  public static void main(String[] args) {
    java.awt.EventQueue.invokeLater(new Runnable() {
      public void run() {
        createAndShowUI();
      }
    });
  }
}