problem with JDialog

[Randall Twede]

i dont get this at all. i have used JOptionPane in the past with no problem. here is the relevant code

   private Paint parent;    // Reference to the JFrame containing this Panel

        if(parent.file != null)	// Open the file
        {
            BufferedImage image;

            try
            {  
                image = ImageIO.read(parent.file);
                if(image == null)
		{
                String message = "This program can only open image files, like gif or jpg";
                JOptionPane.showMessageDialog( parent, message, "Error", JOptionPane.ERROR_MESSAGE );
		}
		else
		{
                g.drawImage(image, 0, 0, null);
		}
            }
            catch (IOException e)
            {
                // handle exception...
            }
        }

if the file is an image file it opens it. but if it is not it displays a blank modal dialog with the title Error, but no message and no ok button so i cant close it. i have checked the API and the tutorial but they both say i am doing it correctly
it also throws exceptions the one at the top says:

exception in thread "AWT-EventQueue-0" java.lang.NullPointerEception

i tried replaceing the parameter message with the string literal with the same result. it didnt throw an exception at first and i managed to get it to close using the X in the title bar(which it should'nt do). i kept toying with it and the next time it did throw the exception. it says it is in paintComponent() line 75. im not using an IDE so im not sure but it has to be the line that opens the JDialog. what can possibly be null??? not parent since it displays the image file. not the String. so what is going on???

[Michael Dunn]

> problem with JDialog

so, where's the JDialog?

if the code you posted is inside a method (unlikely), then:

private Paint parent;
followed by
if(parent.file != null)
has to throw a NPE

> g.drawImage(image, 0, 0, null);

I hope you're not using getGraphics(), but if it's in paintComponent() and
you have a JOptionPane there, slash your wrists and start again.

[Rob Spoor]

Michael is right. getGraphics() is an evil method that should never be used. He's also right about not displaying any dialogs from the painting methods. You should initialize the image from somewhere (a button press, constructor, etc), then call repaint(). Your overridden paintComponent method should then draw the image if it's not null.

[Randall Twede]

ok, i'll post the entire class:

/*
 * DrawingArea.java
 * Class for drawing shapes
 * Created on February 4, 2004, 12:03 PM
 */

/**
 *
 * Randall Twede
 */
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.util.Vector;
import java.io.IOException;
import javax.imageio.ImageIO;
import java.awt.image.BufferedImage;

public class DrawingArea extends JPanel implements MouseListener, MouseMotionListener
{
   private Vector shapes = new Vector();
   private Point start = new Point();
   private Point end = new Point();
   private Paint parent;    // Reference to the JFrame containing this Panel
   private Shape current;
   private int x1 = 0;
   private int x2 = 0;
   private int y1 = 0;
   private int y2 = 0;
   private int w = 0;
   private int h = 0;
    
    /** Creates a new instance of DrawingArea */
    public DrawingArea(Paint p)
    {
        parent = p;
        setBorder( BorderFactory.createTitledBorder( "Drawing Area" ) );
        addMouseListener(this);
	addMouseMotionListener(this);
    }
    
    // Draws all the shapes stored in the vector
    public void paintComponent(Graphics g)
    {
        Shape shape;
        
        super.paintComponent(g);
        setBackground(parent.getBack());    // Set the background color
        if(parent.file != null)	// Open the file
        {
            BufferedImage image;

            try
            {  
                image = ImageIO.read(parent.file);
                if(image != null)
		{
                g.drawImage(image, 0, 0, null);
		}
		else
		{
                String message = "This program can only open image files, like gif or jpg";
                JOptionPane.showMessageDialog( parent, message, "Error", JOptionPane.ERROR_MESSAGE );
		}
            }
            catch (IOException e)
            {
                // handle exception...
            }
        }
        // Loop through the vector
        for(int i = 0; i < shapes.size(); i++)
        {
            shape = (Shape)shapes.elementAt(i);
            shape.draw(g);
        }
	  if(current != null)
	  {
	      current.draw(g);
	  }
    }
 
    public void undo()
    {
        shapes.removeElementAt(shapes.size() - 1);
        current = null;
        repaint();
    }
   
    // Clears the drawing area
    public void clear()
    {
        shapes.clear();
	current = null;
        repaint();
    }
    
    // Gets the starting point
    public void mousePressed(java.awt.event.MouseEvent mouseEvent)
    {
        start = mouseEvent.getPoint();
    }

    public void mouseReleased(java.awt.event.MouseEvent mouseEvent)
    {
        //Stores the Shape in a Vector
        shapes.add(current);
    }
    
    public void mouseClicked(java.awt.event.MouseEvent mouseEvent)
    {
    }
    
    public void mouseEntered(java.awt.event.MouseEvent mouseEvent)
    {
    }
    
    public void mouseExited(java.awt.event.MouseEvent mouseEvent)
    {
    }
    
    public void mouseDragged(java.awt.event.MouseEvent mouseEvent)
    {
        JRadioButton shape;
        String type;
        Color color;
        // Gets the ending point
        end = mouseEvent.getPoint();
        // Calls a private method
        setParameters();
        // Retrieves a referenced to the selected JRadioButton
        shape = parent.getSelected();
        // Sets the type of shape to draw
        type = shape.getText();
        // Sets the drawing color
        color = parent.getFore();
        // Creates a Shape object
	if (type == "Draw Line")
	{
	    current = new Shape(start.x, start.y, end.x, end.y, type, color);
	}
	else
	{
            current = new Shape(x1, y1, w, h, type, color);
	}
        // Call paintComponent()
        repaint();
    }

    public void mouseMoved(java.awt.event.MouseEvent mouseEvent)
    {
    }

    // Determines top left coordinates, width, and height
    private void setParameters()
    {
        x1 = start.x;
        y1 = start.y;
        x2 = end.x;
        y2 = end.y;
        int temp = 0;
        // Swap if necessary
        if (x1 > x2)
        {
            temp = x1;
            x1 = x2;
            x2 = temp;
        }
        // Swap if necessary
        if (y1 > y2)
        {
            temp = y1;
            y1 = y2;
            y2 = temp;
        }
        w = x2 - x1;
        h = y2 - y1;
    }
}

you say it is because i am displaying the dialog from inside paintComponent() so i will see what i can do with that knowledge.

[Randall Twede]

i did as you suggested and moved most of the code to actionPerformed() in the JFrame. in paintComponent() i only have this now:

        if(parent.image != null)
	{
            g.drawImage(parent.image, 0, 0, null);
	}

it works fine now

[Darryl Burke]

Just one point: Paint is a bad choice of name for a JFrame subclass, as there is already a java.awt.Paint interface in the standard JDK.

[Rob Camick]

If you want to get a file name the use a JFileChooser.

Also, you should never display a dialog in any painting method.