Nested canvas in GridLayout can't get its width and height

Question

Hello,
I have a class entitled DisplayCanvas, which will accept some parameters from an invocation in another class including a shape and message parameters. These parameters must be centered in the instance of DisplayCanvas.
For some reason, when I use this.getWidth() and this.getHeight() within my DisplayCanvas class, I get 0 and 0! I need the width and height in order to center the parameters the user will enter.
Why does the width and height result at 0? What can I do to get the width and height?
In my DisplayCanvas class notice the lines:
canWidth = this.getWidth();
canHeight = this.getHeight();
For some reason the result is 0 for each!
Here is my code for the DisplayCanvas class:
//import the necessary clases
import java.awt.*;
import java.applet.*;
import javax.swing.*;
//begin the DisplayCanvas
public class DisplayCanvas extends Canvas
  //declare private data members to house the width and height of the canvas
  private int canWidth;
  private int canHeight;
  //declare private data members for the shape and message
  private String message;
  private String shape;
  private Color sColor;
  private int sWidth;
  private int sHeight;
  private String font;
  private Color ftColor;
  private int ftSize;
  //declare public data members
  //constructor of DisplayCanvas
  public DisplayCanvas()
     //set the width and height
     canWidth = this.getWidth();
     canHeight = this.getHeight();
     //set all data members to defaults
     message = "";
     shape = "";
     sColor = null;
     sWidth = 0;
     sHeight = 0;
     font = "";
     ftColor = null;
     ftSize = 0;
  } //end the constructor
  //begin the setParams function
  public void setParams(String m, String s, Color c, int w, int h,
                        String f, Color ftC, int ftS)
      //set all private data members of DisplayShape to the arguments
      //this function assumes error checking was done by DemoShape
      message = m;
      shape = s;
      sColor = c;
      sWidth = w;
      sHeight = h;
      font = f;
      ftColor = ftC;
      ftSize = ftS;
  } //end the setParams function
  //begin the public paint function of ShowShape
  public void paint(Graphics g)
      //set and output the shape according to the arguments
      //determine the x and y of the shape
      int x = (canWidth - sWidth) / 2;
      int y = (canHeight - sHeight) / 2;
      //set the color for the graphic object
      g.setColor(sColor);
      //output the shape
      g.drawRect(x, y, sWidth, sHeight);
      //set and output the message according to the arguments
      //set the color and the font for the graphic object
      g.setColor(ftColor);
      g.setFont(new Font(font, Font.PLAIN, ftSize));
      //determine the centering of the message
      //output the message with the settings
      g.drawString(canWidth + " " + canHeight, 10, 10);
  } //end the paint function of ShowShape class
} //end the DisplayCanvas classHere is my form entry class using the nested DisplayCanvas instance entitled drawCanvas:
//import the necessary java packages
import java.awt.*;                  //for the awt widgets
import javax.swing.*;               //for the swing widgets
import java.awt.event.*;            //for the event handler interfaces
//no import is needed for the DisplayCanvas class
//if in the same directory as the DemoShape class
public class DemoShape extends JApplet
    //declare private data members of the DemoShape class
    //declare the entry and display panel containers
    private Container entire;           //houses entryPanel and displayCanvas
    private JPanel entryPanel;          //accepts the user entries into widgets
    private DisplayCanvas drawCanvas;   //displays the response of entries
    //required control buttons for the entryPanel
    private JTextField xShapeText, yShapeText, messageText, fontSizeText;
    private ButtonGroup shapeRadio;
    private JRadioButton rect, oval, roundRect;
    private JComboBox shapeColorDrop, fontTypeDrop, fontColorDrop;
    //declare public data members of the DemoShape class
    //init method to initialize the applet objects
    public void init()
        //arrays of string to be used later in combo boxes
        //some are used more than once
        String fonts[] = {"Dialog", "Dialog Input", "Monospaced",
                            "Serif", "Sans Serif"};
        String shapes[] = {"Rectangle", "Round Rectangle", "Oval"};   
        String colors[] = {"Black", "Blue", "Cyan", "Dark Gray",
                            "Gray", "Green", "Light Gray", "Magenta", "Orange",
                            "Pink", "Red", "White", "Yellow"};
        //declare variables to assist with the layout
        //these are the left and right justified x coordinates
        int ljX = 10; int rjX = 150;
        //this is the y coordinates for the rows
        int yRow1 = 10;     //the shape rows
        int yRow2 = 40;
        int yRow3 = 60;
        int yRow4 = 130;
        int yRow5 = 150;
        int yRow6 = 210;    //the message rows
        int yRow7 = 240;
        int yRow8 = 260;
        int yRow9 = 300;
        int yRow10 = 320;
        int yRow11 = 360;
        int yRow12 = 380;
        //these are the widths for the text boxes, drop downs
        //message entry,  big message entry and radio buttons
        int tWidth = 30; int dWidth = 100;
        int mWidth = 250; int bmWidth = 250;
        int rWidth = 125;
        //the height is universal, even for the messages!
        int height = 25;
        //set a content pane for the entire applet
        //set the size of the entire window and show the entire applet
        entire = this.getContentPane();
        entire.setLayout(new GridLayout(1, 2));
        //create the entry panel and add it to the entire pane
        entryPanel = new JPanel();
        entryPanel.setLayout(null);
        entire.add(entryPanel);
        //create the display canvas and add it to the entire pane
        //this will display the output
        drawCanvas = new DisplayCanvas();
        entire.add(drawCanvas);       
        //entry panel code
        //add the form elements in the form of rows
        //the first row (label)
        JLabel entryLabel = new JLabel("Enter Shape Parameters:");
        entryPanel.add(entryLabel);
        entryLabel.setBounds(ljX, yRow1, bmWidth, height);
        //second row (labels)
        JLabel shapeTypeLabel = new JLabel("Select Shape:");
        shapeTypeLabel.setBounds(ljX, yRow2, mWidth, height);
        entryPanel.add(shapeTypeLabel);
        JLabel shapeColorLabel = new JLabel("Select Shape Color:");
        shapeColorLabel.setBounds(rjX, yRow2, mWidth, height);
        entryPanel.add(shapeColorLabel);
        //third row (entry)        
        rect = new JRadioButton("Rectangle", true);
        oval = new JRadioButton("Oval", false);
        roundRect = new JRadioButton("Round Rectangle", false);
        rect.setBounds(ljX, yRow3, rWidth, height);
        oval.setBounds(ljX, yRow3 + 20, rWidth, height);
        roundRect.setBounds(ljX, yRow3 + 40, rWidth, height);
        shapeRadio = new ButtonGroup();
        shapeRadio.add(rect);
        shapeRadio.add(oval);
        shapeRadio.add(roundRect);
        entryPanel.add(rect);
        entryPanel.add(oval);
        entryPanel.add(roundRect);       
        shapeColorDrop = new JComboBox(colors);
        shapeColorDrop.setBounds(rjX, yRow3, dWidth, height);
        shapeColorDrop.addFocusListener(new focusListen());
        entryPanel.add(shapeColorDrop);
        //the fourth row (labels)
        JLabel xShapeLabel = new JLabel("Enter Width:");
        xShapeLabel.setBounds(ljX, yRow4, mWidth, height);
        entryPanel.add(xShapeLabel);
        JLabel yShapeLabel = new JLabel("Enter Height:");
        yShapeLabel.setBounds(rjX, yRow4, mWidth, height);
        entryPanel.add(yShapeLabel);
        //the fifth row (entry)
        xShapeText = new JTextField("200", 3);
        xShapeText.setBounds(ljX, yRow5, tWidth, height);
        xShapeText.addFocusListener(new focusListen());
        entryPanel.add(xShapeText);        
        yShapeText = new JTextField("200", 3);
        yShapeText.setBounds(rjX, yRow5, tWidth, height);
        yShapeText.addFocusListener(new focusListen());
        entryPanel.add(yShapeText);
        //the sixth row (label)
        JLabel messageLabel = new JLabel("Enter Message Parameters:");
        messageLabel.setBounds(ljX, yRow6, bmWidth, height);
        entryPanel.add(messageLabel);
        //the seventh row (labels)   
        JLabel messageEntryLabel= new JLabel("Enter Message:");
        messageEntryLabel.setBounds(ljX, yRow7, mWidth, height);
        entryPanel.add(messageEntryLabel);
        //the eighth row (entry)
        messageText = new JTextField("Enter your message here.");
        messageText.setBounds(ljX, yRow8, mWidth, height);
        messageText.addFocusListener(new focusListen());
        entryPanel.add(messageText);
        //the ninth row (label)
        JLabel fontTypeLabel = new JLabel("Select Font:");
        fontTypeLabel.setBounds(ljX, yRow9, mWidth, height);
        entryPanel.add(fontTypeLabel);
        JLabel fontColorLabel = new JLabel("Select Font Color:");
        fontColorLabel.setBounds(rjX, yRow9, mWidth, height);
        entryPanel.add(fontColorLabel);
        //the tenth row (entry)
        fontTypeDrop = new JComboBox(fonts);
        fontTypeDrop.setBounds(ljX, yRow10, dWidth, height);
        fontTypeDrop.addFocusListener(new focusListen());
        entryPanel.add(fontTypeDrop);       
        fontColorDrop = new JComboBox(colors);
        fontColorDrop.setBounds(rjX, yRow10, dWidth, height);
        fontColorDrop.addFocusListener(new focusListen());
        entryPanel.add(fontColorDrop);
        //the eleventh row (label)
        JLabel fontSizeLabel = new JLabel("Select Font Size:");
        fontSizeLabel.setBounds(ljX, yRow11, mWidth, height);
        entryPanel.add(fontSizeLabel);
        //the final row (entry)
        fontSizeText = new JTextField("12", 2);
        fontSizeText.setBounds(ljX, yRow12, tWidth, height);
        fontSizeText.addFocusListener(new focusListen());
        entryPanel.add(fontSizeText);
        //display panel code
        //use test parameters
        //these will later be retrieved from the entries
        drawCanvas.setParams("Hello", "roundRect", Color.red,
                            100, 100, "Serif", Color.black, 12);
        //set the applet to visible
        //set to visible and display
        entire.setSize(800, 600);
        entire.setVisible(true);
    }   //end the init method
    //declare an inner class to handle events
    private class focusListen implements FocusListener
        //supply the implementation of the actionPerformed method
        //pass an event variable as the argument
        public void focusLost(FocusEvent e)
        { JOptionPane.showMessageDialog(null, "Focus lost."); } 
        //declare an empty focus gained function
        public void focusGained(FocusEvent e) {}      
    }   //end testListen class
}   //end DemoShape class

Answer 1

Sorry for glossing over your code sample, particularly as it looks like one of the best I've seen so far on the forums, but I'm pretty sure the answer you are looking for is as follows:
Java doesn't render a component until paint() is called so until then you are not going to have any size settings because the jvm simply doesn't know how big the visual component is. This makes sense when you think about what the jvm is doing. The layout manager controls the display of the components depending on the settings it is supplied. So until it knows how many components you want, where, what kind of spacing, etc, etc, etc, how can the size be determined.
The true cycle of events is therefore:
create an instance of DisplayCanvas,
add it to your container,
make the container visible (which renders the component),
get the size of the DisplayCanvas instance.
You are being hampered because your desired chain of events is:
create an instance of DisplayCanvas,
get the size of the DisplayCanvas instance,
add it to your container,
make the container visible.
This state of affairs is highly annoying and then leads to the next question "what do we do about that?". There is a cunning trick which is to get the jvm to render the component to an off-screen image, thus calculating the dimensions of the component so that you can do precisely the kind of enquiry on the object that you have been looking for. It should be noted that this may not be the visual size for all the reasons given above, but it is the "preferred size" for the component. Check the Swing tutorials and have a look at the section on Layout Managers for a run down on what that means.
Anyway, we can use this handy code sample to determine the preferred size for your component. Notice the call to "paint()" (normally you would never make a direct call to paint() in swing) and the "g.dispose()" to free resources:
package com.coda.swing.desktool.gui;
import java.awt.Component;
import java.awt.Dimension;
import java.awt.Graphics2D;
import java.awt.image.BufferedImage;
public class PaintUtil
     public PaintUtil()
          super();
     public static Component paintBuffer(Component comp)
          Dimension size = comp.getPreferredSize();
          comp.setSize(size);
          BufferedImage img = new BufferedImage(size.width, size.height, BufferedImage.TYPE_INT_ARGB);
          Graphics2D g2 = img.createGraphics();
          comp.paint(g2);
          g2.dispose();
          return comp;
}Before you make a call to getWidth() or getHeight() put in a call to "PaintUtil.paintBuffer(this);" and see what happens.
By the way, I can't claim credit for this code ... and I really wish I could remember where I got it from myself so I can thank them :)