Thread: (Total begginner) simple GUI program nt displayin

dis code compiles w/o errors and runs buh doesnt produce d desired effect...its supposed to paint lines like d ones in the picture

Capture.JPG
pls kindly check it out

d DrawPanelTest class just runs d program n displays d DrawPanel, d DrawPanel class is d main problem

 
import java.awt.Graphics;
import javax.swing.JPanel;
 
//modified DrawPanel Class for esercise 4.1
public class DrawPanel extends JPanel
{
	public void paintComponent(Graphics g)
	{
	 super.paintComponent( g );//pls explain y dis shud ALWAYS be here
 
	 int width = getWidth(); 
	 int height = getHeight();
 
	 int widthCounter = 0;
	 int heightCounter = 10;
 
 
	 g.drawLine(0,0,width,height); // found out it painted dis line
 
	 //mayb am wrong buh i fink d probs 4rm here
	 while (widthCounter <= 10)
		{
		 width = (width*widthCounter)/10;
		 height = (height*heightCounter)/10;
 
		 g.drawLine(0,0,width,height);
 
		 ++widthCounter;
		 --heightCounter;
		}
	}
}

 
import javax.swing.JFrame;
 
public class DrawPanelTest
{
	public static void main( String args[] )
	{
	 DrawPanel panel = new DrawPanel();
	 JFrame application = new JFrame();
 
	 application.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
 
	 application.add(panel);
	 application.setSize( 300,300); 
	 application.setVisible( true );
	}
}

I thought paintComponent was a protected method.

----Edit-----
It appears to draw a line across the screen of the window. What should it be doing?

I'm wondering if it's going wrong about here:

 while (widthCounter <= 10)
		{
		 width = (width*widthCounter)/10;
		 height = (height*heightCounter)/10;
 
		 g.drawLine(0,0,width,height);
 
		 ++widthCounter;
		 --heightCounter;
		}

width = 0 * widthCounter / 10 = 0;
height = (getHeight() * 10) /10 = getHeight();
g.drawLine(0,0, 0, getHeight());

width = width /10;
height = (height * 9) / 10;
g.drawLine(0,0,width/10, (height*9)/10);

As for why the super should be there, JPanel, while it actually doesn't define a method called paintComponent(Graphics g), it inherits one from its parent, the grandparent of your class, JComponent. JComponent already has a method, though it's a protected method, so you should probably change the type from public to protected, which does the painting. If you leave out super, it will basically be you overriding the JComponent one but not getting any of its features. Calling super enables the paint to get all the conditions that the JComponent paint method has.

Kind of like this:

 
public class Square 
{
 
private double width;
private double height;
 
public Square()
{
setWidth(0.0);
setHeight(0.0);
}
 
public Square(double width, double height)
{
setWidth(width);
setHeight(height);
}
 
public void setWidth(double width)
{
this.width = width;
}
 
public double getWidth()
{
return width;
}
 
public void setHeight(double height)
{
this.height = height;
}
 
public double getHeight()
{
return height;
}
 
}

 
public class Cube extends Square
{
 
private double depth;
 
public Cube()
{
super(); // calls the Square default constructor
setDepth(0.0);
}
 
public Cube(double width, double height, double depth)
{
super(width, height);
setDepth(depth);
}
 
public void setDepth(double depth)
{
this.depth = depth;
}
 
public double getDepth()
{
return depth;
}
}

yeah, netbeans said sumtin abt it bein overridden

Here lies a problem, though I'm not sure it's THE problem.

int width = getWidth(); 
int height = getHeight();
 
int widthCounter = 0;
int heightCounter = 10;
//... other code ...
while (widthCounter <= 10)
{
	 width = (width*widthCounter)/10;
	 height = (height*heightCounter)/10;
 
	 g.drawLine(0,0,width,height);
 
	 ++widthCounter;
	 --heightCounter;
}

Think about what happens when Java does this:

width = (width*widthCounter)/10;
height = (height*heightCounter)/10;

First, it substitutes the values (we'll say the width and height are 100, for ease):

width = (100 * 0) / 10
height = (100 * 10) / 10

...

width = 0
height = 100

If you think about it, now since your width is zero, it will remain zero for the rest of the program because you use multiplication to change its value.
When I follow height's value, I find...
90... 72... 50.4 (50)... 30.24 (30)...

My brain tells me that this pattern will not result in the lines you want drawn. I suggest revising your formula. By the looks of that example, I would suggest trying to avoid multiplication altogether.

Oh! I see what you want! You want ten lines to be drawn total, as fractions of the width of the panel, I think. To do this, you can't just continue using...
width = (100 * 0) / 10

Width keeps getting changed, and from the first time you change it, it no longer represents the actual width of the panel. So, when you reassign width's value, you may have to call again the panel's getWidth() method. The same goes for height. I think this would fix your problem.

Thanks all of you. It was a logical error...
My first time in a forum and I was'nt disappointed...Nice!!!

Also, what does the /this/ keyword mean

Java's this keyword makes a reference to the object that is current being coded.

It is most commonly used when you have a parameter variable that shadows a field, like this:

//Let's say name is an object field
String name;
 
public void setName(String name) //Parameter is named same thing as field
{
   //"this" tells the program to set the name variable in THIS object
   //to the value of the parameter name. It makes things more concise.
   this.name = name;
}

For more information on the this keywoard, see Oracle's tutorial.