Java Help needed: Problem with using graphics

Solved by BIT0102-Mohaimin.

Code:

import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.BoxLayout;

class LeftPanel extends JPanel {

   private BoxLayout leftPanelLayout;

   public void leftPanelFormatter() {
      setSize(100, 400);
      
   }

   public LeftPanel() {
      
      leftPanelFormatter();
   }

}

class RightPanel extends JPanel {
   public void paintComponent(Graphics g) {
      super.paintComponent(g);
      Graphics2D g2 = (Graphics2D) g;
      g2.setColor(Color.white);
      g2.fillRect(0, 0, getWidth(), getHeight());
      g2.setColor(Color.black);
      g2.drawLine(0, 0, getWidth(), getHeight());
   }
}

class MainFrame extends JFrame {
   private LeftPanel leftPanel;
   private RightPanel rightPanel;

   public MainFrame() {
      setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

      setSize(400, 400);
      setResizable(false);
      setTitle("Composite Pattern");
      setLocationRelativeTo(null);
      leftPanel = new LeftPanel();
      rightPanel = new RightPanel();

      add(leftPanel);
      add(rightPanel);
      setVisible(true);

   }
}

public class GrrClass {
   public static void main(String[] args) {
      MainFrame mainFrame = new MainFrame();
   }

}



This code shows an output like -


But I think there is something wrong with this output Why on earth is it measuring the co-ordinates system from the Frame window?
I want it to measure it from the RightPanel.

(and erm, I know. This code smells of stupid names. But.. lets concentrate on the problem here )

I found it out
In the MainFrame class you wrote:

Code:


add(leftPanel);
add(rightPanel);


The default layout of JFrame is BorderLayout. BorderLayout expects a constraints as the second parameter of the add() method. There are 5 possible constraints, EAST, WEST, NORTH, SOUTH and CENTER.
It you do not mention any, the fifth one is taken by default.
So, both your JPanels are added in the center. One on other.
You should make it like this.

Code:

add(leftPanel, BorderLayout.WEST);
add(rightPanel);

Again, it will not work! Your line will be drawn fine, but you will no longer see the leftPanel. Because you need to change the class LeftPanel.

Code:

public void leftPanelFormatter() {
setSize(100, 400);
}

It should be replaced with

Code:

public void leftPanelFormatter() {
setPreferredSize(new Dimension(100, 400));
}

When you are using a LayoutManager, it does not care about what size you have set to a Component. However, it tries to respect preferredSize as much possible.

I hope it will work now