Drawing Rectangles - NullPointerExceptions

[cselic]

Hi. I have a problem with java.lang.NullPointerException.
This is my program.

class: GUI.java

import java.awt.BorderLayout;
import java.awt.Container;
import java.awt.Dimension;
import java.awt.Graphics;

import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JToolBar;
import javax.swing.SwingUtilities;

public class GUI {
	public static void main(String[] args) {
		SwingUtilities.invokeLater(new Runnable() {
			
			@Override
			public void run() {
				DrawFrame frame = new DrawFrame();
				frame.showGUI();
			}
		});
	}
}

class DrawFrame extends JFrame {
	MainPanel mainPanel;
	
	public DrawFrame() {
		MainPanel mainPanel = new MainPanel();
		Container content = getContentPane();
		content.add(mainPanel);
	}
	
	public void showGUI() {
		setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		setPreferredSize(new Dimension(500,500));
		pack();
		setVisible(true);
	}
}

class MainPanel extends JPanel {
	DrawPanel drawPanel = new DrawPanel();
	JToolBar toolBar = new JToolBar();
	
	public MainPanel() {
		add(toolBar, BorderLayout.NORTH);
		add(drawPanel, BorderLayout.CENTER);
	}
}

class DrawPanel extends JPanel {
	public DrawPanel() {
		
	}
	
	public void paintComponent(Graphics g) {
		super.paintComponent(g);
		Rectangle rect = new Rectangle(80,80,70,89);
		rect.drawGeomObj(g);
	}
}

class: Rectangle.java

import java.awt.Point;
import java.awt.Graphics2D;
import java.awt.Graphics;

public class Rectangle {
	public Rectangle(Point p1, Point p2) {
		leftUpPoint = p1;
		width = (int)(Math.abs(p1.x - p2.x));
		height = (int)(Math.abs(p1.y - p2.y));
	}
	
	public Rectangle(int x, int y, int width, int height) {
		leftUpPoint.x = x;
		leftUpPoint.y = y;
		this.width = width;
		this.height = height;
	}
	
	public void drawGeomObj(Graphics g)
	{
		Graphics2D g2 = (Graphics2D) g;
		g2.drawOval(leftUpPoint.x, leftUpPoint.y, width, height);
	}
	
	public boolean containPoint(Point point) {
		if(leftUpPoint.x < point.x && (leftUpPoint.x+width > point.x)
			&& (leftUpPoint.y < point.y) && (leftUpPoint.y + height) > point.y)
				return true;
		
		return false;
	}
	
	public boolean isPointOnBorder(Point point) {
		Rectangle interiorRect = new Rectangle(leftUpPoint.x - toleranceInPixels, leftUpPoint.y - toleranceInPixels,
				width - toleranceInPixels, height - toleranceInPixels);
		Rectangle exteriorRect = new Rectangle(leftUpPoint.x + toleranceInPixels, leftUpPoint.y + toleranceInPixels,
				width + toleranceInPixels, height + toleranceInPixels);
		
		if(!interiorRect.containPoint(point) && exteriorRect.containPoint(point))
			return true;
		
		return false;
	}

	Point leftUpPoint;
	int width, height;
	int toleranceInPixels = 4;
}

[r035198x]

You must read the message when you get an exception to see the line number reported and try to fix it yourself.

Anyway, you have

//...
       //let's call this panelA
        MainPanel mainPanel;
	
	public DrawFrame() {
                //this is a brand new mainPanel
                //not the same as panelA
		MainPanel mainPanel = new MainPanel();
		Container content = getContentPane();
		content.add(mainPanel);
	}
//....

So what you declare inside the constructor is not the same as what you declared outside it. That means what you declared outside the constructor is never initialized inside the constructor.
These scoping rules must be learnt before trying to write GUI programs.

[cselic]

You must read the message when you get an exception to see the line number reported and try to fix it yourself.

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at Rectangle.<init>(Rectangle.java:13)
at DrawPanel.paintComponent(GUI.java:58)

Rectangle.java 13th line:

leftUpPoint.x = x;

GUI.java 58th line:

Rectangle rect = new Rectangle(80,80,70,89);

So, I don't see any nullPointException problem here. My opinion is that constuctor is nice defined.

[cselic]

So what you declare inside the constructor is not the same as what you declared outside it.

Do you suggest me something like that:

public DrawFrame() {
              mainPanel = new JPanel() {
			@Override
                        public mainPanel() {
                         
                        }
	      }
	}

[cselic]

Problem is solved. I've changed constructor:

public Rectangle(int x, int y, int width, int height) {
	leftUpPoint = new Point(x,y);
	this.width = width;
	this.height = height;
}