Thread: Perpendicular Lines

I'm trying to write a program that can create lines in a JPanel by clicking and dragging the mouse. That part I have down. The next thing I'm trying to do is create a perpendicular line to the line that I just drew by clicking randomly somewhere on the JPanel. So basically like this:

Untitled.jpg
If I were to click where that black dot was. It would draw the red line connecting to the black line and they would meet at a 90 degree angle (perpendicular).

I'm having trouble figuring out how to implement this into my program.
Here is my GUI class

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Container;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Point;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;
import java.awt.event.MouseMotionListener;
import java.util.ArrayList;
import java.util.Random;
 
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JMenu;
import javax.swing.JMenuBar;
import javax.swing.JMenuItem;
import javax.swing.JPanel;
import javax.swing.border.BevelBorder;
 
 
public class BasicLinePix extends JFrame {
 
	private JPanel drawingPanel;
 
	private JLabel statusLabel;
 
	private EventHandler eh;
 
	private Line myLine;
 
	private ArrayList<Line> storedLines = new ArrayList<Line>();
 
	public static void main(String[] args) {
		// TODO Auto-generated method stub
 
		BasicLinePix lp = new BasicLinePix();
		lp.setVisible(true);
 
	}
 
	public BasicLinePix() {
		setTitle("Basic Line Pix 1.0");
		setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		Container cp = getContentPane();
		cp.setLayout(new BorderLayout());
 
		eh = new EventHandler();
 
		drawingPanel = makeDrawingPanel();
		drawingPanel.addMouseListener(eh);
		drawingPanel.addMouseMotionListener(eh);
 
		JPanel statusBar = createStatusBar();
 
		cp.add(drawingPanel, BorderLayout.CENTER);
		cp.add(statusBar, BorderLayout.SOUTH);
 
		buildMenu();
 
		pack();
	}
 
	public void paint(Graphics g) {
		super.paint(g);
 
		Graphics gl = drawingPanel.getGraphics();
 
		if(myLine != null)
			for(Line l : storedLines)
				l.draw(gl);
 
	}
 
	private void buildMenu() {
		JMenuBar menuBar = new JMenuBar();
 
		JMenu fileMenu = buildFileMenu();
		JMenu editMenu = buildEditMenu();
 
		menuBar.add(fileMenu);
		menuBar.add(editMenu);
 
		setJMenuBar(menuBar);
 
	}
 
	private JMenu buildEditMenu() {
		JMenu editMenu = new JMenu("Edit");
 
		JMenuItem menuItem = new JMenuItem("Cut");
 
		editMenu.add(menuItem);
		return editMenu;
	}
 
	private JMenu buildFileMenu() {
		JMenu fileMenu = new JMenu("File");
 
		JMenuItem menuItem = new JMenuItem("New");
		menuItem.addActionListener(eh);
		fileMenu.add(menuItem);
 
		menuItem = new JMenuItem("Open");
		fileMenu.add(menuItem);
 
		menuItem = new JMenuItem("Save");
		fileMenu.add(menuItem);
 
		menuItem = new JMenuItem("Exit");
		menuItem.addActionListener(eh);
		fileMenu.add(menuItem);
		return fileMenu;
	}
 
	private JPanel makeDrawingPanel() {
		// TODO Auto-generated method stub
		JPanel p = new JPanel();
		p.setPreferredSize(new Dimension(500, 400));
		p.setBackground(Color.YELLOW);
 
		return p;
	}
 
	private JPanel createStatusBar() {
		JPanel panel = new JPanel();
		panel.setLayout(new BorderLayout());
		statusLabel = new JLabel("No message");
		panel.add(statusLabel, BorderLayout.CENTER);
 
		panel.setBorder(new BevelBorder(BevelBorder.LOWERED));
 
		return panel;
	}
 
	private class EventHandler implements ActionListener, MouseListener,
			MouseMotionListener {
 
		private Point startPoint = null; // line's start point
		private Point endPoint = null; // line's most recent end point
 
		public void actionPerformed(ActionEvent arg0) {
			if (arg0.getActionCommand().equals("Exit")) {
				statusLabel.setText("Exiting program...");
				System.exit(0);
			}
			else if (arg0.getActionCommand().equals("New")) {
				//clears the board of all lines
				storedLines.clear();
				drawingPanel.removeAll();
				drawingPanel.updateUI();
			}
 
		}
 
		@Override
		public void mouseClicked(MouseEvent arg0) {
			// TODO Auto-generated method stub
 
		}
 
		@Override
		public void mouseEntered(MouseEvent arg0) {
			// TODO Auto-generated method stub
 
		}
 
		@Override
		public void mouseExited(MouseEvent arg0) {
			// TODO Auto-generated method stub
 
		}
 
		@Override
		public void mousePressed(MouseEvent e) {
 
			if (e.isShiftDown()) {
				// record starting point for line
				startPoint = new Point(e.getX(), e.getY());
 
				// initialize endPoint
				endPoint = startPoint;
			}
 
			if (e.isControlDown()) {
				Graphics g = drawingPanel.getGraphics();
				//draw the perpendicular line
			}
 
		}
 
		@Override
		public void mouseReleased(MouseEvent e) {
 
			if (e.isShiftDown()) {
				//stores the line in an arrayList
				storedLines.add(myLine);
				startPoint = null;
				endPoint = null;
			}
		}
 
		@Override
		public void mouseDragged(MouseEvent e) {
 
			if (e.isShiftDown()) {
				// Implement rubber-band cursor
				Graphics g = drawingPanel.getGraphics();
				g.setColor(Color.black);
 
				g.setXORMode(drawingPanel.getBackground());
 
				// REDRAW the line that was drawn
				// most recently during this drag
				// XOR mode means that yellow pixels turn black
				// essentially erasing the existing line
				drawLine(g, startPoint, endPoint);
 
				// Update the end point of the line to current mouse position
				endPoint = new Point(e.getX(), e.getY());
 
				// Draw line to current mouse position
				// XOR mode: yellow pixels become black
				// black pixels, like those from existing lines, temporarily
				// become
				// yellow
				drawLine(g, startPoint, endPoint);
 
			}
 
		}
 
		@Override
		public void mouseMoved(MouseEvent arg0) {
			// TODO Auto-generated method stub
 
		}
 
		private void drawLine(Graphics g, Point start, Point end) {
			if (startPoint != null && endPoint != null) {
				int startX = ((Double) start.getX()).intValue();
				int startY = ((Double) start.getY()).intValue();
 
				int endX = ((Double) end.getX()).intValue();
				int endY = ((Double) end.getY()).intValue();
 
				g.drawLine(startX, startY, endX, endY);
			}
		}
 
	}
 
}

and here is my line classe

import java.awt.Graphics;
import java.awt.Point;
import java.util.ArrayList;
 
 
public class Line {
	private Point startPoint, endPoint;
	double slope;
	double length;
 
	public Line(Point start, Point end) {
		startPoint = start;
		endPoint = end;
	}
 
	public double getSlope() {
		return slope;
	}
 
	public double getLength() {
		return length;
	}
 
	public void draw(Graphics g) {
		drawLine(g, startPoint, endPoint);
	}
 
	private void drawLine(Graphics g, Point start, Point end) {
		if (startPoint != null && endPoint != null) {
			int startX = ((Double) start.getX()).intValue();
			int startY = ((Double) start.getY()).intValue();
 
			int endX = ((Double) end.getX()).intValue();
			int endY = ((Double) end.getY()).intValue();
			g.drawLine(startX, startY, endX, endY);
		}
	}
}

If anyone could lead me into the right direction that would be great. Thanks!

create a perpendicular line to the line that I just drew by clicking randomly somewhere on the JPanel.

Since the coordinates of the screen are down and to the right, to draw a line from the point to the line all you need to know is where is the line relative to the point. Then draw the line keeping the x or y value constant until the drawn red line touches the existing black line.
For example if the black line was horizontal, you would use the point's x value to draw the line
If the line was vertical use the point's y value.

Draw a grid on a piece of paper, add a line and a point and mark their x,y values.
Look at the line's x,y end points and use one of them to know where to end the drawn line.

If the black line is diagonal, you will need to use some trigonometry to compute the values. Consult a math book for the formulas to use.