drawlines...

hvordan gør jeg det?

Kode 

import java.awt.*;

public class Test extends Frame {
    Thread thread;
    TestCanvas canvas;
    int state;
    public Test() {
        setSize(400, 400);
        canvas = new TestCanvas();
        add(canvas);
        state = 0;
        thread = new Thread() {
            public void run() {
                while (true) {
                    canvas.repaint();
                    try {
                        sleep(1000);
                    } catch (Exception e) {
                    }
                    state = (state + 1) % 4;
                }
            }
        };
        setVisible(true);
        thread.start();
    }

    private class TestCanvas extends Canvas {
        public void paint(Graphics g) {
            update(g);
        }

        public void update(Graphics g) {
            g.setColor(Color.BLACK);
            g.fillRect(0,0,getSize().width,getSize().height);
            g.setColor(Color.white);
            int x1, y1, x2, y2;
            switch (state) {
                case 0:
                    x1 = x2 = getSize().width/2;
                    y1 = 0;
                    y2 = getSize().height/2;
                    break;
                case 1:
                    x1 = getSize().width/2;
                    x2 = getSize().width;
                    y1 = y2 = getSize().height/2;
                    break;
                case 2:
                    x1 = x2 = getSize().width/2;
                    y1 = getSize().height/2;
                    y2 = getSize().height;
                    break;
                default:
                    x1 = 0;
                    x2 = getSize().width/2;
                    y1 = y2 = getSize().height/2;
                    break;
            }
            g.drawLine(x1, y1, x2, y2);
        }
    }

    public static void main(String args[]) {
        new Test();
    }
}

hvordan fjerner jeg en drawLine uden den fjerner det i baggrunden.

Kode 

import java.awt.*;
import java.awt.event.*;
import java.util.LinkedList;

public class Test extends Frame {
    private MyCanvas canvas;
    private LinkedList<Line> lines;

    private Point current = null;
    private Point dragDestination = null;

    public Test() {
        setSize(400, 400);
        lines = new LinkedList<Line>();
        canvas = new MyCanvas();
        add(canvas);

        setVisible(true);
    }

    private class Line {
        private Point origin;
        private Point destination;
        
        public Line(Point origin, Point destination) {
            this.origin = origin;
            this.destination = destination;
        }

        public Point getOrigin() {
            return origin;
        }

        public Point getDestination() {
            return destination;
        }
    }

    private class MyMouseListener extends MouseAdapter {
        public void mousePressed(MouseEvent e) {
            if (e.getButton() == MouseEvent.BUTTON1) {
                current = new Point(e.getX(), e.getY());
            } else if (lines.size() > 0) {
                lines.removeLast();
                canvas.repaint();
            }
        }

        public void mouseReleased(MouseEvent e) {
            if (e.getButton() == MouseEvent.BUTTON1) {
                lines.add(new Line(current, new Point(e.getX(), e.getY())));
                current = null;
                canvas.repaint();
            }
        }
    }

    private class MyMouseMotionListener extends MouseMotionAdapter {
        public void mouseDragged(MouseEvent e) {
            if (current != null) {
                dragDestination = new Point(e.getX(), e.getY());
                canvas.repaint();
            }
        }
    }

    private class MyCanvas extends Canvas {
        public MyCanvas() {
            addMouseListener(new MyMouseListener());
            addMouseMotionListener(new MyMouseMotionListener());
        }

        public void paint(Graphics g) {
            update(g);
        }

        public void update(Graphics g) {
            g.setColor(Color.BLACK);
            g.fillRect(0,0,getSize().width,getSize().height);

            g.setColor(Color.WHITE);
            for (Line l : lines) {
                g.drawLine(l.getOrigin().x, l.getOrigin().y, l.getDestination().x, l.getDestination().y);
            }
            if (current != null && dragDestination != null) {
                g.drawLine(current.x, current.y, dragDestination.x, dragDestination.y);
            }
        }
    }

    public static void main(String args[]) {
        new Test();
    }
}