Question

I have a problem with this… When I draw a line the character follows the line I drew but the computer paints a line somewhere else:

So does anyone know what is going on?
My code:

@SuppressWarnings({"serial","rawtypes","unchecked"})
public class someGame extends JFrame implements MouseListener, KeyListener{

ArrayList lines = new ArrayList();
Point2D.Double start;
final Color BROWN = new Color(156,93,82);
Slider thread;
Rectangle cow = null;
boolean drawGuy = false;
public someGame(){
    super("Some Game");
    setSize(700,700);
    setVisible(true);
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    addMouseListener(this);
    addKeyListener(this);
}

public void paint(Graphics g){
    super.paint(g);
    g.setColor(Color.black);
    for(int i = 0; i < lines.size(); i++){
        Line2D.Double temp = (Line2D.Double) lines.get(i);
        int x1 = Integer.parseInt(""+Math.round(temp.getX1()));
        int x2 = Integer.parseInt(""+Math.round(temp.getX2()));
        int y1 = Integer.parseInt(""+Math.round(temp.getY1()));
        int y2 = Integer.parseInt(""+Math.round(temp.getY2()));

        g.drawLine(x1,x2,y1,y2);
    }

    if(drawGuy){
        try{
            URL url = this.getClass().getResource("resources/img/world/char.png");
            Image image = Toolkit.getDefaultToolkit().getImage(url);
            g.drawImage(image, cow.x, cow.y, this);
        } catch(Exception exc){}
    }
}

public void mouseClicked(MouseEvent e){}
public void mouseEntered(MouseEvent e){}
public void mouseExited(MouseEvent e){}
public void mousePressed(MouseEvent e){
    start = new Point2D.Double(e.getX(),e.getY());
}
public void mouseReleased(MouseEvent e){
    Point2D.Double end = new Point2D.Double(e.getX(),e.getY());
    lines.add(new Line2D.Double(start,end));
    repaint();
}
public void keyPressed(KeyEvent e){}
public void keyReleased(KeyEvent e){}
public void keyTyped(KeyEvent e){
    if(e.getKeyChar()=='p'||e.getKeyChar()=='P'){
        thread = new Slider();
        thread.start();
        thread.action(true);
    }
    if(e.getKeyChar()=='q'||e.getKeyChar()=='Q'){
        thread.action(false);
        drawGuy = false;
        thread = null;
    }
}
private class Slider extends Thread{
    double velocity, gravity;
    boolean go = false;
    public void run(){
        if(go){
            initGuy();
            velocity = 0;
            gravity = 1;
        }
        while(go){
            try{
                Line2D.Double lineTaken = null;
                boolean onLine = false;
                int firstOnLine = -1;
                for(int i = lines.size()-1; i>=0; i--){
                    Line2D.Double temp = (Line2D.Double) lines.get(i);
                    if(temp.intersects(cow.x,cow.y,50,50)){
                        lineTaken = temp;
                        onLine = true;
                        if(firstOnLine!=i){
                            firstOnLine = i;
                            gravity = 0;
                        }
                        break;
                    }
                }
                if(onLine){
                    double grav = (lineTaken.y2-lineTaken.y1)/50;
                    double vlct = (lineTaken.x2-lineTaken.x1)/100;
                    if(velocity<5)velocity+=vlct;
                    if(gravity<2.5)gravity+=grav;
                }
                else{
                    gravity+=.2;
                }
                cow.x+=velocity;
                cow.y+=gravity;

                Thread.sleep(75);
                repaint();
            }catch(Exception e){break;}
        }
    }
    public void action(boolean b){
        go = b;
    }
    public void initGuy(){
        Line2D.Double firstLine = (Line2D.Double) lines.get(0);
        int x = Integer.parseInt(""+Math.round(firstLine.x1));
        int y = Integer.parseInt(""+Math.round(firstLine.y1));
        cow = new Rectangle(x+30,y-20,30,30);
        drawGuy = true;
    }
}

Sorry for the large code but I have no idea what went wrong…
Big thanks to anyone who answers 🙂
…and yes, the main method is someGame g = new someGame();

Answer 1

1) You should never use paint(..) without a good reason. rather extend JPanel class and override paintComponent(...).

2) Dont extend JFrame class unless adding functionality

3) Dont forget all Swing component creation and manipulation should be done on EDT – Event Dispatch Thread

4) Java naming convention is CamelCase, and each new word for a class name should begin with capitals i.e SomeGame

5) Dont use setSize rather override component getPreferredSize and return desired size and than call pack() on JFrame before setting visible.

6) Use Keybindings instead of KeyListener

7) Use MouseAdapter instead of MouseListener

8) Do not extend Thread class rather implement Runnable and create start() method to start thread in runnable class (I have not implemented this in below code)

9) Not a good idea to do long running tasks like load images or do any other work besides the necessaries for painting objects in paint method you should load picture etc on class initialization or at least outside of a components paint methods. (I have not implemented this in below code)

10) Your biggest problem about lines not being drawing correctly was:

g.drawLine(x1,x2,y1,y2);

it should be drawLine(int x1,int y1,int x2,int y2):

g.drawLine(x1,y1,x2,y2);

Here is the code (Dont have time to explain right now hope you can understand):

I added some extra stuff like casting to Graphics2D and using RenderHints for Anti-Aliasing as suggested by @MadProgrammer in his comment below.

import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Image;
import java.awt.Rectangle;
import java.awt.RenderingHints;
import java.awt.Toolkit;
import java.awt.event.ActionEvent;
import java.awt.event.KeyEvent;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.awt.geom.Line2D;
import java.awt.geom.Point2D;
import java.net.URL;
import java.util.ArrayList;
import javax.swing.AbstractAction;
import javax.swing.JComponent;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.KeyStroke;
import javax.swing.SwingUtilities;

public class SomeGame {

    public SomeGame() {
        JFrame frame = new JFrame("Some Game");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        DrawPanel dp = new DrawPanel();
        frame.add(dp);

        frame.pack();
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                new SomeGame();
            }
        });

    }
}

class DrawPanel extends JPanel {

    ArrayList lines = new ArrayList();
    Point2D.Double start;
    final Color BROWN = new Color(156, 93, 82);
    Slider thread;
    Rectangle cow = null;
    boolean drawGuy = false;

    public DrawPanel() {
        addMouseListener(new MouseAdapter() {
            @Override
            public void mouseReleased(MouseEvent me) {
                super.mouseReleased(me);
                Point2D.Double end = new Point2D.Double(me.getX(), me.getY());
                lines.add(new Line2D.Double(start, end));
                repaint();
            }

            @Override
            public void mousePressed(MouseEvent me) {
                super.mousePressed(me);
                start = new Point2D.Double(me.getX(), me.getY());
            }
        });
        setKeyBindings();
    }

    private void setKeyBindings() {
        getInputMap(JComponent.WHEN_IN_FOCUSED_WINDOW).put(KeyStroke.getKeyStroke(KeyEvent.VK_P, 0), "P");
        getActionMap().put("P", new AbstractAction() {
            @Override
            public void actionPerformed(ActionEvent ae) {
                thread = new Slider(DrawPanel.this);
                thread.action(true);
                thread.start();
            }
        });
        getInputMap(JComponent.WHEN_IN_FOCUSED_WINDOW).put(KeyStroke.getKeyStroke(KeyEvent.VK_Q, 0), "q");
        getActionMap().put("Q", new AbstractAction() {
            @Override
            public void actionPerformed(ActionEvent ae) {
                thread.action(false);
                drawGuy = false;
                thread = null;
            }
        });
    }

    @Override
    public Dimension getPreferredSize() {
        return new Dimension(700, 700);
    }

    @Override
    public void paintComponent(Graphics g) {
        super.paintComponent(g);

        //lets go 2D :)
        Graphics2D g2d = (Graphics2D) g;

        //turn on anti aliasing
        g2d.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);


        g2d.setColor(Color.black);

        for (int i = 0; i < lines.size(); i++) {
            Line2D.Double temp = (Line2D.Double) lines.get(i);
            int x1 = Integer.parseInt("" + Math.round(temp.getX1()));
            int x2 = Integer.parseInt("" + Math.round(temp.getX2()));
            int y1 = Integer.parseInt("" + Math.round(temp.getY1()));
            int y2 = Integer.parseInt("" + Math.round(temp.getY2()));

            g2d.drawLine(x1, y1, x2, y2);
        }

        if (drawGuy) {
            try {
                URL url = this.getClass().getResource("resources/img/world/char.png");
                Image image = Toolkit.getDefaultToolkit().getImage(url);
                g.drawImage(image, cow.x, cow.y, this);
            } catch (Exception exc) {
            }
        }
    }

    private class Slider extends Thread {

        double velocity, gravity;
        boolean go = false;
        private final DrawPanel dp;

        private Slider(DrawPanel dp) {
            this.dp = dp;
        }

        public void run() {
            if (go) {
                initGuy();
                velocity = 0;
                gravity = 1;
            }
            while (go) {
                try {
                    Line2D.Double lineTaken = null;
                    boolean onLine = false;
                    int firstOnLine = -1;
                    for (int i = lines.size() - 1; i >= 0; i--) {
                        Line2D.Double temp = (Line2D.Double) lines.get(i);
                        if (temp.intersects(cow.x, cow.y, 50, 50)) {
                            lineTaken = temp;
                            onLine = true;
                            if (firstOnLine != i) {
                                firstOnLine = i;
                                gravity = 0;
                            }
                            break;
                        }
                    }
                    if (onLine) {
                        double grav = (lineTaken.y2 - lineTaken.y1) / 50;
                        double vlct = (lineTaken.x2 - lineTaken.x1) / 100;
                        if (velocity < 5) {
                            velocity += vlct;
                        }
                        if (gravity < 2.5) {
                            gravity += grav;
                        }
                    } else {
                        gravity += .2;
                    }
                    cow.x += velocity;
                    cow.y += gravity;

                    Thread.sleep(75);
                    SwingUtilities.invokeLater(new Runnable() {
                        @Override
                        public void run() {
                            dp.repaint();
                        }
                    });
                } catch (Exception e) {
                    break;
                }
            }
        }

        public void action(boolean b) {
            go = b;
        }

        public void initGuy() {
            Line2D.Double firstLine = (Line2D.Double) lines.get(0);
            int x = Integer.parseInt("" + Math.round(firstLine.x1));
            int y = Integer.parseInt("" + Math.round(firstLine.y1));
            cow = new Rectangle(x + 30, y - 20, 30, 30);
            drawGuy = true;
        }
    }
}