Java rectangle collision detection confusion

collision-detection, java, swing

Solution

Here is an example of a Game Loop / Game Logic and collision detection via `Rectangle2D#intersects(..)` method .

It uses `JPanel` to draw everything on and `Rectangle2D` is used for `Entity` class (which is any object needed to be drawn to `GamePanel` which is our JPanel where everything is drawn).

The `updateGame()` method is where you will find the collision checking:

    private void updateGame() {

        if (entities.get(0).intersects(entities.get(1))) {
            System.out.println("Intersecting");
        }
         ....
    }

You are Player 1 and move with W,A,S,D. When you intersect Player 2, a `println()` will confirm the intersection.

GameLogic.java:

import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.RenderingHints;
import java.awt.event.ActionEvent;
import java.awt.geom.Rectangle2D;
import java.awt.image.BufferedImage;
import java.util.ArrayList;
import java.util.concurrent.atomic.AtomicBoolean;
import javax.swing.AbstractAction;
import javax.swing.JComponent;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.KeyStroke;
import javax.swing.SwingUtilities;

/**
 *
 * @author David Kroukamp
 */
public class GameLogic {

    public GameLogic() {
        initComponents();
    }
    final GamePanel gp = new GamePanel(500, 500);

    private void initComponents() {

        JFrame frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        Entity entity1 = new Entity(100, 100, 100, 100, createWhiteImage());
        Entity entity2 = new Entity(300, 200, 100, 100, createBlackImage());

        gp.addEntity(entity1);
        gp.addEntity(entity2);//just a standing still JPanel

        setGamePanelKeyBindings(gp, entity1);

        frame.add(gp);

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

        //start the game loop which will repaint the screen
        runGameLoop();
    }
    //Starts a new thread and runs the game loop in it.

    public void runGameLoop() {
        Thread loop = new Thread(new Runnable() {
            @Override
            public void run() {
                gp.running.set(true);
                gp.gameLoop();
            }
        });
        loop.start();
    }

    private void setGamePanelKeyBindings(GamePanel gp, final Entity entity) {
        gp.getInputMap(JComponent.WHEN_IN_FOCUSED_WINDOW).put(KeyStroke.getKeyStroke("D"), "D pressed");
        gp.getActionMap().put("D pressed", new AbstractAction() {
            @Override
            public void actionPerformed(ActionEvent ae) {
                entity.RIGHT = true;
            }
        });

        gp.getInputMap(JComponent.WHEN_IN_FOCUSED_WINDOW).put(KeyStroke.getKeyStroke("A"), "A pressed");
        gp.getActionMap().put("A pressed", new AbstractAction() {
            @Override
            public void actionPerformed(ActionEvent ae) {
                entity.LEFT = true;
            }
        });

        gp.getInputMap(JComponent.WHEN_IN_FOCUSED_WINDOW).put(KeyStroke.getKeyStroke("W"), "W pressed");
        gp.getActionMap().put("W pressed", new AbstractAction() {
            @Override
            public void actionPerformed(ActionEvent ae) {
                entity.UP = true;
            }
        });

        gp.getInputMap(JComponent.WHEN_IN_FOCUSED_WINDOW).put(KeyStroke.getKeyStroke("S"), "S pressed");
        gp.getActionMap().put("S pressed", new AbstractAction() {
            @Override
            public void actionPerformed(ActionEvent ae) {
                entity.DOWN = true;
            }
        });
        gp.getInputMap(JComponent.WHEN_IN_FOCUSED_WINDOW).put(KeyStroke.getKeyStroke("released D"), "D released");
        gp.getActionMap().put("D released", new AbstractAction() {
            @Override
            public void actionPerformed(ActionEvent ae) {
                entity.RIGHT = false;
            }
        });

        gp.getInputMap(JComponent.WHEN_IN_FOCUSED_WINDOW).put(KeyStroke.getKeyStroke("released A"), "A released");
        gp.getActionMap().put("A released", new AbstractAction() {
            @Override
            public void actionPerformed(ActionEvent ae) {
                entity.LEFT = false;
            }
        });

        gp.getInputMap(JComponent.WHEN_IN_FOCUSED_WINDOW).put(KeyStroke.getKeyStroke("released W"), "W released");
        gp.getActionMap().put("W released", new AbstractAction() {
            @Override
            public void actionPerformed(ActionEvent ae) {
                entity.UP = false;
            }
        });

        gp.getInputMap(JComponent.WHEN_IN_FOCUSED_WINDOW).put(KeyStroke.getKeyStroke("released S"), "S released");
        gp.getActionMap().put("S released", new AbstractAction() {
            @Override
            public void actionPerformed(ActionEvent ae) {
                entity.DOWN = false;
            }
        });
    }

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

    private BufferedImage createWhiteImage() {
        BufferedImage img = new BufferedImage(100, 100, BufferedImage.TYPE_INT_ARGB);
        Graphics2D g2 = img.createGraphics();
        g2.setColor(Color.WHITE);
        g2.fillRect(0, 0, img.getWidth(), img.getHeight());
        return img;
    }

    private BufferedImage createBlackImage() {
        BufferedImage img = new BufferedImage(100, 100, BufferedImage.TYPE_INT_ARGB);
        Graphics2D g2 = img.createGraphics();
        g2.setColor(Color.BLACK);
        g2.fillRect(0, 0, img.getWidth(), img.getHeight());
        return img;
    }
}

class Entity extends Rectangle2D.Double {

    private int speed = 5;
    public boolean UP = false,
            DOWN = false,
            LEFT = false,
            RIGHT = false;
    private final BufferedImage image;

    public Entity(int x, int y, int width, int height, BufferedImage image) {
        super(x, y, width, height);
        this.width = width;
        this.height = height;
        this.image = image;
    }

    public BufferedImage getImage() {
        return image;
    }

    public void move() {
        if (UP) {
            y -= speed;
        }
        if (DOWN) {
            y += speed;
        }
        if (LEFT) {
            x -= speed;
        }
        if (RIGHT) {
            x += speed;
        }

    }
}

class GamePanel extends JPanel {

    private int width, height;
    private int frameCount = 0;
    private int fps = 0;
    public static AtomicBoolean running = new AtomicBoolean(false), paused = new AtomicBoolean(false);
    final ArrayList<Entity> entities = new ArrayList<>();

    GamePanel(int w, int h) {
        super(true);
        setIgnoreRepaint(true);//mustnt repaint itself the gameloop will do that
        setLayout(null);
        width = w;
        height = h;
    }

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

    public void addEntity(Entity e) {
        entities.add(e);
    }

    //Only run this in another Thread!
    public void gameLoop() {
        //This value would probably be stored elsewhere.
        final double GAME_HERTZ = 30.0;
        //Calculate how many ns each frame should take for our target game hertz.
        final double TIME_BETWEEN_UPDATES = 1000000000 / GAME_HERTZ;
        //At the very most we will update the game this many times before a new render.
        //If you're worried about visual hitches more than perfect timing, set this to 1.
        final int MAX_UPDATES_BEFORE_RENDER = 5;
        //We will need the last update time.
        double lastUpdateTime = System.nanoTime();
        //Store the last time we rendered.
        double lastRenderTime = System.nanoTime();

        //If we are able to get as high as this FPS, don't render again.
        final double TARGET_FPS = 60;
        final double TARGET_TIME_BETWEEN_RENDERS = 1000000000 / TARGET_FPS;

        //Simple way of finding FPS.
        int lastSecondTime = (int) (lastUpdateTime / 1000000000);

        while (running.get()) {
            double now = System.nanoTime();
            int updateCount = 0;

            if (!paused.get()) {
                //Do as many game updates as we need to, potentially playing catchup.
                while (now - lastUpdateTime > TIME_BETWEEN_UPDATES && updateCount < MAX_UPDATES_BEFORE_RENDER) {
                    updateGame();
                    lastUpdateTime += TIME_BETWEEN_UPDATES;
                    updateCount++;
                }

                //If for some reason an update takes forever, we don't want to do an insane number of catchups.
                //If you were doing some sort of game that needed to keep EXACT time, you would get rid of this.
                if (now - lastUpdateTime > TIME_BETWEEN_UPDATES) {
                    lastUpdateTime = now - TIME_BETWEEN_UPDATES;
                }

                drawGame();
                lastRenderTime = now;

                //Update the frames we got.
                int thisSecond = (int) (lastUpdateTime / 1000000000);

                if (thisSecond > lastSecondTime) {
                    System.out.println("NEW SECOND " + thisSecond + " " + frameCount);
                    fps = frameCount;
                    frameCount = 0;
                    lastSecondTime = thisSecond;
                }

                //Yield until it has been at least the target time between renders. This saves the CPU from hogging.
                while (now - lastRenderTime < TARGET_TIME_BETWEEN_RENDERS && now - lastUpdateTime < TIME_BETWEEN_UPDATES) {
                    //allow the threading system to play threads that are waiting to run.
                    Thread.yield();

                    //This stops the app from consuming all your CPU. It makes this slightly less accurate, but is worth it.
                    //You can remove this line and it will still work (better), your CPU just climbs on certain OSes.
                    //FYI on some OS's this can cause pretty bad stuttering. Scroll down and have a look at different peoples' solutions to this.
                    //On my OS it does not unpuase the game if i take this away
                    try {
                        Thread.sleep(1);
                    } catch (Exception e) {
                    }

                    now = System.nanoTime();
                }
            }
        }
    }

    private void drawGame() {
        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                repaint();
            }
        });
    }

    private void updateGame() {

        if (entities.get(0).intersects(entities.get(1))) {
            System.out.println("Intersecting");
        }

        for (Entity e : entities) {
            e.move();
        }
    }

    @Override
    protected void paintComponent(Graphics grphcs) {
        super.paintComponent(grphcs);
        Graphics2D g2d = (Graphics2D) grphcs;

        applyRenderHints(g2d);

        g2d.setColor(Color.GREEN);
        g2d.fillRect(0, 0, getWidth(), getHeight());

        for (Entity e : entities) {
            g2d.drawImage(e.getImage(), (int) e.getX(), (int) e.getY(), null);
        }

        g2d.setColor(Color.BLACK);
        g2d.drawString("FPS: " + fps, 5, 10);

        frameCount++;
    }
    private final static RenderingHints textRenderHints = new RenderingHints(RenderingHints.KEY_TEXT_ANTIALIASING, RenderingHints.VALUE_TEXT_ANTIALIAS_ON);
    private final static RenderingHints imageRenderHints = new RenderingHints(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
    private final static RenderingHints colorRenderHints = new RenderingHints(RenderingHints.KEY_COLOR_RENDERING, RenderingHints.VALUE_COLOR_RENDER_QUALITY);
    private final static RenderingHints interpolationRenderHints = new RenderingHints(RenderingHints.KEY_INTERPOLATION, RenderingHints.VALUE_INTERPOLATION_BILINEAR);
    private final static RenderingHints renderHints = new RenderingHints(RenderingHints.KEY_RENDERING, RenderingHints.VALUE_RENDER_QUALITY);

    public static void applyRenderHints(Graphics2D g2d) {
        g2d.setRenderingHints(textRenderHints);
        g2d.setRenderingHints(imageRenderHints);
        g2d.setRenderingHints(colorRenderHints);
        g2d.setRenderingHints(interpolationRenderHints);
        g2d.setRenderingHints(renderHints);
    }
}

Problem

Original source