java arrayList不同地对待不同元素

Question

So, I'm trying to make a simple platformer, here is the code:

package Main;

import java.applet.Applet;
import java.awt.Color;
import java.awt.Font;
import java.awt.Frame;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Image;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;
import java.awt.geom.Rectangle2D;
import java.util.ArrayList;

public class Platformer extends Applet implements Runnable, KeyListener{

    // buffering variables
    private Image dbImage;
    private Graphics dbg;

    // running boolean
    private boolean running = true;

    // Player coords
    private int rectX = 100;
    private int rectY = 420;

    // initialise dx and dy at zero
    private double dx = 0;
    private double dy = 0;

    private boolean jumping = false;

    //debug Strings
    private String inter = new String();
    private String coords = new String();

    //for background scrolling
    private int worldShift;

    //sprites
    Rectangle2D player = new Rectangle2D.Double(rectX, rectY, 10, 10);
    Rectangle2D ground = new Rectangle2D.Double(0, 420, 800, 150);
    int[][] platforms = { {300 + worldShift, 300, 100, 20}, { 450 + worldShift, 200, 100, 20 },     { 500 + worldShift, 250, 100, 20 }, { 100 + worldShift, 200, 100, 20 } };
    ArrayList<Rectangle2D> platformList = new ArrayList<Rectangle2D>();

    int level = 1;

    @Override
    public void init() {

        setSize(800, 480);
        setFocusable(true);
        addKeyListener(this);
        Frame frame = (Frame) this.getParent().getParent();
        frame.setTitle("Test");
        setVisible(true);

        for (int i = 0; i < platforms.length; i++){
            // platforms
            Rectangle2D platform = new Rectangle2D.Double();
            platform.setRect(platforms[i][0] + worldShift, platforms[i][1], platforms[i][2], platforms[i][3]);
            platformList.add(0, platform);
        }
    }

    @Override
    public void start() {
        Thread thread = new Thread(this);
        thread.start();
    }

    @Override
    public void stop() {
        // TODO Auto-generated method stub
    }

    @Override
    public void destroy() {
        // TODO Auto-generated method stub
    }

    public void paint(Graphics g) {
        //Double Buffering
        Graphics2D g2 = (Graphics2D) g;
        dbImage = createImage(getWidth(), getHeight());
        dbg = dbImage.getGraphics();
        paintComponent(dbg);
        g2.drawImage(dbImage, 0, 0, this);
    }

    // Paint objects
    public void paintComponent(Graphics g) {
        Graphics2D g2 = (Graphics2D) g;

        g2.setColor(Color.white);
        g2.draw(player);
        g2.fill(player);

        if (level == 1) {
            g2.setColor(Color.blue);
            g2.draw(ground);
            g2.fill(ground);

            for (Rectangle2D plat : platformList){
                g2.setColor(Color.red);
                g2.draw(plat);
                g2.fill(plat);
            }

            //debug printouts
            g.drawString(inter, 400, 30);
            g.drawString(coords, 20, 150);

            Font bold = new Font("Liberation Sans", Font.BOLD, 36);
            setFont(bold);
            g.drawString("dx, dy: (" + dx + ", " + dy + ")", 30, 30);
            g.drawString("x, y: (" + player.getX() + ", " + player.getY() + ")", 30, 60);
        }
    }

    @Override
    public void run() {
        while (running) {

            update();
            repaint();

            try {
                Thread.sleep(17);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }

    public void update() {
        worldShift = 0;

        rectX += dx;
        rectY += dy + 1;

        // set sprite coords
        player.setRect(rectX, rectY, 10, 10);
        ground.setRect(0, 420, 800, 480);

        // stops player running off screen
        if (rectX < 1)  rectX = 0;
        if (rectX > 790) rectX = 790;

        // collision detection
        if (player.intersects(ground)) {
            rectY = (int) (ground.getY() - player.getHeight());
            jumping = false;
            dy = 0;
        }

        for (int i = 0; i < platformList.size(); i++){
            Rectangle2D plat = platformList.get(i);

            //sets platform's coordinates
            plat.setRect(plat.getX() + worldShift, plat.getY(), plat.getWidth(), plat.getHeight());

            //sets debug Strings
            coords = "Not on platform";
            inter = " " + player.intersects(plat);

            //for moving the world
            if (rectX > 700 && dx == 5)
                worldShift = -5;
            if (rectX < 100 && dx == -5)
                worldShift = 5;

            //collision detection
            if (player.intersects(plat)) {
                System.out.println(plat);
                jumping = false;
                dy = 0;
                coords = " " + plat.getBounds();
                System.out.println(platformList.indexOf(plat));
                // if player coming from top
                if (rectY < plat.getY())
                    rectY = (int) (plat.getY() - player.getHeight());
                    // if player coming from bottom
                else if (rectY > plat.getY() + plat.getHeight())
                    rectY = (int) (plat.getY() + plat.getHeight());
            }
            else if (!player.intersects(plat) && !player.intersects(ground)) jumping = true;
        }

        //implements falling
        if (jumping)
            dy += 1;
    }

    //button presses
    @Override
    public void keyPressed(KeyEvent e) {
        System.out.println("Key pressed code=" + e.getKeyCode() + ", char="+ e.getKeyChar());
        if (e.getKeyCode() == 37)
            dx -= 5; // 37 is left
        if (e.getKeyCode() == 39)
            dx += 5; // 39 is right
        // if (e.getKeyCode() == 40) dy -= 5; //40 is down
        if (e.getKeyCode() == 38 && jumping == false){
            // if on the ground,
            // i.e. only jump if on the ground
            dy -= 20; // 38 is up
            jumping = true;
            System.out.println("Jumping");
        }
    }

    //key release
    @Override
    public void keyReleased(KeyEvent e) {
        if (e.getKeyCode() == 37)
            dx = 0; // 37 is left
        if (e.getKeyCode() == 39)
            dx = 0; // 39 is right
        if (e.getKeyCode() == 40)
            dy = 0; // 40 is down
        // if (e.getKeyCode() == 38)
    }

    @Override
    public void keyTyped(KeyEvent e) {
        // TODO Auto-generated method stub

    }

}

The problem is that when the player jumps on the platforms, the platforms act differently depending on the platform. If I jump on the last drawn one, it acts how I want (jumping is set to false , the player.intersects(plat) prints true to screen and the platform's bounds are printed both to the screen and the console). Then, when I jump on a different platform, jumping stays true, player.intersects(plat) prints false to screen and the platform's bounds are only printed to the console, not the screen.

I think I've narrowed it down to being something to do with having more than one element in the arrayList, but I don't know the specific issue.

Answer 1

there is a simple reason why jump is always true - in the last line of your complex logic you write

 if (!player.intersects(plat) && !player.intersects(ground)) jumping = true;

This will overwrite the jumping=false you set in a loop run before.

If you intersect with a platform which is not the last then you will not intersect with the last platform in the ArrayList - but the logic code above will set jump to true in the last loop run.

I assume your player is only intersecting one platform at once - so your code should work if you insert a break statement after the last operation in the if(player.intersects(plat)) { ... } code block.

You should reengineer your logic part and write JUnit tests for it - but anyway you should refactor your code. If your game is getting more complex you will get problems with adapting new features. Please use constants for the key codes so that it is more easy for readers to understand what you are doing.