为什么我的射弹（图像对象）不动？ Java 摇摆

Question

我正在尝试制作一个演示 spaceship shooter。每次我按下 space 条时，我都会在 paintComponent(Graphics g) 方法中绘制一个新的射弹（Image）并调用 moveProjectile() 方法。问题是 moveProjectile() 方法似乎已关闭。

MyJPanel.java

import java.awt.Cursor;
import java.awt.Graphics;
import java.awt.Image;
import java.awt.Point;
import java.awt.Toolkit;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.KeyAdapter;
import java.awt.event.KeyEvent;
import java.awt.event.MouseEvent;
import java.awt.event.MouseMotionListener;
import java.awt.image.BufferedImage;
import java.util.ArrayList;

import javax.swing.ImageIcon;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.Timer;

public class MyJPanel extends JPanel implements ActionListener
{
    private static final long serialVersionUID = 1L;

    private Timer timer;
    private Image backgroundImage;
    private Image player;
    private int playerX, playerY;
    private int projectileX,projectileY;
    private Image projectileImage;
    private ArrayList<Image> projectiles = new ArrayList<Image>();

    boolean flag = false;

    public MyJPanel(Image backgroundImage, Image player,Image projectileImage)
    {
        this.backgroundImage = backgroundImage;
        this.player = player;
        this.projectileImage = projectileImage;
        this.setLayout(null);

        timer = new Timer(50, this);
        timer.start();

        this.addKeyListener(new KeyAdapter() // Listens for a keyboard event
        {
            public void keyPressed(KeyEvent e) 
            {
                if (e.getKeyCode() == KeyEvent.VK_SPACE) // If pressing space - shoot
                {
                    flag = true;
                    moveProjectile();
                }
                repaint();
            }
        });

        // Mouse listener
        this.addMouseMotionListener(new MouseMotionListener()
        {   
            @Override
            public void mouseMoved(MouseEvent e)
            {
                playerX = e.getX();
                playerY = e.getY();
            }

            @Override
            public void mouseDragged(MouseEvent e)
            {

            }
        });
        hideMouseCursor();

        this.setFocusable(true);
        this.setVisible(true);
    } // End of JPanle constructor

    public void paintComponent(Graphics graphics)
    {
        super.paintComponent(graphics);

        graphics.drawImage(backgroundImage,0,0,this.getWidth(),this.getHeight(),null); // Draw the background
        graphics.drawImage(player,playerX,playerY,null); // Draw the player
        if (flag)
        {
            projectileX = playerX + player.getWidth(null);
            projectileY = playerY + player.getHeight(null) / 2 - 27;
            graphics.drawImage(projectileImage,projectileX,projectileY,null);
        }
    }

    public void moveProjectile()
    {
        while (projectileX < this.getWidth())
        {
            this.projectileX += 2;
            repaint();
        }
    }

    public void  hideMouseCursor() // Hides the mouse cursor
    {
        //Transparent 16 x 16 pixel cursor image.
        BufferedImage cursorbackgroundImgage = new BufferedImage(16, 16, BufferedImage.TYPE_INT_ARGB);

        // Create a new blank cursor.
        Cursor blankCursor = Toolkit.getDefaultToolkit().createCustomCursor(
                cursorbackgroundImgage, new Point(0, 0), "Blank Cursor");

        // Set the blank cursor to the JPanel.
        this.setCursor(blankCursor);
    }

    @Override
    public void actionPerformed(ActionEvent actionEvent) // Without the method and the repaint() the mouse listener will not work 
    {
        repaint();
    }

    public class ProjectileThread extends Thread
    {
        @Override
        public void run()
        {
            projectileX = playerX + player.getWidth(null);
            projectileY = playerY + player.getHeight(null) / 2;

        }
    }

    public static void main(String[] args)
    {
        JFrame frame = new JFrame("A Game by me");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setExtendedState(JFrame.MAXIMIZED_BOTH); // Making the frame take a full screen

        ImageIcon backgroundImageIcon = new ImageIcon("space_background_2.jpg");
        Image backgroundImgage = backgroundImageIcon.getImage();
        ImageIcon playerImageIcon = new ImageIcon("spaceship_1.png");
        Image playerImage = playerImageIcon.getImage();
        ImageIcon projectileIcon = new ImageIcon("spaceship_projectile_1.png");
        Image projectileImage = projectileIcon.getImage();  

        frame.add(new MyJPanel(backgroundImgage,playerImage,projectileImage));
        frame.setVisible(true);
    }
} // End of MyJPanel

有些变量和方法我不使用，请不要介意。代码中需要注意的地方：

if (e.getKeyCode() == KeyEvent.VK_SPACE) // If pressing space - shoot
{
    flag = true;
    moveProjectile();
}
repaint();

如果按 space -> flag = true 表示

if (flag)
{
    projectileX = playerX + player.getWidth(null);
    projectileY = playerY + player.getHeight(null) / 2 - 27;
    graphics.drawImage(projectileImage,projectileX,projectileY,null);
}

绘制弹丸并将其向右移动。问题是它从来没有向右移动过。相反，它跟随由鼠标移动的 space飞船。

如有任何建议，我们将不胜感激。

public class ProjectileThread extends Thread
{
    public ProjectileThread(int playerX,int playerY)
    {
        projectileX = playerX + player.getWidth(null);
        projectileY = playerY + player.getHeight(null) / 2;
    }
    @Override
    public void run()
    {
        while (projectileX < getWidth())
        {
            projectileX += 2;
        }
    }
}



public void keyPressed(KeyEvent e) 
        {
            if (e.getKeyCode() == KeyEvent.VK_SPACE) // If pressing space - shoot
            {
                ProjectileThread projectileThread = new ProjectileThread(playerX,playerY);
                projectileThread.start();
            }
            repaint();
        }

Answer 1

您的 moveProjectile 方法通过在其 X 坐标上加 2 水平移动射弹

this.projectileX += 2;

但是，您的绘制方法会用您的玩家 X 坐标覆盖此值：

projectileX = playerX + player.getWidth(null);

所以每次你绘制弹丸时，它都在相对于玩家的相同位置。您需要先使用玩家坐标绘制弹丸，然后在不重置其 X 坐标的情况下移动它。

你有一个线程应该在射弹发射时启动。因此，在 space 键的处理程序中，启动线程并将玩家坐标传递给它。这是你的起点。然后在线程的运行方法中，移动弹丸，循环等待一下。

Answer 2

while (projectileX < getWidth())
{
    projectileX += 2;
}

所有这一切都是设置 projectile = getWidth() + 1（或可能+2）。同样，您的 moveProjectile 方法具有以下内容：

while (projectileX < this.getWidth())
{
    this.projectileX += 2;
    repaint();
}

做同样的事情。请注意，循环主体中的 repaint() 基本上什么都不做，因为您阻塞了事件线程，直到 moveProjectile() 方法 returns

为什么我的射弹（图像对象）不动？ Java 摇摆

Why my projectile(Image object) is not moving? Java Swing

java

user-interface

swing

keylistener

jpanel