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我正在尝试将图像加载到我正在开发的游戏中。在我们在课堂上建立的小组游戏之后，我正在设计它。在我的面板类中，我加载了一个图像并将其绘制到背景，这很有效，它找到了我的图像并绘制它。

但是，无论我如何加载放入我的飞船类中的图像并使用飞船实例绘制它，它都会给我一个空指针异常。我猜它永远找不到图像，但是我将它放在源文件夹中成功提取背景图像的相同位置。此外，如果我在我的游戏屏幕类中加载空间图像并将其绘制到面板上，它工作正常。

这两个类的代码如下。我试过用 

spaceship = ImageIO.read(new File("Background.png"));

也：

Image img = Toolkit.getDefaultToolkit().createImage("spaceShip.png");

但是两种方式都得到相同的空指针。我迷路了，希望有人能帮忙。

package temp;

import java.awt.Graphics;
import java.awt.Image;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;
import java.awt.image.BufferedImage;
import java.io.File;
import java.util.ArrayList;
import javax.imageio.ImageIO;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class GameScreen extends JFrame {

    //Create an instance of a spaceship object
    SpaceShip spaceship;

    //Create an instance of a missile object
    Missile missile;

    //Create an instance of a launcher object
    Launcher launcher;

    //create an array that holds missile objects
    ArrayList<Missile> missileArray = new ArrayList<Missile>();

    //Creates an array to hold spaceship objects
    ArrayList<SpaceShip> enemyArray = new ArrayList<SpaceShip>();

    int lives = 3;

    public GameScreen() {
        //sets up the panel and the parent frame

        //sets the default specs of the JFrame and adds the panel to it
        this.setSize(600, 700);
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        this.setVisible(true);

        //Creates a panel object and adds it to the main frame
        panel panel1 = new panel(launcher, enemyArray, missileArray);
        this.add(panel1);
    }
}

class panel extends JPanel {
    //creates a new launcher object
    Launcher launcher;

    //creates a spaceship object
    SpaceShip space;

    //creates an arraylist of spaceship objects
    static ArrayList<SpaceShip> ships;

    //creates an arraylist of missile objects
    ArrayList<Missile> missiles;

    //loads images to draw 
    Image backGround;

    public panel(Launcher la, ArrayList<SpaceShip> ss, ArrayList<Missile> mi) {
        try {
            launcher = la;
            ships = ss;
            missiles = mi;

            backGround = ImageIO.read(new File("Background.png"));
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    public void paintComponent(Graphics g) {
        BufferedImage x = new BufferedImage(600, 700, BufferedImage.TYPE_INT_RGB);
        Graphics g2 = x.getGraphics();

        g2.drawImage(backGround, 0, 0, this);

        g2.drawImage(space.img, 0, 0, this);

        System.out.println(ships.size());

        g.drawImage(x, 0, 0, this);
    }
}

宇宙飞船.java

import java.awt.Image;
import java.awt.Toolkit;
import java.io.File;
import java.io.IOException;
import java.util.logging.Level;
import java.util.logging.Logger;
import javax.imageio.ImageIO;

/**
 * @author WymoreJW
 */
class SpaceShip extends Character {

    Image img = Toolkit.getDefaultToolkit().createImage("spaceShip.png");

    public void SpaceShip() {

        this.health = 10;
        this.speed = 2;
    }
}

堆栈跟踪错误

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at missiledefense.panel.paintComponent(GameScreen.java:108)
    at javax.swing.JComponent.paint(JComponent.java:1029)
    at javax.swing.JComponent.paintChildren(JComponent.java:862)
    at javax.swing.JComponent.paint(JComponent.java:1038)
    at javax.swing.JComponent.paintChildren(JComponent.java:862)
    at javax.swing.JComponent.paint(JComponent.java:1038)
    at javax.swing.JLayeredPane.paint(JLayeredPane.java:567)
    at javax.swing.JComponent.paintChildren(JComponent.java:862)
    at javax.swing.JComponent.paintToOffscreen(JComponent.java:5131)

    at    javax.swing.RepaintManager$PaintManager.paintDoubleBuffered
(RepaintManager.java:1479)

    at javax.swing.RepaintManager$PaintManager.paint(RepaintManager.java:1410)
    at javax.swing.RepaintManager.paint(RepaintManager.java:1224)
    at javax.swing.JComponent.paint(JComponent.java:1015)
    at java.awt.GraphicsCallback$PaintCallback.run(GraphicsCallback.java:21)
    at sun.awt.SunGraphicsCallback.runOneComponent(SunGraphicsCallback.java:60)
    at sun.awt.SunGraphicsCallback.runComponents(SunGraphicsCallback.java:97)
    at java.awt.Container.paint(Container.java:1780)
    at java.awt.Window.paint(Window.java:3375)
    at javax.swing.RepaintManager.paintDirtyRegions(RepaintManager.java:796)
    at javax.swing.RepaintManager.paintDirtyRegions(RepaintManager.java:713)
    at javax.swing.RepaintManager.seqPaintDirtyRegions(RepaintManager.java:693)
    at javax.swing.SystemEventQueueUtilities$ComponentWorkRequest.run
(SystemEventQueueUtilities.java:125)
    at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:209)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:597)
    at java.awt.EventDispatchThread.pumpOneEventForFilters
(EventDispatchThread.java:269)
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:184)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy
(EventDispatchThread.java:174)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:169)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:161)
    at java.awt.EventDispatchThread.run(EventDispatchThread.java:122)
4

2 回答 2

3

最大的问题是您的评论与代码不一致（语义上）：

//creates a spaceship object
SpaceShip space;

这不会创建实例。这会创建对实例的引用——但没有实例。是null。您将它传递给（名称不佳的）panel类构造函数，并表现得好像其中应该有一个值。但是没有。

，NPE或至少其中之一：

g2.drawImage(space.img, 0, 0, this); // *foom*

至少在提供的代码中，space从未初始化过。

我还建议不要将所有这些内容合并到一个文件中。

于 2011-12-05T14:43:41.497 回答
0

你的变量 spaceShip、missile 和 launcher 都是空的。要实例化一个变量，您需要调用它的构造函数，例如

//Actually Create an instance of a spaceship object
SpaceShip spaceship = new SpaceShip();

//Actually Create an instance of a missile object
Missile missile = new Missile();

//Actually Create an instance of a launcher object
Launcher launcher = new Launcher();

如果您不这样做，那么当该行panel panel1 = new panel(launcher, enemyArray, missileArray);运行时，面板构造函数将尝试设置launcher = la;. 问题来了，因为 la 当前为空。

我还建议将您的面板类重命名为更具描述性的名称，或者至少重命名为 Panel（因为 Java 中的类应始终以大写字母开头）。

希望这可以帮助。

于 2011-12-05T14:54:36.247 回答