我正试图在我正在制作的游戏中加载图像。我在模仿我们在课堂上做的一个小组游戏。在我的panel类中，我加载一个图像并将其绘制到背景中，这是有效的，它找到我的图像并绘制它。
然而，无论我如何加载我放在我的宇宙飞船类中的图像并使用宇宙飞船的实例绘制它，它都会给我一个空指针异常。它从来没有找到图像，我猜，但是我把它放在源文件夹的相同位置，背景图像被成功地拉出来。同样，如果我在gamesscreen类中加载空间图像并将其绘制到面板上，它也能正常工作。
下面是这两个类的代码。我尝试用 加载图像spaceship = ImageIO.read(new File("Background.png"));
以及:
Image img = Toolkit.getDefaultToolkit().createImage("spaceShip.png");
但得到相同的空指针两种方式。我迷路了，希望有人能帮帮我。
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);
}
}
SpaceShip.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)
最大的问题是你的注释不与代码对齐(语义上):
//creates a spaceship object
SpaceShip space;
这不会创建实例。这将创建对实例的引用——但不会创建实例。是null。您将它传递给(名字不好的)panel类构造函数，并表现得好像它应该有一个值。但是没有。
NPE，或至少其中一个:
g2.drawImage(space.img, 0, 0, this); // *foom*
至少在提供的代码中，space从未初始化。
我还建议不要把所有这些塞进一个文件。
变量太空船，导弹和发射器都是空的。要实例化一个变量，你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();
如果你不这样做，那么当panel panel1 = new panel(launcher, enemyArray, missileArray);行运行时，面板构造函数将尝试设置launcher = la;。这里出现了问题，因为la当前为null。
我还建议将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中的类总是以大写字母开头)。