我正在制作“太空侵略者”类型的游戏，目前正在尝试实现碰撞检测，以便如果镜头击中目标，则将目标从帧中删除。但是我在下面的标记行上得到了空指针异常（我还将发布异常跟踪）。这是因为我没有正确设置射击对象的y坐标吗？我尝试修复此问题，但是虽然可能没有采取适当的措施，但是没有任何反应。
这是两个相关的类，如有必要，我可以发布更多内容：
public class GamePanel extends JPanel {
Launcher launcher1;
Background bground1;
public static Shot shot;
public int shotCounter;
public int rCount;
public int cCount;
Timer timer;
Timer eTimer;
Timer rTimer;
Timer cTimer;
Russia russia;
China china;
public ArrayList<Object> enemies;
public GamePanel() throws IOException {
super();
enemies = new ArrayList<>();
this.shotCounter = 0;
launcher1 = new Launcher();
bground1 = new Background();
timer = new Timer(10, new ActionListener() {
@Override
public void actionPerformed(ActionEvent e) {
componentMove(3);
repaint();
}
});
rTimer = new Timer(10, new ActionListener() {
@Override
public void actionPerformed(ActionEvent e) {
componentMove(1);
repaint();
}
});
cTimer = new Timer(10, new ActionListener() {
@Override
public void actionPerformed(ActionEvent e) {
componentMove(2);
repaint();
}
});
eTimer = new Timer(10, new ActionListener() {
@Override
public void actionPerformed(ActionEvent e) {
addEnemy();
eTimer.setDelay(7000);
repaint();
}
});
eTimer.start();
}//end constructor
@Override
protected void paintComponent(Graphics g) {
super.paintComponent(g);
g.drawImage(bground1.background, 0, 0, getWidth(), getHeight(), null);
g.drawImage(launcher1.baldEagleImage, launcher1.getLxCoord(), launcher1.lyCoord, null);//paint the launcher
if (rCount == 1) {
g.drawImage(russia.image, russia.getXCoord(), russia.getYCoord(), null);
rTimer.start();
}
if (cCount == 1) {
g.drawImage(china.image, china.getXCoord(), china.getYCoord(), null);
cTimer.start();
}
if (shotCounter == 1) {
g.drawImage(shot.mcDShotImage, shot.staticXLauncherCoord, shot.getSyCoord(), null);
timer.start();
}
}//end paintComponent method
public void move(GamePanel gamePanel) {
launcher1.moveX();
repaint();
}//end move method
public void componentMove(int c) {
if (c == 1) {
do {
russia.move();
} while (!collisionDetected(russia));
} else if (c == 2) {
do {
china.move();
} while (!collisionDetected(china));
} else if (c == 3) {
shot.moveY();
}
}
public boolean collisionDetected(Entity object) {
// Create variables to hold temporary values to check for ball colision
int oTempX = object.getXCoord();
int oTempY = object.getYCoord();
int oTempW = object.getImageWidth();
int oTempH = object.getImageHeight();
int sTempX = shot.staticXLauncherCoord;
int sTempY = shot.getSyCoord(); // THIS IS THE EXCEPTION
int sTempH = shot.mcDShotImage.getHeight();
int sTempW = shot.mcDShotImage.getWidth();
double xDiff = Math.pow((oTempX - sTempX), 2);
double yDiff = Math.pow((oTempY - sTempY), 2);
double hSum = Math.pow(oTempH + sTempH, 2);
double wSum = Math.pow(oTempW + sTempW, 2);
if (shotCounter == 1) {
if ((xDiff + yDiff) <= hSum) {//use the ball heights to check if two balls intersect
if ((xDiff + yDiff) <= wSum) {//use the ball widths to check if two balls intersect
enemies.remove(this);
repaint();
return true;
} else {
return false;
}
} else {
return false;
}
} else {
return false;
}
}
public void addShot() {
try {
shot = new Shot();
shotCounter = 1;
repaint();
} catch (IOException ex) {
}
}
public void addEnemy() {
int enemy = (int) (Math.round(Math.random()));
if (enemy == 0) {
try {
enemies.add(russia = new Russia());
rCount = 1;
repaint();
} catch (IOException ex) {
}
} else {
try {
enemies.add(china = new China());
cCount = 1;
repaint();
} catch (IOException ex) {
}
}
}
}//end GamePanel class
public class Shot {
public int syCoord;
public int sRise = 1;
public BufferedImage mcDShotImage;
GamePanel gPanel;
public static int staticXLauncherCoord;
public Shot() throws IOException {
staticXLauncherCoord = Launcher.getLxCoord() + 10;
syCoord = 381;
mcDShotImage = ImageIO.read(new File("mcdonaldsarchesshot.jpg"));
}//end constructor
public void moveY() {
do {
syCoord -= sRise;
setSyCoord(syCoord);
} while (syCoord <= 1);
}//end moveY method
public void setSyCoord(int syCoord) {
this.syCoord = syCoord;
}
public int getSyCoord() {
return syCoord;
}
}//end Shot class
这是异常打印输出：
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at GamePanel.collisionDetected(GamePanel.java:125)
at GamePanel.componentMove(GamePanel.java:112)
at GamePanel$3.actionPerformed(GamePanel.java:65)
at javax.swing.Timer.fireActionPerformed(Timer.java:312)
at javax.swing.Timer$DoPostEvent.run(Timer.java:244)
at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:251)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:733)
at java.awt.EventQueue.access$200(EventQueue.java:103)
at java.awt.EventQueue$3.run(EventQueue.java:694)
at java.awt.EventQueue$3.run(EventQueue.java:692)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:703)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:242)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:161)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:150)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:146)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:138)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:91)
最佳答案
您在哪里/什么时候调用addShot？这是显示shot初始化（通过shot = new Shot()）的唯一逻辑。
从堆栈跟踪看来，actionPerformed调用了componentMove，后者又调用了collisionDetected，但是这些都不调用addShot，因此您的公共静态shot永远不会初始化并且为null（因此是例外）。
请注意，访问shot的公共静态staticXLauncherCoord是没有问题的，因为它是静态的，并且没有绑定到Shot的实例。