蛇的正确运动工作正常但是当我按下任何其他键（向上或向下）改变运动的方向时，它会抛出错误并且蛇不会移动。以下是错误：
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at GamePlay.paint(GamePlay.java:76)
at javax.swing.JComponent.paintToOffscreen(JComponent.java:5210)
at javax.swing.BufferStrategyPaintManager.paint(BufferStrategyPaintManager.java:290)
at javax.swing.RepaintManager.paint(RepaintManager.java:1272)
at javax.swing.JComponent._paintImmediately(JComponent.java:5158)
at javax.swing.JComponent.paintImmediately(JComponent.java:4969)
at javax.swing.RepaintManager$4.run(RepaintManager.java:831)
at javax.swing.RepaintManager$4.run(RepaintManager.java:814)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:76)
at javax.swing.RepaintManager.paintDirtyRegions(RepaintManager.java:814)
at javax.swing.RepaintManager.paintDirtyRegions(RepaintManager.java:789)
at javax.swing.RepaintManager.prePaintDirtyRegions(RepaintManager.java:738)
at javax.swing.RepaintManager.access$1200(RepaintManager.java:64)
at javax.swing.RepaintManager$ProcessingRunnable.run(RepaintManager.java:1732)
at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:311)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:756)
at java.awt.EventQueue.access$500(EventQueue.java:97)
at java.awt.EventQueue$3.run(EventQueue.java:709)
at java.awt.EventQueue$3.run(EventQueue.java:703)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:726)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:201)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class GamePlay extends JPanel implements KeyListener, ActionListener{
private ImageIcon titleImage;
private int[] snakexlength= new int[750];
private int[] snakeylength= new int[750];
int lengthOfSnake= 3;
int moves=0;
private boolean left= false;
private boolean right= false;
private boolean up= false;
private boolean down= false;
private ImageIcon rightmouth;
private ImageIcon upmouth;
private ImageIcon downmouth;
private ImageIcon leftmouth;
private Timer timer;
private int delay= 100;
private ImageIcon snakeimage;
public GamePlay(){
addKeyListener(this);
setFocusable(true);
setFocusTraversalKeysEnabled(false);
timer= new Timer(delay, this);
timer.start();
}
public void paint(Graphics g){
if(moves==0){
snakexlength[2]=50;
snakexlength[1]= 75;
snakexlength[0]=100;
snakeylength[2]= 100;
snakeylength[1]=100;
snakeylength[0]= 100;
}
//draw title image border
g.setColor(Color.WHITE);
g.drawRect(25, 100, 851, 55);
//draw the title image
titleImage= new ImageIcon("snaketitle.jpg");
titleImage.paintIcon(this, g, 25, 13);
//draw border for gameplay
g.setColor(Color.black);
g.drawRect(25, 75, 850, 575);
//draw background for the gameplay
g.setColor(Color.black);
g.fillRect(25,75,850,580);
rightmouth= new ImageIcon("rightmouth.png");
rightmouth.paintIcon(this, g, snakexlength[0], snakeylength[0]);
for(int a=0; a<lengthOfSnake; a++){
if(a==0 && down){
downmouth.paintIcon(this, g, snakexlength[a], snakeylength[a]);
}
if(a==0 && up){
upmouth.paintIcon(this, g, snakexlength[a], snakeylength[a]);
}
if(a==0 && right){
rightmouth.paintIcon(this, g, snakexlength[a], snakeylength[a]);
}
if(a==0 && left){
leftmouth.paintIcon(this, g, snakexlength[a], snakeylength[a]);
}
if(a!=0){
snakeimage= new ImageIcon("snakeimage.png");
snakeimage.paintIcon(this, g,snakexlength[a], snakeylength[a]) ;
}
}
g.dispose();
}
@Override
public void actionPerformed(ActionEvent e) {
timer.start();
if(right){
for(int r= lengthOfSnake-1; r>=0; r--){
snakeylength[r+1]= snakeylength[r];
}
for(int r=lengthOfSnake; r>=0; r--){
if(r==0){
snakexlength[r]= snakexlength[r]+25;
}
else{
snakexlength[r]= snakexlength[r-1];
}
if(snakexlength[r]> 850){
snakexlength[r]= 25;
}
}
repaint();
}
if(left){
for(int r= lengthOfSnake-1; r>=0; r--){
snakeylength[r+1]= snakeylength[r];
}
for(int r=lengthOfSnake; r>=0; r--){
if(r==0){
snakexlength[r]= snakexlength[r]-25;
}
else{
snakexlength[r]= snakexlength[r-1];
}
if(snakexlength[r]> 25){
snakexlength[r]= 850;
}
}
repaint();
}
if(up){
for(int r= lengthOfSnake-1; r>=0; r--){
snakexlength[r+1]= snakexlength[r];
}
for(int r=lengthOfSnake; r>=0; r--){
if(r==0){
snakeylength[r]= snakeylength[r]+25;
}
else{
snakeylength[r]= snakeylength[r-1];
}
if(snakeylength[r]< 75){
snakeylength[r]= 625;
}
}
repaint();
}
if(down){
for(int r= lengthOfSnake-1; r>=0; r--){
snakexlength[r+1]= snakexlength[r];
}
for(int r=lengthOfSnake; r>=0; r--){
if(r==0){
snakeylength[r]= snakeylength[r]+25;
}
else{
snakeylength[r]= snakeylength[r-1];
}
if(snakeylength[r]> 625){
snakeylength[r]= 75;
}
}
repaint();
}
}
@Override
public void keyTyped(KeyEvent e) {
// TODO Auto-generated method stub
}
@Override
public void keyReleased(KeyEvent e) {
// TODO Auto-generated method stub
}
@Override
public void keyPressed(KeyEvent e) {
if(e.getKeyCode()== KeyEvent.VK_RIGHT){
moves++;
right= true;
if(!left){
right=true;
}
else{
right= false;
left= true;
}
}
if(e.getKeyCode()== KeyEvent.VK_LEFT){
moves++;
left= true;
if(!right){
left=true;
}
else{
left= false;
right= true;
}
up=false;
down=false;
}
if(e.getKeyCode()== KeyEvent.VK_UP){
moves++;
up= true;
if(!down){
up=true;
}
else{
up= false;
down= true;
}
left=false;
right=false;
}
if(e.getKeyCode()== KeyEvent.VK_DOWN){
moves++;
down= true;
if(!up){
down=true;
}
else{
down= false;
up= true;
}
left=false;
right=false;
}
}
}
答案 0 :(得分：1)
private ImageIcon upmouth;
private ImageIcon downmouth;
private ImageIcon leftmouth;
添加：
rightmouth = new ImageIcon("rightmouth.png");
leftmouth= new ImageIcon("leftmouth.png");
upmouth = new ImageIcon("upmouth.png");
downmouth = new ImageIcon("downmouth.png");
最好在构造函数中，因此不要在每次paint()调用时加载图像。