有没人知道马士兵贪吃蛇哪里怎改才不报空指针异常吗

Question

求助啊，各位大佬们........

Answer 1

import java.awt.Color;
import java.awt.Graphics;
import java.awt.event.*;
public class Snake {
private Node head = null;
private Node tail = null;
private int size = 0;
private Node node = new Node(20,30,Dir.L);
public Snake(){
head = node;
tail = node;
size = 1;
}

public void addToTail(){
Node node = null;
switch(tail.dir){
case L:
node = new Node(tail.rows,tail.cols + 1,tail.dir);
break;
case U:
node = new Node(tail.rows + 1,tail.cols,tail.dir);
break;
case R:
node = new Node(tail.rows,tail.cols - 1,tail.dir);
break;
case D:
node = new Node(tail.rows - 1,tail.cols,tail.dir);
break;
}

tail.next = node;
node.prev = tail;
tail = node;
size++;
}
public void addToHead(){
Node node = null;
switch(head.dir){
case L:
node = new Node(head.rows,head.cols - 1,head.dir);
break;
case U:
node = new Node(head.rows - 1,head.cols,head.dir);
break;
case R:
node = new Node(head.rows,head.cols + 1 ,head.dir);
break;
case D:
node = new Node(head.rows + 1,head.cols,head.dir);
break;
}
node.next = head;
node.prev = node;
head = node;
size++;
}
//画蛇的方法
public void draw(Graphics g){
if(size<=0) return;//蛇的长度小于等于0，即蛇还没创建
move();
for(Node node = head;node != null;node = node.next){
node.draw(g);
}
}
private void move(){
addToHead();
deletFromTail();
}
private void deletFromTail(){

if(size == 0) 
return;

tail = tail.prev;
tail.next = null;
}

//蛇的一截
private class Node{
 int w = Yard.BLOCK_SIZE;//蛇的大小
 int h = Yard.BLOCK_SIZE;
 int rows ,cols;//蛇的位置
Dir dir = Dir.L;//蛇的方向
 Node next = null;
 Node prev = null;

public Node(int rows,int cols,Dir dir){
this.rows = rows;
this.cols = cols;
this.dir = dir;
}
protected void draw(Graphics g){
Color c = g.getColor();
g.setColor(Color.BLACK);
g.fillRect(cols * Yard.BLOCK_SIZE, rows * Yard.BLOCK_SIZE, w, h);
g.setColor(c);
}

}
public void keyPressed(KeyEvent e){
int key = e.getKeyCode();
switch(key){
case KeyEvent.VK_LEFT:
head.dir = Dir.L;
break;
case KeyEvent.VK_UP:
head.dir = Dir.U;
break;
case KeyEvent.VK_RIGHT:
head.dir = Dir.R;
break;
case KeyEvent.VK_DOWN:
head.dir = Dir.D;
break;
}
}

}Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at Snake.deletFromTail(Snake.java:76)
at Snake.move(Snake.java:68)
at Snake.draw(Snake.java:61)
at Yard.paint(Yard.java:44)
at sun.awt.RepaintArea.paintComponent(RepaintArea.java:264)
at sun.awt.RepaintArea.paint(RepaintArea.java:240)
at sun.awt.windows.WComponentPeer.handleEvent(WComponentPeer.java:347)
at java.awt.Component.dispatchEventImpl(Component.java:4937)
at java.awt.Container.dispatchEventImpl(Container.java:2287)
at java.awt.Window.dispatchEventImpl(Window.java:2719)
at java.awt.Component.dispatchEvent(Component.java:4687)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:735)
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.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:87)
at java.awt.EventQueue$4.run(EventQueue.java:708)
at java.awt.EventQueue$4.run(EventQueue.java:706)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:705)
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)
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at Snake.deletFromTail(Snake.java:75)
at Snake.move(Snake.java:68)
at Snake.draw(Snake.java:61)
at Yard.paint(Yard.java:44)
at sun.awt.RepaintArea.paintComponent(RepaintArea.java:264)
at sun.awt.RepaintArea.paint(RepaintArea.java:240)
at sun.awt.windows.WComponentPeer.handleEvent(WComponentPeer.java:347)
at java.awt.Component.dispatchEventImpl(Component.java:4937)
at java.awt.Container.dispatchEventImpl(Container.java:2287)
at java.awt.Window.dispatchEventImpl(Window.java:2719)
at java.awt.Component.dispatchEvent(Component.java:4687)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:735)
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.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:87)
at java.awt.EventQueue$4.run(EventQueue.java:708)
at java.awt.EventQueue$4.run(EventQueue.java:706)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:705)
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)
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at Snake.deletFromTail(Snake.java:75)
at Snake.move(Snake.java:68)
at Snake.draw(Snake.java:61)
at Yard.paint(Yard.java:44)
at sun.awt.RepaintArea.paintComponent(RepaintArea.java:264)
at sun.awt.RepaintArea.paint(RepaintArea.java:240)
at sun.awt.windows.WComponentPeer.handleEvent(WComponentPeer.java:347)
at java.awt.Component.dispatchEventImpl(Component.java:4937)
at java.awt.Container.dispatchEventImpl(Container.java:2287)
at java.awt.Window.dispatchEventImpl(Window.java:2719)
at java.awt.Component.dispatchEvent(Component.java:4687)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:735)
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.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:87)
at java.awt.EventQueue$4.run(EventQueue.java:708)
at java.awt.EventQueue$4.run(EventQueue.java:706)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:705)
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)
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at Snake.deletFromTail(Snake.java:75)
at Snake.move(Snake.java:68)
at Snake.draw(Snake.java:61)
at Yard.paint(Yard.java:44)
at sun.awt.RepaintArea.paintComponent(RepaintArea.java:264)
at sun.awt.RepaintArea.paint(RepaintArea.java:240)
at sun.awt.windows.WComponentPeer.handleEvent(WComponentPeer.java:347)
at java.awt.Component.dispatchEventImpl(Component.java:4937)
at java.awt.Container.dispatchEventImpl(Container.java:2287)
at java.awt.Window.dispatchEventImpl(Window.java:2719)
at java.awt.Component.dispatchEvent(Component.java:4687)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:735)
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.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:87)
at java.awt.EventQueue$4.run(EventQueue.java:708)
at java.awt.EventQueue$4.run(EventQueue.java:706)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:705)
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)