所以我正在做一个叫做闪避的小游戏，长方形从“天空”掉下来，你必须躲闪它们。当我运行游戏时，这个错误消息刚刚弹出，我不知道为什么。每当我运行程序时，屏幕也会变白。
我无法弄清楚这一点，我在代码中搜索了几个小时，无法弄清楚原因。请停止
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at dodge.Enemy.move(Enemy.java:22)
at dodge.Enemy.draw(Enemy.java:32)
at dodge.EnemyManager.draw(EnemyManager.java:34)
at dodge.Dodge.paint(Dodge.java:38)
at javax.swing.JComponent.paintChildren(JComponent.java:887)
at javax.swing.JComponent.paint(JComponent.java:1063)
at javax.swing.JComponent.paintChildren(JComponent.java:887)
at javax.swing.JComponent.paint(JComponent.java:1063)
at javax.swing.JLayeredPane.paint(JLayeredPane.java:585)
at javax.swing.JComponent.paintChildren(JComponent.java:887)
at javax.swing.JComponent.paintToOffscreen(JComponent.java:5226)
at
很抱歉，很久了。但这是我的代码。
Dodge.java
package dodge;
import java.awt.event.KeyListener;
import java.awt.Color;
import java.awt.Dimension;
import javax.swing.*;
import java.awt.Graphics;
import java.awt.event.*;
public class Dodge extends JPanel implements KeyListener{
private Player player;
private Stage stage;
private EnemyManager manager;
public Dodge(){
setSize(new Dimension(800, 600));
setPreferredSize(new Dimension(800, 600));
addKeyListener(this);
setFocusable(true);
stage = new Stage();
player = new Player(this, 200, 200);
manager = new EnemyManager(this, 10);
}
@Override
public void update(Graphics g){
paint(g);
}
@Override
public void paint(Graphics g){
g.setColor(Color.BLACK);
g.fillRect(0, 0, getWidth(), getHeight());
stage.draw(g);
player.draw(g);
manager.draw(g);
g.dispose();
repaint();
}
@Override
public void keyReleased(KeyEvent e){
player.setXD(0);
player.setYD(0);
}
@Override
public void keyTyped(KeyEvent e){
}
public Stage getStage(){
return stage;
}
@Override
public void keyPressed(KeyEvent e){
int c = e.getKeyCode();
if(c == KeyEvent.VK_W){
}
if(c ==KeyEvent.VK_A){
player.setXD(-1);
}
if(c == KeyEvent.VK_S){
}
if(c ==KeyEvent.VK_D){
player.setXD(1);
}
}
public static void main(String[] args) {
JFrame frame = new JFrame();
frame.setTitle("Dodge the Rectangles");
frame.add(new Dodge());
frame.pack();
frame.setResizable(false);
frame.setSize(new Dimension(800, 600));
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setLocationRelativeTo(null);
frame.setVisible(true);
}
}
EnemyManager.java
package dodge;
import java.awt.*;
import java.util.ArrayList;
import java.util.List;
import java.util.*;
public class EnemyManager {
private int amount;
private List<Enemy> enemies = new ArrayList<Enemy>();
private Dodge instance;
public EnemyManager(Dodge instance, int a){
this.amount = a;
spawn();
this.instance = instance;
}
private void spawn(){
Random random = new Random();
int ss = enemies.size();
if(ss<amount){
for(int i = 0; i < amount - ss; i++){
enemies.add(new Enemy(instance, random.nextInt(778), 10));
}
}else if(ss>amount){
for(int i = 0; i < ss - amount; i++){
enemies.remove(i);
}
}
}
public void draw(Graphics g){
for(Enemy e : enemies) e.draw(g);
}
}
Enemy.java
package dodge;
import java.awt.*;
public class Enemy extends Entity{
private Rectangle hitbox;
private int ix, iy;
private boolean dead = false;
private Dodge instance;
public Enemy(Dodge instance, int x, int y){
super(x, y);
this.instance = instance;
hitbox = new Rectangle (x, y, 32, 32);
ix = 0;
iy = 1;
}
private void move(){
if(instance.getStage().isCollided(hitbox)){
iy = 0;
dead = true;
}
hitbox.x += ix;
hitbox.y +=iy;
}
private boolean isDead() {return dead;}
@Override
public void draw(Graphics g){
move();
g.setColor(Color.CYAN);
g.fillRect(hitbox.x, hitbox.y, hitbox.width, hitbox.height);
}
}
Entity.java
package dodge;
import java.awt.*;
public abstract class Entity {
protected int x, y, w, h;
protected boolean removed = false;
public Entity(int x, int y){
this.x = x;
this.y = y;
}
public void draw(Graphics g){
}
public int getX() {return x;}
public int getY(){return y;}
public int getW() {return w;}
public int getH(){return h;}
}
Player.java
package dodge;
import java.awt.*;
public class Player extends Entity{
private int xd, yd;
private Dodge instance;
private Rectangle hitbox;
public Player(Dodge instance, int x , int y){
super(x, y);
this.instance = instance;
w = 16; h = 16;
hitbox = new Rectangle(x, y, w, h);
}
public void draw(Graphics g){
move();
if(!instance.getStage().isCollided(hitbox)){
yd = 1;
}else yd = 0;
g.setColor(Color.ORANGE);
g.fillOval(hitbox.x, hitbox.y, hitbox.width, hitbox.height);
}
private void move(){
hitbox.x += xd;
hitbox.y +=yd;
}
public void setXD(int value){
xd = value;
}
public void setYD(int value){
yd = value;
}
}
Stage.java
package dodge;
import java.awt.*;
public class Stage {
private Rectangle platform = new Rectangle(0, 540, 800, 100);
public Stage(){
}
public boolean isCollided(Rectangle entity){
return platform.intersects(entity);
}
public void draw(Graphics g){
g.setColor(Color.WHITE);
g.fillRect(platform.x, platform.y, platform.width, platform.height);
}
}
我希望这不会太久。感谢
答案 0 :(得分：3)
阅读stacktrace。 Enemy.instance或Enemy.instance.getStage()为null。
答案 1 :(得分：1)
每当我运行程序时，屏幕也会变白。
不要覆盖update（）和paint（）。这是在AWT中使用的旧代码，而不是在Swing中完成自定义绘制的方式。在Swing中，您只需覆盖JPanel的paintComponent（）方法，并且不要忘记调用super.paintComponent（）。阅读Custom Painting上Swing教程中的部分，了解更多信息和示例。
不要在绘画方法中调用repaint（）。这将导致无限循环。这不是做动画的方法。使用Swing Timer或单独的Thread来安排动画。