所以我试图制作一个游戏,我会产生20个敌人。敌人应该加载到一系列敌人中,然后游戏就是在for循环中绘制它们,唯一的问题是当我尝试运行for循环时我不断获得NullPointerException
以下代码中的行:
enemy[i] = new Enemy(enemyX, enemyY, enemySize, vx, vy);
是抛出异常的人。我也列出了下面的完整信息源,以便您可以看到我尝试做什么,我只是卡住了,我需要一些帮助,有没有其他方法可以创建敌人并将它们放在屏幕上?
主类:
//define package
package auctus;
//imports
import org.newdawn.slick.AppGameContainer;
import org.newdawn.slick.BasicGame;
import org.newdawn.slick.Color;
import org.newdawn.slick.GameContainer;
import org.newdawn.slick.Graphics;
import org.newdawn.slick.SlickException;
//create class
public class Auctus extends BasicGame{
//Window dimensions and title
public static final int WIDTH = 800;
public static final int HEIGHT = 600;
public static final String TITLE = "Auctus 0.1";
//Defining the player variables, obviously only one player to create so no need for
//automation in this area
public float playerX, playerY, playerSize;
//Creating the enemy array
public Enemy enemy[];
//declaring a global variable for the number of enemies
public int numberOfEnemies;
//slick2D API stuff
public Auctus(String title) {
super(TITLE);
}
//initialisation method
public void init(GameContainer gc) throws SlickException {
//setting the number of enemies
numberOfEnemies = 20;
//setting the player variables dynamically each time the game is started
playerX = (float)Math.random() * WIDTH;
playerY = (float)Math.random() * HEIGHT;
playerSize = (float)Math.random() * 50;
//creating the enemies
for(int i = 0; i < numberOfEnemies; i++){
float enemyX = (float)Math.random() * WIDTH;
float enemyY = (float)Math.random() * HEIGHT;
float enemySize = (float)Math.random() * 50;
float vx = (float)Math.random() * 2;
float vy = (float)Math.random() * 2;
//this is the line that's giving me trouble
enemy[i] = new Enemy(enemyX, enemyY, enemySize, vx, vy);
}
}
//from here it doesn't matter, this is error free both syntactically and logically
public void render(GameContainer gc, Graphics g) throws SlickException {
drawPlayer(g, playerX, playerY, playerSize);
for(int i = 0; i < numberOfEnemies; i++){
enemy[i].drawEnemies(g);
}
}
public void update(GameContainer gc, int delta) throws SlickException {
updatePlayer(gc, delta, playerX, playerY, playerSize);
for(int i = 0;i < numberOfEnemies; i++){
enemy[i].updateEnemies(delta);
}
}
public void drawPlayer(Graphics g, float playerX, float playerY, float playerSize){
}
public void updatePlayer(GameContainer gc, int delta, float playerX, float playerY, float playerSize){
}
public void updateEnemies(GameContainer gc, int delta, float enemyX, float enemyY, float enemySize, float vx, float vy){
for(int i = 0; i < numberOfEnemies; i++){
enemyX += vx;
enemyY += vy;
}
}
public static void main(String[] args){
try{
AppGameContainer app = new AppGameContainer(new Auctus(TITLE));
app.start();
} catch(SlickException e){
e.printStackTrace();
}
}
}
敌人类:
package auctus;
//imports
import org.newdawn.slick.Color;
import org.newdawn.slick.Graphics;
//class declaration
public class Enemy {
//declaring variables
public float enemyX, enemyY, enemySize, vx, vy;
public Enemy(float x, float y, float size, float dx, float dy){
enemyX = x;
enemyY = y;
enemySize = size;
vx = dx;
vy = dy;
}
public float enemyX(){
return enemyY;
}
public float enemyY(){
return enemyY;
}
public float enemySize(){
return enemySize;
}
public float vx(){
return vx;
}
public float vy(){
return vy;
}
public void setX(float x){
enemyX = x;
}
public void setY(float y){
enemyY = y;
}
public void setSize(float size){
enemySize = size;
}
public void setVX(float dx){
vx = dx;
}
public void setVY(float dy){
vy = dy;
}
public void drawEnemies(Graphics g){
g.setColor(Color.red);
g.fillRect(enemyX, enemyY, enemySize, enemySize);
}
public void updateEnemies(int delta){
if(enemyX < 800 || enemyX > 0){
enemyX += vx * delta;
}
if(enemyX > 800 || enemyX < 0){
vx = -vx;
}
if(enemyY < 600 || enemyY > 0){
enemyY += vy * delta;
}
if(enemyY > 600 || enemyY < 0){
vy = - vy;
}
}
}
敌人的阶级很直接。我无法理解为什么它会变成NullPointerException
,但也许其他人可以发现它。
我做错了什么?
答案 0 :(得分:8)
您永远不会实例化数组enemy
。
在某些时候,在使用 enemy
之前,您需要以下内容:enemy = new Enemy[numEnemies];
。您可以在声明中执行此操作:
// use a constant for the number of enemies:
public static final int NUM_ENEMIES = 20;
Enemy enemy[] = new Enemy[NUM_ENEMIES];
否则,由于enemy
为null
- 因为您尚未对其进行实例化 - 当您尝试将其编入索引时,您将获得NullPointerException
。