通过扩展类调用方法仅在方法被java重写时有效
我有4个类(核心类、游戏主类、精灵类、角色类)
-GameMain扩展核心(核心>;>;GameMain)
-字符扩展精灵(精灵>;>;字符)
如果我调用-b.get,我会得到一个NullPointerException。。。()-在角色中不重写它们的方法(它们最初在Spriter中,我通过角色从GameMain调用它们)
我在内核中创建角色对象,并将其放入ArrayList中
public abstract class Core {
ArrayList<Character> mon = new ArrayList<Character>();
void gameloop(){
while(running){
//some code
while(mon.size() < 2){
mon.add(new Character(blk_1,5,1));// Character(Spriter, Long, Long){}
}
draw(g);
}
}
}
然后在GameMain中,我调用了一个最初在Spriter中的方法,但我通过角色调用它
public class GameMain extends Core{
public void draw(Graphics2D g){
for(Character b : mon){ //mon is the ArrayList<Character>
if(test.getCoordY() <= b.getCoordY() - (b.getHeight()/2)){ //<<<< NullPointerException caused by these two Methods , test is a Spriter class
g.drawImage(chChange(), Math.round(test.getX()), Math.round(test.getY()), test.getWidth(), (test.getHeight()), null);
g.drawImage(b.getImage(), Math.round(b.getX()), Math.round(b.getY()), null);
}else {
g.drawImage(b.getImage(), Math.round(b.getX()), Math.round(b.getY()), null);
g.drawImage(chChange(), Math.round(test.getX()), Math.round(test.getY()), null);
}
}
}
}
这是Spriter的方法,除非我重写它,否则我将得到错误
public class Spriter {
Spriter(Image i){
this.i = i;
scenes = new ArrayList();
}
Spriter(){
scenes = new ArrayList();
start();
}
public int getHeight(){
return i.getHeight(null);
}
public float getCoordY(){
float cy;
cy = y + i.getHeight(null); //<<<< NullPointerException happens here, Image i;
return cy;
}
public void setX(float x){
this.x = x;
}
public void setY(float y){
this.y = y;
}
// other similar Methods but no problem with them
//-------------------------------------- Animation Part ----------------------------------------------
public synchronized void addScene(Image i,long t){
total_t+=t;
scenes.add(new oneScene(i, total_t));
}
//start animation
public synchronized void start(){
mov_time = 0;
scn_indx = 0;
}
// get current scene
public synchronized Image getAnimeImage(){
if(scenes.size()==0){
return null;
}else{
return getScene(scn_indx).pic;
}
}
//get the scene
private oneScene getScene(int x){
return (oneScene)scenes.get(x);
}
private class oneScene{
Image pic;
long endTime;
public oneScene(Image pic, long endTime){
this.pic = pic;
this.endTime = endTime;
}
}
}
如果我这样做,它会起作用:
public class Character extends Spriter{
public Character(Spriter s, long health, long power) {
this.s = s;
this.health = health;
this.power = power;
s.setX(randomY());
s.setY(randomY());
}
public float getCoordY(){
return s.getCoordY();
}
public float getHeight(){
return s.getgetHeight();
}
//some other methods for health and power
}
但是如果我这样做,它能工作吗(它已经给出了错误,但如何避免它):
public class Character extends Spriter{
public Character(Spriter s, long health, long power) {
this.s = s;
this.health = health;
this.power = power;
s.setX(randomY()); //setting x for the dynamiclly generated Character Object
s.setY(randomY()); // same for y
}
//some methods for power and health
}
作为测试设置x,y(它是一个精灵,工作非常好)
public class GameMain extends Core{
public void init(){
test.setX(500); test.setY(488);
}
public void draw(Graphics2D g){
//the above code
}
}
如果它们完全相同,我看不出有什么必要推翻它们
# 1 楼答案
我相信你的问题是你的类
Character
是一个Spriter
并且有一个Spriter
(而且可能不应该“有一个”Spriter
,但我不知道,因为你没有发布所有的代码)。您从未初始化Character
的super
实例# 2 楼答案
问题是,当创建
Character
时,调用的是Spriter
的无参数构造函数,而不是带有Image
参数的构造函数。这意味着你的形象永远不会定型;这就是为什么会出现空指针异常您可能应该向
Character
的构造函数参数添加一个Image
,并将其传递给超类构造函数,如下所示@ElliottFrisch-如果这是你试图用你的答案说的话,我很抱歉。我不想复制;我只是不确定你是不是这个意思。无论如何,这绝对是问题所在