我正在尝试构建一个新手应用程序，它从屏幕顶部删除一组披萨主题，用户应该避免它们，但由于某种原因，我在logcat中得到NullPointerException并且应用程序崩溃了。 。 它说它发生在firstSet（）中，当我试图将值放入point.y时没有意义，因为我在约束器中初始化它们。
package com.example.secondapp;
import java.util.Random;
import android.annotation.SuppressLint;
import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Point;
import android.graphics.drawable.Drawable;
import android.view.View;
public class EnemyTopics extends View {
private Drawable[] topic;
private Point[] cords;
private int width,height,screenHgt,screenWid,ClosestTopic,score;
private int tomatos,olives,mushroom,onion;
private UserCharacter character;
private Context context;
@SuppressLint("NewApi")
public EnemyTopics(Context context,int wid,int hgt,int screenHgt,int screenWid,UserCharacter uc) {
super(context);
this.cords = new Point[20];
for(Point point : cords) {
point = new Point();
point.x = 0;
point.y = 0;
}
this.width = wid;
this.height = hgt;
this.topic = new Drawable[20];
this.tomatos = R.drawable.tomatos;
this.olives = R.drawable.olives;
this.mushroom = R.drawable.mushroom;
this.onion = R.drawable.onion;
this.ClosestTopic = 0;
this.screenHgt = screenHgt;
this.screenWid = screenWid;
this.character = uc;
this.score = 0;
this.context = context;
this.firstSet();
this.setBounds();
}
public int maxLeft(int position) {
return this.cords[position].x - (this.width/2);
}
public int maxRight(int position) {
return this.cords[position].x + (this.width/2);
}
public int maxTop(int position) {
return this.cords[position].y - (this.height/2);
}
public int maxBottom(int position) {
return this.cords[position].y + (this.height/2);
}
public void setBounds() {
int pos = 0;
for (Drawable topics : this.topic) {
topics.setBounds(maxLeft(pos), maxTop(pos), maxRight(pos), maxBottom(pos));
pos++;
}
}
public void moveTopics() {
int pos = 0;
for (Point p : this.cords) {
if(maxBottom(pos) <= screenHgt) {
p.y++;
}
else {
p.y = -200;
}
pos++;
}
if(maxTop(ClosestTopic) > character.maxBottom()) {
ClosestTopic++;
score++;
}
if(ClosestTopic <= 19)
ClosestTopic = 0;
}
public void checkHit() {
if(maxBottom(ClosestTopic) >= character.maxTop() && maxTop(ClosestTopic) <= character.maxBottom() && maxLeft(ClosestTopic) <= character.maxRight() && maxRight(ClosestTopic) >= character.maxLeft()) {
score = 0;
this.firstSet();
character.restart();
}
}
@SuppressLint("NewApi")
public void firstSet() {
Random r = new Random();
int random = r.nextInt(4);
int pos = 0;
for (Point p : cords) {
p.y = (0 - pos*15);
p.x = screenWid/2;
switch (random) {
case 0:
topic[pos]= context.getResources().getDrawable(tomatos,null);
break;
case 1:
topic[pos]= context.getResources().getDrawable(olives,null);
break;
case 2:
topic[pos]= context.getResources().getDrawable(mushroom,null);
break;
case 3:
topic[pos]= context.getResources().getDrawable(onion,null);
break;
default:
topic[pos]= context.getResources().getDrawable(tomatos,null);
}
pos++;
random = r.nextInt(4);
}
}
public void draw (Canvas canvas) {
for (Drawable topics : this.topic) {
topics.draw(canvas);
}
}
}
我禁用了移动它们的线程，这意味着问题是在构造函数，firstSet或setBound上，但我无法找到它，所以我会appriciate帮助:) 提前谢谢。
logcat:
09-20 14:13:53.725: E/AndroidRuntime(1154): FATAL EXCEPTION: main
09-20 14:13:53.725: E/AndroidRuntime(1154): Process: com.example.secondapp, PID: 1154
09-20 14:13:53.725: E/AndroidRuntime(1154): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.secondapp/com.example.secondapp.MainActivity}: java.lang.NullPointerException: Attempt to write to field 'int android.graphics.Point.y' on a null object reference
09-20 14:13:53.725: E/AndroidRuntime(1154): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416)
09-20 14:13:53.725: E/AndroidRuntime(1154): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
09-20 14:13:53.725: E/AndroidRuntime(1154): at android.app.ActivityThread.-wrap11(ActivityThread.java)
09-20 14:13:53.725: E/AndroidRuntime(1154): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
09-20 14:13:53.725: E/AndroidRuntime(1154): at android.os.Handler.dispatchMessage(Handler.java:102)
09-20 14:13:53.725: E/AndroidRuntime(1154): at android.os.Looper.loop(Looper.java:148)
09-20 14:13:53.725: E/AndroidRuntime(1154): at android.app.ActivityThread.main(ActivityThread.java:5417)
09-20 14:13:53.725: E/AndroidRuntime(1154): at java.lang.reflect.Method.invoke(Native Method)
09-20 14:13:53.725: E/AndroidRuntime(1154): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
09-20 14:13:53.725: E/AndroidRuntime(1154): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
09-20 14:13:53.725: E/AndroidRuntime(1154): Caused by: java.lang.NullPointerException: Attempt to write to field 'int android.graphics.Point.y' on a null object reference
09-20 14:13:53.725: E/AndroidRuntime(1154): at com.example.secondapp.EnemyTopics.firstSet(EnemyTopics.java:96)
09-20 14:13:53.725: E/AndroidRuntime(1154): at com.example.secondapp.EnemyTopics.<init>(EnemyTopics.java:42)
09-20 14:13:53.725: E/AndroidRuntime(1154): at com.example.secondapp.MainActivity.onCreate(MainActivity.java:19)
09-20 14:13:53.725: E/AndroidRuntime(1154): at android.app.Activity.performCreate(Activity.java:6237)
09-20 14:13:53.725: E/AndroidRuntime(1154): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1107)
09-20 14:13:53.725: E/AndroidRuntime(1154): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2369)
09-20 14:13:53.725: E/AndroidRuntime(1154): ... 9 more
答案 0 :(得分：5)
使用这种循环不能将对象分配给数组。 point是引用的副本，不会将其复制回数组。
this.cords = new Point[20];
for(Point point : cords) {
point = new Point();
point.x = 0;
point.y = 0;
}
使用“老式”方式：
this.cords = new Point[20];
for(int i = 0; i < cords.length; ++i ) {
cords[i] = new Point();
cords[i].x = 0;
cords[i].y = 0;
}