ヌルオブジェクト参照（NullpointerException）で仮想メソッドを呼び出そうとしました

Question

誰でも助けてもらえますか？アンドロイドスタジオで視差画像ビューを実行しようとしましたが、「ヌルオブジェクト参照で仮想メソッドを呼び出そうとしました」という例外メッセージが表示され続けます。それは、それがNullPointerExceptionであることを明らかにする。青色で強調表示されたメッセージは、発生したようです。

これはラインである：私のParallaxViewクラスで

backgrounds.add(new Background(this.context, screenWidth, screenHeight, "skyline", 0, 80, 50)); 

。

そしてラインで：私のParallaxActivityクラスで

setContentView(parallaxView); 

。

これは例外が発生する場所です。私はコピー＆ペーストフル例外メッセージを、以下のクラス： 1.

package com.hfad.parallaxproject; 

import android.content.Context; 
import android.graphics.Canvas; 
import android.graphics.Color; 
import android.graphics.Paint; 
import android.graphics.Rect; 
import android.view.SurfaceHolder; 
import android.view.SurfaceView; 
import java.util.ArrayList; 

public class ParallaxView extends SurfaceView implements Runnable { 

ArrayList<Background> backgrounds; 

private volatile boolean running; 
private Thread gameThread = null; 

//For drawing 
private Paint paint; 
private Canvas canvas; 
private SurfaceHolder ourHolder; 

//Holds a reference to the Activity 
Context context; 

//Control the fps 
long fps = 60; 

//Screen resolution 
int screenWidth; 
int screenHeight; 

ParallaxView(Context context, int screenWidth, int screenHeight) { 
    super(context); 

    this.context = context ; 

    this.screenWidth = screenWidth; 
    this.screenHeight = screenHeight; 

    //Initialize our drawing objects 
    ourHolder = getHolder(); 
    paint = new Paint(); 

    //load the background data into the Background objects and 
    //place them in our GameObject arraylist 

    backgrounds.add(new Background(this.context, screenWidth, screenHeight, "skyline", 0, 80, 50)) ; 
    backgrounds.add(new Background(this.context, screenWidth, screenHeight, "grass", 70, 110, 200)) ; 
    //Add more backgrounds here 
} 

private void drawBackground(int position) { 

    //Make a copy of the relevant background 
    Background bg = backgrounds.get(position) ; 

    //define what portion of images to capture and 
    //what coordinates of screen to draw them at 

    //For the regular bitmap 
    Rect fromRect1 = new Rect(0, 0, bg.width - bg.xClip, bg.height); 
    Rect toRect1 = new Rect(bg.xClip, bg.startY, bg.width, bg.endY); 

    //For the reversed background 
    Rect fromRect2 = new Rect(bg.width - bg.xClip, 0, bg.width, bg.height); 
    Rect toRect2 = new Rect(0, bg.startY, bg.xClip, bg.endY) ; 

    //draw the two background bitmaps 
    if(!bg.reversedFirst) { 
     canvas.drawBitmap(bg.bitmap, fromRect1, toRect1, paint) ; 
     canvas.drawBitmap(bg.bitmapReversed, fromRect2, toRect2, paint); 
    } else { 
     canvas.drawBitmap(bg.bitmap, fromRect2, toRect2, paint) ; 
     canvas.drawBitmap(bg.bitmapReversed, fromRect1, toRect1, paint) ; 
    } 

} 

@Override 
public void run() { 

    while(running) { 
     long startFrameTime = System.currentTimeMillis(); 

     update(); 

     draw(); 

     //Calculate the fps this frame 
     long timeThisFrame = System.currentTimeMillis() - startFrameTime; 
     if (timeThisFrame >= 1) { 
      fps = 1000/timeThisFrame; 
     } 
    } 
} 

private void update() { 
    //Update all the background positions 
    for(Background bg : backgrounds) { 
     bg.update(fps) ; 
    } 
} 

private void draw() { 
    if(ourHolder.getSurface().isValid()) { 
     //First we lock the area of memory we will be drawing to 
     canvas = ourHolder.lockCanvas(); 

     //draw a background color 
     canvas.drawColor(Color.argb(255, 0, 3, 70)); 

     //Draw the background parallax 
     drawBackground(0) ; 

     //Draw the rest of the game 
     paint.setTextSize(60); 
     paint.setColor(Color.argb(255, 255, 255, 255)); 
     canvas.drawText("I am a plane", 350, screenHeight/100 * 5, paint); 
     paint.setTextSize(220); 
     canvas.drawText("I'm a train", 50, screenHeight/100 * 80, paint); 

     drawBackground(1) ; 
     //Draw the foreground parallax 

     //Unlock and draw the scene 
     ourHolder.unlockCanvasAndPost(canvas); 
    } 

} 

//Clean up our thread if the game is stopped 
public void pause() { 
    running = false; 
    try{ 
     gameThread.join(); 
    } catch (InterruptedException e) { 
     //Error 
    } 
} 

//Make a new thread and start it 
//Execution moves to our run method 

public void resume() { 
    running = true; 
    gameThread = new Thread(this); 
    gameThread.start() ; 
} 
} //End of ParallaxView Class 

そして 2.

package com.hfad.parallaxproject; 

import android.app.Activity; 
import android.graphics.Point; 
import android.os.Bundle; 
import android.view.Display; 

public class ParallaxActivity extends Activity { 

//Our object to handle the View 
private ParallaxView parallaxView; 

@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 

    //Get a Display object to access screen details 
    Display display = getWindowManager().getDefaultDisplay(); 

    //Load the resolution into a Point object 
    Point resolution = new Point(); 
    display.getSize(resolution); 

    //And finally set the view for our game 
    parallaxView = new ParallaxView(this, resolution.x, resolution.y); 

    //Make our parallaxView the view for the Activity 
    setContentView(parallaxView); 
} 

//If the Activity is paused make sure to pause our thread 
@Override 
protected void onPause() { 
    super.onPause(); 
    parallaxView.pause(); 
} 

//If the Activity is resumed make sure to resume our thread 
@Override 
protected void onResume() { 
    super.onResume(); 
    parallaxView.resume(); 
} 
} 

：

06-28 20:11:48.297 2420-2420/com.hfad.parallaxproject E/AndroidRuntime: FATAL EXCEPTION: main 
Process: com.hfad.parallaxproject, PID: 2420 
java.lang.RuntimeException: Unable to start activity 
ComponentInfo{com.hfad.parallaxproject/com.hfad.parallaxproject 
.ParallaxActivity}: java.lang.NullPointerException: 
Attempt to invoke virtual method 'boolean java.util.ArrayList.add(java.lang.Object)' on a null object reference  
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2646 
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707) 
at android.app.ActivityThread.-wrap12(ActivityThread.java) 
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460) 
at android.os.Handler.dispatchMessage(Handler.java:102) 
at android.os.Looper.loop(Looper.java:154) 
at android.app.ActivityThread.main(ActivityThread.java:6077) 
at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865) 
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)                  
Caused by: java.lang.NullPointerException: 
Attempt to invoke virtual method 'boolean java.util.ArrayList.add(java.lang.Object)' on a null object reference 
at com.hfad.parallaxproject.ParallaxView.<init>(ParallaxView.java:49) 
at com.hfad.parallaxproject.ParallaxActivity.onCreate(ParallaxActivity.java:25) 
at android.app.Activity.performCreate(Activity.java:6662) 
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:111 
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2599 
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707) 
at android.app.ActivityThread.-wrap12(ActivityThread.java) 
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460) 
at android.os.Handler.dispatchMessage(Handler.java:102) 
at android.os.Looper.loop(Looper.java:154) 
at android.app.ActivityThread.main(ActivityThread.java:6077) 
at java.lang.reflect.Method.invoke(Native Method) 
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865) 
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)  

これらは2つのエラーがで発生したクラスです

Answer 1

ArrayListを初期化するのを忘れてしまいます。

ArrayList<Background> backgrounds = new ArrayList<>();