如何检查图像是否放在正确的容器中？

Question

我正在开发一个应用程序，用户必须将一个对象（例如塑料瓶，罐头，易碎包装）拖到正确的垃圾箱（垃圾箱，回收箱）中。我正在尝试这样做，以便一旦用户将对象拖到正确的容器中，下一个对象就会在同一图像视图中弹出，并告诉用户是否选择了正确的容器。

在我看来，我已经正确地完成了所有操作，但是只要将对象拖到其中一个垃圾箱，该应用程序就会崩溃。我在做什么错了？

MainActivity.java

public class MainActivity extends AppCompatActivity {

    private ImageView question, option1, option2, option3;
    private Map<Integer, Integer> drawableValues;
    private int currentIndex = 0;

    private int[] textureArrayWin = {
            R.drawable.image_0,
            R.drawable.image_1,
            R.drawable.image_2,
            R.drawable.image_3,
            R.drawable.image_4,
            R.drawable.image_5,
            R.drawable.image_6,
            R.drawable.image_7,
            R.drawable.image_8,
            R.drawable.image_9,
            R.drawable.image_10,
    };

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        drawableValues = new HashMap<>();
        drawableValues.put(R.drawable.image_0, 1);
        drawableValues.put(R.drawable.image_1, 2);
        drawableValues.put(R.drawable.image_2, 3);
        drawableValues.put(R.drawable.image_3, 2);
        drawableValues.put(R.drawable.image_4, 3);
        drawableValues.put(R.drawable.image_5, 3);
        drawableValues.put(R.drawable.image_6, 1);
        drawableValues.put(R.drawable.image_7, 1);
        drawableValues.put(R.drawable.image_8, 1);
        drawableValues.put(R.drawable.image_9, 1);
        drawableValues.put(R.drawable.image_10, 1);

        question = (ImageView) findViewById(R.id.qImageView);
        question.setImageDrawable(getDrawable(textureArrayWin[currentIndex]));
        option1 = (ImageView) findViewById(R.id.gWBinImageView);
        option2 = (ImageView) findViewById(R.id.rBImageView);
        option3 = (ImageView) findViewById(R.id.oWImageView);

        option1.setOnDragListener(dragListener);
        option2.setOnDragListener(dragListener);
        option3.setOnDragListener(dragListener);

        question.setOnLongClickListener(longClickListener);
    }

    View.OnLongClickListener longClickListener = new View.OnLongClickListener(){
        @Override
        public boolean onLongClick(View v) {
            ClipData data = ClipData.newPlainText("", "");
            View.DragShadowBuilder myShadowBuilder = new View.DragShadowBuilder(v);
            v.startDrag(data, myShadowBuilder, v, 0);
            return true;
        }
    };

    View.OnDragListener dragListener = new View.OnDragListener() {
        @Override
        public boolean onDrag(View v, DragEvent event) {
            int dragEvent = event.getAction();

            switch (dragEvent){

                case DragEvent.ACTION_DROP:
                    final View view = (View) event.getLocalState();
                    //view.setVisibility(View.INVISIBLE);

                    ImageView dropTarget = (ImageView) v;
                    ImageView dropped = (ImageView) view;

                    int answer = drawableValues.get(((ImageView) dropped).getDrawable());
                    int tagDropTarget = Integer.parseInt((String)dropTarget.getTag());

                    if (answer == tagDropTarget ){
                        Toast.makeText(MainActivity.this,R.string.correctMessage, Toast.LENGTH_SHORT).show();
                     }
                    else {
                        Toast.makeText(MainActivity.this,R.string.incorrectMessage, Toast.LENGTH_SHORT).show();
                    }
                    currentIndex = (currentIndex + 1);
                    Drawable imageQuestion = getResources().getDrawable(textureArrayWin[currentIndex]);
                    question.setImageDrawable(imageQuestion);
                    break;
            }
            return true;
        }
    };
}

activity_main.xml（相关部分）

        <ImageView
        android:tag="1"
        android:id="@+id/gWBinImageView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_weight="1"
        app:srcCompat="@drawable/general_waste" />

    <ImageView
        android:tag="2"
        android:id="@+id/rBImageView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_weight="1"
        app:srcCompat="@drawable/recycle_bin" />

    <ImageView
        android:tag="3"
        android:id="@+id/oWImageView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_weight="1"
        app:srcCompat="@drawable/food_waste" />
</LinearLayout>

错误日志

2019-06-12 23:26:14.261 12092-12092/com.example.myapplication E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.myapplication, PID: 12092
java.lang.NullPointerException: Attempt to invoke virtual method 'int java.lang.Integer.intValue()' on a null object reference
    at com.example.myapplication.MainActivity$2.onDrag(MainActivity.java:93)
    at android.view.View.callDragEventHandler(View.java:26134)
    at android.view.View.dispatchDragEvent(View.java:26125)
    at android.view.ViewGroup.dispatchDragEvent(ViewGroup.java:1784)
    at android.view.ViewGroup.dispatchDragEvent(ViewGroup.java:1784)
    at android.view.ViewGroup.dispatchDragEvent(ViewGroup.java:1784)
    at android.view.ViewGroup.dispatchDragEvent(ViewGroup.java:1784)
    at android.view.ViewGroup.dispatchDragEvent(ViewGroup.java:1784)
    at android.view.ViewGroup.dispatchDragEvent(ViewGroup.java:1784)
    at android.view.ViewGroup.dispatchDragEvent(ViewGroup.java:1784)
    at android.view.ViewRootImpl.handleDragEvent(ViewRootImpl.java:6856)
    at android.view.ViewRootImpl.access$1200(ViewRootImpl.java:146)
    at android.view.ViewRootImpl$ViewRootHandler.handleMessage(ViewRootImpl.java:4719)
    at android.os.Handler.dispatchMessage(Handler.java:107)
    at android.os.Looper.loop(Looper.java:214)
    at android.app.ActivityThread.main(ActivityThread.java:7116)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:925)