安卓应用程序开发"NullPointerException on setBackgroundResource(xx)“

Question

我试着为飞盘高尔夫球杆制作一个应用程序。使用这个应用程序，我正在使用本教程来做一个gallery刷卡，看看不同的课程。但是在运行应用程序时，我会在我的NullpointerException方法中得到一个getView。我不明白为什么会发生这种事。需要别人的帮助来指出出了什么问题。这是一个例外：

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ImageView.setBackgroundResource(int)' on a null object reference

我的MainActivity：

public class MainActivity extends ActionBarActivity {




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


}

public void OpenActivityBanen(View view) {

    Intent visBane = new Intent(this,banen.class);
    startActivity(visBane);
}

}

我的SecondActivity：(在这里我希望这个库滑动器能够运行)这就是我在方法getView中获得iv.setBackgroundResource(thumb[p]);的地方

public class banen extends MainActivity {

private GridView gv;
private Animator mCurrentAnimator;
private int mShortAnimationDuration;
private int j = 0;

private GestureDetector detector;
private static final int SWIPE_MIN_DISTANCE = 120;
private static final int SWIPE_THRESHOLD_VELOCITY = 200;

private int thumb[] =
        {
                R.drawable.hull1, R.drawable.kartinfo , R.drawable.logo , R.drawable.logo1234, R.drawable.logo3 , R.drawable.logo4
        };

private ImageView expandedImageView;


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

    detector = new GestureDetector(this, new SwipeGestureDetector());

    gv = (GridView) findViewById(R.id.grid_view);
    gv.setAdapter(new ImageAdapter(this));
    gv.setOnItemClickListener(new AdapterView.OnItemClickListener() {
        @Override
        public void onItemClick(AdapterView<?> parent, View v, int pos, long     id) {

            j = pos;
            zoomImageFromThumb(v, thumb[pos]);
        }
    });
    mShortAnimationDuration =      getResources().getInteger(android.R.integer.config_shortAnimTime);
}

class ImageAdapter extends BaseAdapter
{

    private LayoutInflater layoutInflater;

    public ImageAdapter(banen activity)
    {
        layoutInflater = (LayoutInflater) activity
                .getSystemService(Context.LAYOUT_INFLATER_SERVICE);
    }

    @Override
    public int getCount() {
        return thumb.length;
    }

    @Override
    public Object getItem(int pos) {
        return pos;
    }

    @Override
    public long getItemId(int pos) {
        return pos;
    }

    @Override
    public View getView(int pos, View convertView, ViewGroup parent) {

        View listItem = convertView;
        int p = pos;

        if(listItem == null)
        {
            listItem= layoutInflater.inflate(R.layout.banen, null);
        }

        ImageView iv = (ImageView) listItem.findViewById(R.id.thumb);

        iv.setBackgroundResource(thumb[p]);

        return listItem;
    }
}

private void zoomImageFromThumb(final View thumbView, int imageResId)
{
    if(mCurrentAnimator != null){
        mCurrentAnimator.cancel();
    }

    expandedImageView = (ImageView) findViewById(R.id.imageView2);

    expandedImageView.setOnTouchListener(new View.OnTouchListener() {

        @Override
        public boolean onTouch(View v, MotionEvent event) {
            if (detector.onTouchEvent(event)) {
                return true;
            } else {
                return false;
            }

        }
    });

    expandedImageView.setImageResource(imageResId);

    final Rect startBounds = new Rect();
    final Rect finalBounds = new Rect();
    final Point globalOffset = new Point();

    thumbView.getGlobalVisibleRect(startBounds);
       findViewById(R.id.container).getGlobalVisibleRect(finalBounds,globalOffset);

    startBounds.offset(-globalOffset.x, -globalOffset.y);
    finalBounds.offset(-globalOffset.x, -globalOffset.y);

    float startScale;

    if((float) finalBounds.width() / finalBounds.height() > (float)   startBounds
            .width() / startBounds.height()){

        startScale = (float) startBounds.height() / finalBounds.height();
        float startWidth = startScale * finalBounds.width();
        float deltaWidth = (startWidth - startBounds.width()) / 2;
        startBounds.left -= deltaWidth;
        startBounds.right+= deltaWidth;

    }else{

        startScale =(float) startBounds.width()/finalBounds.width();
        float startHeight = startScale * finalBounds.height();
        float deltaHeight = (startHeight = startBounds.height()) / 2;
        startBounds.top -= deltaHeight;
        startBounds.bottom += deltaHeight;

    }

    thumbView.setAlpha(0f);
    expandedImageView.setVisibility(View.VISIBLE);

    expandedImageView.setPivotX(0f);
    expandedImageView.setPivotY(0f);

    AnimatorSet set = new AnimatorSet();

    set.play(ObjectAnimator.ofFloat(expandedImageView, View.X,  startBounds.left, finalBounds.left))
            .with(ObjectAnimator.ofFloat(expandedImageView, View.Y, startBounds.top, finalBounds.top))
            .with(ObjectAnimator.ofFloat(expandedImageView, View.SCALE_X, startScale, 1f))
            .with(ObjectAnimator.ofFloat(expandedImageView, View.SCALE_Y, startScale, 1f));

    set.setDuration(mShortAnimationDuration);
    set.setInterpolator(new DecelerateInterpolator());
    set.addListener(new AnimatorListenerAdapter() {

        @Override
        public void onAnimationCancel(Animator animation) {
            mCurrentAnimator = null;
        }

        @Override
        public void onAnimationEnd(Animator animation) {
            mCurrentAnimator = null;
        }
    });
    set.start();
    mCurrentAnimator = set;

    final float startScaleFinal = startScale;
    expandedImageView.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {

            if(mCurrentAnimator != null)
            {
                mCurrentAnimator.cancel();
            }

            AnimatorSet set = new AnimatorSet();

            set.play(ObjectAnimator.ofFloat(expandedImageView, View.X, startBounds.left))
                    .with(ObjectAnimator.ofFloat(expandedImageView, View.Y, startBounds.top))
                    .with(ObjectAnimator.ofFloat(expandedImageView, View.SCALE_X, startScaleFinal))
                    .with(ObjectAnimator.ofFloat(expandedImageView, View.SCALE_Y, startScaleFinal));

            set.setDuration(mShortAnimationDuration);
            set.setInterpolator(new DecelerateInterpolator());
            set.addListener(new AnimatorListenerAdapter() {

                @Override
                public void onAnimationEnd(Animator animation) {
                    thumbView.setAlpha(1f);
                    expandedImageView.setVisibility(View.GONE);
                    mCurrentAnimator = null;
                }

                @Override
                public void onAnimationCancel(Animator animation) {
                    thumbView.setAlpha(1f);
                    expandedImageView.setVisibility(View.GONE);
                    mCurrentAnimator = null;
                }
            });
            set.start();
            mCurrentAnimator = set;

        }
    });

}

private class SwipeGestureDetector extends  GestureDetector.SimpleOnGestureListener{
    @Override
    public boolean onFling(MotionEvent e1, MotionEvent e2, float velocityX, float velocityY) {

        try{
            if(e1.getX() - e2.getX() > SWIPE_MIN_DISTANCE && Math.abs(velocityX) > SWIPE_THRESHOLD_VELOCITY)
            {
                if(thumb.length >j){
                    j++;

                    if(j < thumb.length){
                        expandedImageView.setImageResource(thumb[j]);
                        return true;
                    } else {
                        j = 0;
                        expandedImageView.setImageResource(thumb[j]);
                        return true;
                    }
                }
            }else if(e1.getX() - e2.getX() > SWIPE_MIN_DISTANCE &&   Math.abs(velocityX) > SWIPE_THRESHOLD_VELOCITY){

                if(j > 0){

                    j--;
                    expandedImageView.setImageResource(thumb[j]);
                    return true;

                }else {
                    j = thumb.length - 1;
                    expandedImageView.setImageResource(thumb[j]);
                    return true;
                }
            }
        }catch(Exception e){
            e.printStackTrace();
        }
        return false;
    }
}

@Override
public void onBackPressed() {
    finish();

}
}

Banen的XML：

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent" android:layout_height="match_parent"
android:id="@+id/container">

<GridView
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:id="@+id/grid_view"
    android:columnWidth="80dp"
    android:horizontalSpacing="10dp"
    android:numColumns="auto_fit"
    android:stretchMode="columnWidth"
    android:verticalSpacing="10dp">
</GridView>


<ImageView
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:id="@+id/imageView2"
    android:visibility="invisible"
    android:contentDescription="@string/app_name"/>
</FrameLayout>

主要活动的XML：

<Button
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="@string/spillKnapp"
    android:onClick="SpillMedNoen"
    android:id="@+id/knapp1"
    android:minWidth="300dp"
    android:padding="20dp"
    android:layout_above="@+id/baneKnapp1"
    android:layout_centerHorizontal="true" />

<ImageView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:id="@+id/imageView"
    android:src="@drawable/logo4"
    android:layout_alignParentStart="true"
    android:layout_above="@+id/knapp1" />

<Button
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="@string/InfoKnapp"
    android:onClick="ShowImage"
    android:id="@+id/knapp2"
    android:minWidth="300dp"
    android:padding="20dp"
    android:layout_alignParentBottom="true"
    android:layout_centerHorizontal="true"
    android:layout_marginBottom="23dp" />

<Button

    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="@string/baneKnapp"
    android:id="@+id/baneKnapp1"
    android:onClick="OpenActivityBanen"
    android:minWidth="300dp"
    android:padding="20dp"
    android:layout_above="@+id/knapp2"
    android:layout_centerHorizontal="true" />
</RelativeLayout>

Single_grid_item的XML：(这是存储“拇指”ID的地方)

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent" android:layout_height="match_parent"
android:padding="5dp">

<ImageView
    android:layout_width="72dp"
    android:layout_height="72dp"
    android:id="@+id/thumb"
    android:layout_centerHorizontal="true"
    android:contentDescription="@string/app_name"/>


</RelativeLayout>

我的LogCat：

Process: com.hatlojohansen.veardiscgolf, PID: 19166
java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ImageView.setBackgroundResource(int)' on a null object reference
        at com.hatlojohansen.veardiscgolf.banen$ImageAdapter.getView(banen.java:103)
        at android.widget.AbsListView.obtainView(AbsListView.java:2347)
        at android.widget.GridView.onMeasure(GridView.java:1060)
        at android.view.View.measure(View.java:17547)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5535)
        at android.widget.FrameLayout.onMeasure(FrameLayout.java:436)
        at android.view.View.measure(View.java:17547)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5535)
        at android.widget.FrameLayout.onMeasure(FrameLayout.java:436)
        at android.support.v7.internal.widget.ContentFrameLayout.onMeasure(ContentFrameLayout.java:124)
        at android.view.View.measure(View.java:17547)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5535)
        at android.support.v7.internal.widget.ActionBarOverlayLayout.onMeasure(ActionBarOverlayLayout.java:444)
        at android.view.View.measure(View.java:17547)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5535)
        at android.widget.FrameLayout.onMeasure(FrameLayout.java:436)
        at android.view.View.measure(View.java:17547)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5535)
        at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1436)
        at android.widget.LinearLayout.measureVertical(LinearLayout.java:722)
        at android.widget.LinearLayout.onMeasure(LinearLayout.java:613)
        at android.view.View.measure(View.java:17547)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5535)
        at android.widget.FrameLayout.onMeasure(FrameLayout.java:436)
        at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2615)
        at android.view.View.measure(View.java:17547)
        at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:2015)
        at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1173)
        at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1379)
        at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1061)
        at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:5885)
        at android.view.Choreographer$CallbackRecord.run(Choreographer.java:767)
        at android.view.Choreographer.doCallbacks(Choreographer.java:580)
        at android.view.Choreographer.doFrame(Choreographer.java:550)
        at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:753)
        at android.os.Handler.handleCallback(Handler.java:739)
        at android.os.Handler.dispatchMessage(Handler.java:95)
        at android.os.Looper.loop(Looper.java:135)
        at android.app.ActivityThread.main(ActivityThread.java:5257)
        at java.lang.reflect.Method.invoke(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:372)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)

Answer 1

在方法getView()中，您已经膨胀了R.layout.Banen，然后调用了listItem.findViewById(R.id.thumb)，但是id拇指的XML元素是在R.layout.single_grid_item中声明的。我猜你可能是想让R.layout.single_grid_item膨胀，而不是Banen。尝试更改以下内容：

listItem= layoutInflater.inflate(R.layout.banen, null);

至

listItem= layoutInflater.inflate(R.layout.single_grid_item, null);

Answer 2

当您膨胀项目的布局时：

listItem = layoutInflater.inflate(R.layout.banen, null);

GridView是空的。因此，在Single_grid_item.xml布局中查找项目将返回null。

也许你是想用imageView2 id代替thumb？或者您可能夸大了错误的布局，而应该这样做：

listItem = layoutInflater.inflate(R.layout.Single_grid_item, null);

Answer 3

以下变动

listItem= layoutInflater.inflate(R.layout.banen, null); 

至

listItem= layoutInflater.inflate(R.layout.Single_grid_item, null);