Android自定义ListView崩溃

Question

When I start my new activity it crashes. I don't know why, i did the same thing I did for my other adapters and they work. I noticed some Measure Error in the log. What does that mean and where is my bug?

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:orientation="horizontal"
    android:padding="5dip" >

    <ImageView
        android:id="@+id/arrow"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentRight="true"
        android:layout_centerVertical="true"
        android:layout_marginRight="5dp"
        android:src="@drawable/arrow" />

    <LinearLayout
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentLeft="true"
        android:layout_centerVertical="true"
        android:layout_toLeftOf="@+id/arrow" >

        <TextView
            android:id="@+id/numberLabel"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="0.4"
            android:textColor="@color/text_color"
            android:gravity="center" />

        <TextView
            android:id="@+id/titleLabel"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="0.6"
            android:textColor="@color/text_color"
            android:singleLine="true"
            android:textAppearance="?android:attr/textAppearanceMedium" />

    </LinearLayout>

</RelativeLayout>

My Adapter :

public class DAlbumlistAdapter extends ArrayAdapter<Song> {

    Context mContext;
    List<Song> mObjects;

    public DAlbumlistAdapter(Context context, List<Song> songList) {
        super(context, R.layout.list_item_albumlist, songList);
        this.mContext = context;
        this.mObjects = songList;
    }

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

        ViewHolder holder;
        Song obj = mObjects.get(position);

        if (convertView == null) {
            holder = new ViewHolder();
            convertView = LayoutInflater.from(mContext).inflate(
                    R.layout.list_item_song, parent, false);

            holder.titleLabel = (TextView) convertView
                    .findViewById(R.id.titleLabel);
            holder.numberLabel = (TextView) convertView
                    .findViewById(R.id.numberLabel);

            convertView.setTag(holder);
        } else {
            holder = (ViewHolder) convertView.getTag();
        }
        holder.numberLabel.setText("HAHA");
        holder.titleLabel.setText("HAHA");

        return convertView;
    }

    class ViewHolder {
        TextView numberLabel;
        TextView titleLabel;
    }

}

This is my error log :

06-29 19:11:45.643: E/AndroidRuntime(3419): FATAL EXCEPTION: main

06-29 19:11:45.643: E/AndroidRuntime(3419): java.lang.NullPointerException

06-29 19:11:45.643: E/AndroidRuntime(3419):     at com.davlog.musicplayer.DAlbumlistAdapter.getView(DAlbumlistAdapter.java:45)

06-29 19:11:45.643: E/AndroidRuntime(3419):     at android.widget.AbsListView.obtainView(AbsListView.java:2619)

06-29 19:11:45.643: E/AndroidRuntime(3419):     at android.widget.ListView.measureHeightOfChildren(ListView.java:1253)

06-29 19:11:45.643: E/AndroidRuntime(3419):     at android.widget.ListView.onMeasure(ListView.java:1165)

06-29 19:11:45.643: E/AndroidRuntime(3419):     at android.view.View.measure(View.java:16142)

06-29 19:11:45.643: E/AndroidRuntime(3419):     at android.widget.RelativeLayout.measureChild(RelativeLayout.java:666)

06-29 19:11:45.643: E/AndroidRuntime(3419):     at android.widget.RelativeLayout.onMeasure(RelativeLayout.java:477)

06-29 19:11:45.643: E/AndroidRuntime(3419):     at android.view.View.measure(View.java:16142)

06-29 19:11:45.643: E/AndroidRuntime(3419):     at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4927)

06-29 19:11:45.643: E/AndroidRuntime(3419):     at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)

06-29 19:11:45.643: E/AndroidRuntime(3419):     at android.view.View.measure(View.java:16142)

06-29 19:11:45.643: E/AndroidRuntime(3419):     at android.widget.LinearLayout.measureVertical(LinearLayout.java:847)

06-29 19:11:45.643: E/AndroidRuntime(3419):     at android.widget.LinearLayout.onMeasure(LinearLayout.java:588)

06-29 19:11:45.643: E/AndroidRuntime(3419):     at android.view.View.measure(View.java:16142)

06-29 19:11:45.643: E/AndroidRuntime(3419):     at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4927)

06-29 19:11:45.643: E/AndroidRuntime(3419):     at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)

06-29 19:11:45.643: E/AndroidRuntime(3419):     at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2471)

06-29 19:11:45.643: E/AndroidRuntime(3419):     at android.view.View.measure(View.java:16142)

06-29 19:11:45.643: E/AndroidRuntime(3419):     at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:2167)

06-29 19:11:45.643: E/AndroidRuntime(3419):     at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1315)

06-29 19:11:45.643: E/AndroidRuntime(3419):     at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1526)

06-29 19:11:45.643: E/AndroidRuntime(3419):     at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1212)

06-29 19:11:45.643: E/AndroidRuntime(3419):     at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:4981)

06-29 19:11:45.643: E/AndroidRuntime(3419):     at android.view.Choreographer$CallbackRecord.run(Choreographer.java:791)

06-29 19:11:45.643: E/AndroidRuntime(3419):     at android.view.Choreographer.doCallbacks(Choreographer.java:591)

06-29 19:11:45.643: E/AndroidRuntime(3419):     at android.view.Choreographer.doFrame(Choreographer.java:561)

06-29 19:11:45.643: E/AndroidRuntime(3419):     at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:777)

06-29 19:11:45.643: E/AndroidRuntime(3419):     at android.os.Handler.handleCallback(Handler.java:725)

06-29 19:11:45.643: E/AndroidRuntime(3419):     at android.os.Handler.dispatchMessage(Handler.java:92)

06-29 19:11:45.643: E/AndroidRuntime(3419):     at android.os.Looper.loop(Looper.java:137)

06-29 19:11:45.643: E/AndroidRuntime(3419):     at android.app.ActivityThread.main(ActivityThread.java:5306)

06-29 19:11:45.643: E/AndroidRuntime(3419):     at java.lang.reflect.Method.invokeNative(Native Method)

06-29 19:11:45.643: E/AndroidRuntime(3419):     at java.lang.reflect.Method.invoke(Method.java:511)

06-29 19:11:45.643: E/AndroidRuntime(3419):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1102)

06-29 19:11:45.643: E/AndroidRuntime(3419):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:869)

06-29 19:11:45.643: E/AndroidRuntime(3419):     at dalvik.system.NativeStart.main(Native Method)

Answer 1

You only create a new ViewHolder object in the if (convertView == null) but you use it outside of that if. Therefore, if you never get in your if statement, then holder is NULL and you get the NULL pointer exception.