java - 尝试实现自定义列表适配器时应用程序崩溃(NullPointerException)

Question

错误:

05-17 11:27:39.986: E/AndroidRuntime(5172): FATAL EXCEPTION: main 05-17 11:27:39.986: E/AndroidRuntime(5172): Process: com.example.yevsnap, PID: 5172 
05-17 11:27:39.986: E/AndroidRuntime(5172): java.lang.NullPointerException 
05-17 11:27:39.986: E/AndroidRuntime(5172): at com.example.yevsnap.MessageAdapter.getView(MessageAdapter.java:46) 
05-17 11:27:39.986: E/AndroidRuntime(5172): at android.widget.AbsListView.obtainView(AbsListView.java:2240) 
05-17 11:27:39.986: E/AndroidRuntime(5172): at android.widget.ListView.makeAndAddView(ListView.java:1790)
 05-17 11:27:39.986: E/AndroidRuntime(5172): at android.widget.ListView.fillDown(ListView.java:691) 
05-17 11:27:39.986: E/AndroidRuntime(5172): at android.widget.ListView.fillFromTop(ListView.java:752) 
05-17 11:27:39.986: E/AndroidRuntime(5172): at android.widget.ListView.layoutChildren(ListView.java:1630) 
05-17 11:27:39.986: E/AndroidRuntime(5172): at android.widget.AbsListView.onLayout(AbsListView.java:2091) 
05-17 11:27:39.986: E/AndroidRuntime(5172): at android.view.View.layout(View.java:14817) 05-17 11:27:39.986: E/AndroidRuntime(5172): at android.view.ViewGroup.layout(ViewGroup.java:4631) 
05-17 11:27:39.986: E/AndroidRuntime(5172): at android.widget.RelativeLayout.onLayout(RelativeLayout.java:1055) 
05-17 11:27:39.986: E/AndroidRuntime(5172): at android.view.View.layout(View.java:14817) 05-17 11:27:39.986: E/AndroidRuntime(5172): at android.view.ViewGroup.layout(ViewGroup.java:4631) 
05-17 11:27:39.986: E/AndroidRuntime(5172): at android.widget.FrameLayout.layoutChildren(FrameLayout.java:453) 
05-17 11:27:39.986: E/AndroidRuntime(5172): at android.widget.FrameLayout.onLayout(FrameLayout.java:388) 
05-17 11:27:39.986: E/AndroidRuntime(5172): at android.view.View.layout(View.java:14817) 05-17 11:27:39.986: E/AndroidRuntime(5172): at android.view.ViewGroup.layout(ViewGroup.java:4631) 
05-17 11:27:39.986: E/AndroidRuntime(5172): at android.support.v4.view.ViewPager.onLayout(ViewPager.java:1589)
 05-17 11:27:39.986: E/AndroidRuntime(5172): at android.view.View.layout(View.java:14817) 05-17 11:27:39.986: E/AndroidRuntime(5172): at android.view.ViewGroup.layout(ViewGroup.java:4631) 
05-17 11:27:39.986: E/AndroidRuntime(5172): at android.widget.FrameLayout.layoutChildren(FrameLayout.java:453) 
05-17 11:27:39.986: E/AndroidRuntime(5172): at android.widget.FrameLayout.onLayout(FrameLayout.java:388) 
05-17 11:27:39.986: E/AndroidRuntime(5172): at android.view.View.layout(View.java:14817)
 05-17 11:27:39.986: E/AndroidRuntime(5172): at android.view.ViewGroup.layout(ViewGroup.java:4631) 
05-17 11:27:39.986: E/AndroidRuntime(5172): at com.android.internal.widget.ActionBarOverlayLayout.onLayout(ActionBarOverlayLayout.java:374) 05-17 11:27:39.986: E/AndroidRuntime(5172): at android.view.View.layout(View.java:14817) 
05-17 11:27:39.986: E/AndroidRuntime(5172): at android.view.ViewGroup.layout(ViewGroup.java:4631)
05-17 11:27:39.986: E/AndroidRuntime(5172): at android.widget.FrameLayout.layoutChildren(FrameLayout.java:453) 
05-17 11:27:39.986: E/AndroidRuntime(5172): at android.widget.FrameLayout.onLayout(FrameLayout.java:388) 
05-17 11:27:39.986: E/AndroidRuntime(5172): at android.view.View.layout(View.java:14817) 05-17 11:27:39.986: E/AndroidRuntime(5172): at android.view.ViewGroup.layout(ViewGroup.java:4631) 
-17 11:27:39.986: E/AndroidRuntime(5172): at android.view.ViewRootImpl.performLayout(ViewRootImpl.java:1987) 
05-17 11:27:39.986: E/AndroidRuntime(5172): at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1744) 
05-17 11:27:39.986: E/AndroidRuntime(5172): at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1000) 
05-17 11:27:39.986: E/AndroidRuntime(5172): at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:5670) 
05-17 11:27:39.986: E/AndroidRuntime(5172): at android.view.Choreographer$CallbackRecord.run(Choreographer.java:761) 
05-17 11:27:39.986: E/AndroidRuntime(5172): at android.view.Choreographer.doCallbacks(Choreographer.java:574) 
05-17 11:27:39.986: E/AndroidRuntime(5172): at android.view.Choreographer.doFrame(Choreographer.java:544) 05-17 11:27:39.986: E/AndroidRuntime(5172): at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:747) 
05-17 11:27:39.986: E/AndroidRuntime(5172): at android.os.Handler.handleCallback(Handler.java:733) 
-17 11:27:39.986: E/AndroidRuntime(5172): at android.os.Handler.dispatchMessage(Handler.java:95) 
05-17 11:27:39.986: E/AndroidRuntime(5172): at android.os.Looper.loop(Looper.java:136) 
05-17 11:27:39.986: E/AndroidRuntime(5172): at android.app.ActivityThread.main(ActivityThread.java:5017) 
05-17 11:27:39.986: E/AndroidRuntime(5172): at java.lang.reflect.Method.invokeNative(Native Method) 
05-17 11:27:39.986: E/AndroidRuntime(5172): at java.lang.reflect.Method.invoke(Method.java:515) 05-17 11:27:39.986: E/AndroidRuntime(5172): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779) 
05-17 11:27:39.986: E/AndroidRuntime(5172): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595) 
05-17 11:27:39.986: E/AndroidRuntime(5172): at dalvik.system.NativeStart.main(Native Method)

我很确定代码是正确的:

呈现列表 fragment 的主要部分:

@Override
public void onResume() {
    super.onResume();

    getActivity().setProgressBarIndeterminateVisibility(true);

    ParseQuery<ParseObject> query = new ParseQuery<ParseObject>(
            ParseConstants.CLASS_MESSAGES);
    query.whereEqualTo(ParseConstants.KEY_RECIPIENT_IDS, ParseUser
            .getCurrentUser().getObjectId());
    query.addDescendingOrder(ParseConstants.KEY_CREATED_AT);
    query.findInBackground(new FindCallback<ParseObject>() {
        @Override
        public void done(List<ParseObject> messages, ParseException e) {
            getActivity().setProgressBarIndeterminateVisibility(false);

            if (e == null) {
                // We found messages!
                mMessages = messages;

                String[] usernames = new String[mMessages.size()];
                int i = 0;
                for (ParseObject message : mMessages) {
                    usernames[i] = message
                            .getString(ParseConstants.KEY_SENDER_NAME);
                    i++;
                }
                //SIMPLE LIST WORKS:
                //ArrayAdapter<String> adapter = new ArrayAdapter<String>(getListView().getContext(), android.R.layout.simple_list_item_1, usernames);

                //this crashes:
                MessageAdapter adapter = new MessageAdapter(getListView()
                        .getContext(), mMessages);

                setListAdapter(adapter);
            }
        }
    });
}

以及自定义的 arrayAdapter 类。 MessageAdapter.java:

package com.example.yevsnap;

import java.util.List;

import android.content.Context;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.ImageView;
import android.widget.TextView;

import com.parse.ParseObject;

public class MessageAdapter extends ArrayAdapter<ParseObject> {

    protected Context mContext;
    protected List<ParseObject> mMessages;

    public MessageAdapter(Context context, List<ParseObject> messages) {
        super(context, R.layout.message_item, messages);
        mContext = context;
        mMessages = messages;
    }

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

        if (convertView == null) {
            convertView = LayoutInflater.from(mContext).inflate(
                    R.layout.message_item, null);
            holder = new ViewHolder();
            holder.iconImageView = (ImageView) convertView
                    .findViewById(R.id.messageIcon);
            holder.nameLabel = (TextView) convertView
                    .findViewById(R.id.senderLabel);
        } else {
            holder = (ViewHolder) convertView.getTag();
        }

        ParseObject message = mMessages.get(position);

        if (message.getString(ParseConstants.KEY_FILE_TYPE).equals(
                ParseConstants.TYPE_IMAGE)) {
            holder.iconImageView.setImageResource(R.drawable.ic_action_picture);
        } else {
            holder.iconImageView
                    .setImageResource(R.drawable.ic_action_play_over_video);
        }
        holder.nameLabel.setText(message
                .getString(ParseConstants.KEY_SENDER_NAME));

        return convertView;
    }

    private static class ViewHolder {
        ImageView iconImageView;
        TextView nameLabel;
    }
}

Answer 1

您忘记将 holder 存储到 View 中。当 convertView 被给予回收时，尝试检索持有者时会返回 null。

添加

convertView.setTag(holder);

到实例化持有者的 block 。