在运行时将Imageview drawable添加到listview Imageview项会产生空指针异常

Question

这是我的 listviewite.xml ，具有以下结构，当应用程序运行并且它到达getFavourite（）时它强制关闭。我想从任何援助中知道原因。我认为问题出在xml结构中，是吗？帮助

 <?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:baselineAligned="false"
    android:gravity="left|center"
    android:paddingBottom="5px"
    android:paddingLeft="5px"
    android:paddingTop="5px" >

    <RelativeLayout
        android:layout_width="wrap_content"
        android:layout_height="fill_parent" >

        <ImageView
            android:id="@+id/avatar"
            android:layout_width="wrap_content"
            android:layout_height="fill_parent"
            android:layout_centerVertical="true"
            android:layout_marginRight="6dip"
            android:visibility="gone" />

        <ImageView
            android:id="@+id/imgbtn_favorite"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_centerVertical="true"
            android:layout_alignBottom="@+id/avatar"
            android:layout_alignParentRight="true"
            android:layout_marginRight="6dp"
            android:indeterminate="true"
            android:maxHeight="30dip"
            android:maxWidth="30dip"
            android:minHeight="30dip"
            android:minWidth="30dip"
            android:contentDescription="@string/favorites" />

        <ProgressBar
            android:id="@+id/progress_bar"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_centerVertical="true"
            android:layout_marginRight="6dip"
            android:indeterminate="true"
            android:maxHeight="30dip"
            android:maxWidth="30dip"
            android:minHeight="30dip"
            android:minWidth="30dip" />
    </RelativeLayout>

    <LinearLayout
        android:layout_width="0dip"
        android:layout_height="fill_parent"
        android:layout_weight="1"
        android:orientation="vertical" >

        <TextView
            android:id="@+id/username"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:gravity="center" />

        <TextView
            android:id="@+id/message"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_marginLeft="10px"
            android:textColor="#0099CC" />
    </LinearLayout>

</LinearLayout>

这是下面的 getFavourite（）方法：

imageManager.displayImage(tweet.image_url, activity, holder.image,holder.progress); // CHANGED

            /*If a product exists in shared preferences then set heart_red drawable
             * and set a tag*/
          if (checkFavoriteItem(tweet)) {
                holder.favoriteImg.setImageResource(R.drawable.heart_red);
                holder.favoriteImg.setTag("red");
            } else {
                holder.favoriteImg.setImageResource(R.drawable.heart_grey);
                holder.favoriteImg.setTag("grey");
            }
        }
        return v;
    }

    /* Checks whether a particular product exists in SharedPreferences */
    public boolean checkFavoriteItem(Tweet checkTweet) {

        boolean check = false;
        List<Tweet> favorites = sharedPreference.getFavorites(context);
        if (favorites != null) {
            for (Tweet tweet : favorites) {
                if (tweet.equals(checkTweet)) {
                    check = true;
                    break;
                }
            }
        }
        return check;
    }

public ArrayList<Tweet> getFavorites(Context context) {
        SharedPreferences settings;
        List<Tweet> favorites;

        settings = context.getSharedPreferences(PREFS_NAME, Context.MODE_PRIVATE);

        if (settings.contains(FAVORITES)) {
            String jsonFavorites = settings.getString(FAVORITES, null);
            Gson gson = new Gson();
            Tweet[] favoriteItems = gson.fromJson(jsonFavorites, Tweet[].class);

            favorites = Arrays.asList(favoriteItems);
            favorites = new ArrayList<Tweet>(favorites);
        } else
            return null;

        return (ArrayList<Tweet>) favorites;
    }

这是适配器代码：请求

public class TweetItemAdapter extends ArrayAdapter<Tweet> {

    private ArrayList<Tweet> tweets;
    private Activity activity;
    private Context context;
    public ImageManager imageManager;
    public SharedPreference sharedPreference;

    public TweetItemAdapter(Activity a, int textViewResourceId, ArrayList<Tweet> tweets) {
        super(a, textViewResourceId, tweets);
        this.tweets = tweets;
        activity = a;

        sharedPreference = new SharedPreference();
        imageManager = new ImageManager(activity.getApplicationContext());
    }



    public static class ViewHolder {
        public TextView username;
        public TextView message;
        public ImageView image;
        public ProgressBar progress; // ADDED
        public ImageView favoriteImg;
    }

    @Override
    public int getCount() {
        return tweets.size();
    }

    @Override
    public Tweet getItem(int position) {
        return tweets.get(position);
    }

    @Override
    public long getItemId(int position) {
        return 0;
    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        View v = convertView;
        ViewHolder holder;
        if (v == null) {
            LayoutInflater vi = (LayoutInflater) activity.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
            v = vi.inflate(R.layout.listitem, null);
            holder = new ViewHolder();
            holder.username = (TextView) v.findViewById(R.id.username);
            holder.message = (TextView) v.findViewById(R.id.message);
            holder.image = (ImageView) v.findViewById(R.id.avatar);
            holder.progress = (ProgressBar) v.findViewById(R.id.progress_bar); // ADDED
            holder.favoriteImg = (ImageView) v.findViewById(R.id.imgbtn_favorite);
            v.setTag(holder);
        } else
            holder = (ViewHolder) v.getTag();

        final Tweet tweet = tweets.get(position);
        if (tweet != null) {
            holder.username.setText(tweet.username);
            holder.message.setText(tweet.message);
            holder.image.setTag(tweet.image_url);
            imageManager.displayImage(tweet.image_url, activity, holder.image,holder.progress); // CHANGED

            /*If a product exists in shared preferences then set heart_red drawable
             * and set a tag*/
          if (checkFavoriteItem(tweet)) {
                holder.favoriteImg.setImageResource(R.drawable.heart_red);
                holder.favoriteImg.setTag("red");
            } else {
                holder.favoriteImg.setImageResource(R.drawable.heart_grey);
                holder.favoriteImg.setTag("grey");
            }
        }
        return v;
    }

    /* Checks whether a particular product exists in SharedPreferences */
    public boolean checkFavoriteItem(Tweet checkTweet) {

        boolean check = false;
        List<Tweet> favorites = sharedPreference.getFavorites(context);
        if (favorites != null) {
            for (Tweet tweet : favorites) {
                if (tweet.equals(checkTweet)) {
                    check = true;
                    break;
                }
            }
        }
        return check;
    }

    @Override
    public void add(Tweet tweet) {
        super.add(tweet);
        tweets.add(tweet);
        notifyDataSetChanged();
    }

    @Override
    public void remove(Tweet tweet) {
        super.remove(tweet);
        tweets.remove(tweet);
        notifyDataSetChanged();
    }   

}

logcat的：

07-12 09:49:11.312: E/AndroidRuntime(8857): FATAL EXCEPTION: main
    07-12 09:49:11.312: E/AndroidRuntime(8857): java.lang.NullPointerException
    07-12 09:49:11.312: E/AndroidRuntime(8857):     at com.rowland.preferences.SharedPreference.getFavorites(SharedPreference.java:65)
    07-12 09:49:11.312: E/AndroidRuntime(8857):     at com.rowland.adapters.TweetItemAdapter.checkFavoriteItem(TweetItemAdapter.java:114)
    07-12 09:49:11.312: E/AndroidRuntime(8857):     at com.rowland.adapters.TweetItemAdapter.getView(TweetItemAdapter.java:99)
    07-12 09:49:11.312: E/AndroidRuntime(8857):     at android.widget.AbsListView.obtainView(AbsListView.java:1593)
    07-12 09:49:11.312: E/AndroidRuntime(8857):     at android.widget.ListView.makeAndAddView(ListView.java:1782)
    07-12 09:49:11.312: E/AndroidRuntime(8857):     at android.widget.ListView.fillDown(ListView.java:705)
    07-12 09:49:11.312: E/AndroidRuntime(8857):     at android.widget.ListView.fillFromTop(ListView.java:762)
    07-12 09:49:11.312: E/AndroidRuntime(8857):     at android.widget.ListView.layoutChildren(ListView.java:1633)
    07-12 09:49:11.312: E/AndroidRuntime(8857):     at android.widget.AbsListView.onLayout(AbsListView.java:1423)
    07-12 09:49:11.312: E/AndroidRuntime(8857):     at android.view.View.layout(View.java:7175)
    07-12 09:49:11.312: E/AndroidRuntime(8857):     at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1254)
    07-12 09:49:11.312: E/AndroidRuntime(8857):     at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1130)
    07-12 09:49:11.312: E/AndroidRuntime(8857):     at android.widget.LinearLayout.onLayout(LinearLayout.java:1047)
    07-12 09:49:11.312: E/AndroidRuntime(8857):     at android.view.View.layout(View.java:7175)
    07-12 09:49:11.312: E/AndroidRuntime(8857):     at android.widget.FrameLayout.onLayout(FrameLayout.java:338)
    07-12 09:49:11.312: E/AndroidRuntime(8857):     at android.view.View.layout(View.java:7175)
    07-12 09:49:11.312: E/AndroidRuntime(8857):     at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1254)
    07-12 09:49:11.312: E/AndroidRuntime(8857):     at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1130)
    07-12 09:49:11.312: E/AndroidRuntime(8857):     at android.widget.LinearLayout.onLayout(LinearLayout.java:1047)
    07-12 09:49:11.312: E/AndroidRuntime(8857):     at android.view.View.layout(View.java:7175)
    07-12 09:49:11.312: E/AndroidRuntime(8857):     at android.widget.FrameLayout.onLayout(FrameLayout.java:338)
    07-12 09:49:11.312: E/AndroidRuntime(8857):     at android.view.View.layout(View.java:7175)
    07-12 09:49:11.312: E/AndroidRuntime(8857):     at android.widget.FrameLayout.onLayout(FrameLayout.java:338)
    07-12 09:49:11.312: E/AndroidRuntime(8857):     at android.view.View.layout(View.java:7175)
    07-12 09:49:11.312: E/AndroidRuntime(8857):     at android.view.ViewRoot.performTraversals(ViewRoot.java:1146)
    07-12 09:49:11.312: E/AndroidRuntime(8857):     at android.view.ViewRoot.handleMessage(ViewRoot.java:1865)
    07-12 09:49:11.312: E/AndroidRuntime(8857):     at android.os.Handler.dispatchMessage(Handler.java:99)
    07-12 09:49:11.312: E/AndroidRuntime(8857):     at android.os.Looper.loop(Looper.java:130)
    07-12 09:49:11.312: E/AndroidRuntime(8857):     at android.app.ActivityThread.main(ActivityThread.java:3687)
    07-12 09:49:11.312: E/AndroidRuntime(8857):     at java.lang.reflect.Method.invokeNative(Native Method)
    07-12 09:49:11.312: E/AndroidRuntime(8857):     at java.lang.reflect.Method.invoke(Method.java:507)
    07-12 09:49:11.312: E/AndroidRuntime(8857):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:867)
    07-12 09:49:11.312: E/AndroidRuntime(8857):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:625)
    07-12 09:49:11.312: E/AndroidRuntime(8857):     at dalvik.system.NativeStart.main(Native Method)

Answer 1

您没有在适配器构造函数中初始化context。您将其传递给getFavorites()并在其上调用方法会导致NPE。