我有一个存储JSON内容的ArrayList和一个显示它的ArrayAdapter。现在我试图保存这个JSON内容，以便用户即使在离线时也能看到它。
我按照这个问题来执行此操作：How to Cache Parsed JSON for Offline usage 上面的问题让我们将ArrayList保存在外部存储上的File中 现在，当用户在离线时启动应用程序时，我想在我的arrayAdapter中显示这个存储的ArrayList的内容，即actorList。
这是我的代码到目前为止，当使用NullPointerException
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
final View rootView = inflater.inflate(R.layout.fragment_photos, container, false);
if (isNetworkAvailable()) {
actorsList = new ArrayList<Actors>();
new JSONAsyncTask().execute(baseURL);
listview = (ListView) rootView.findViewById(R.id.list);
adapter = new ActorAdapter(getActivity(), R.layout.row, actorsList);
listview.setAdapter(adapter);
}
else{
Toast.makeText(getActivity(), "Network Unavailable. Please Try Again Later ", Toast.LENGTH_LONG).show();
try {
Toast.makeText(getActivity(), "Reached TRY ", Toast.LENGTH_LONG).show();
actorsList = (ArrayList<Actors>)objectFromFile(dataPath);
// System.out.print(actorsList);
for(int i=0;i<actorsList.size();i++)
{
Toast.makeText(getActivity(), "Reached FOR ", Toast.LENGTH_LONG).show();
actor.setName(actorsList.get(i).getName());
actor.setDescription(actorsList.get(i).getDescription());
actor.setImage(actorsList.get(i).getImage());
}
listview = (ListView) rootView.findViewById(R.id.list);
adapter = new ActorAdapter(getActivity(), R.layout.row, actorsList);
listview.setAdapter(adapter);
} catch (Exception e) {
e.printStackTrace();
}
}
return rootView;
}
class JSONAsyncTask extends AsyncTask<String, Void, Boolean> {
@Override
protected void onPreExecute() {
}
@Override
protected Boolean doInBackground(String... urls) {
try {
//------------------>>
HttpGet httppost = new HttpGet(urls[0]);
HttpClient httpclient = new DefaultHttpClient();
HttpResponse response = httpclient.execute(httppost);
// StatusLine stat = response.getStatusLine();
int status = response.getStatusLine().getStatusCode();
if (status == 200) {
HttpEntity entity = response.getEntity();
String data = EntityUtils.toString(entity);
JSONObject jsono = new JSONObject(data);
jarray = jsono.getJSONArray("posts");
for (int i = 0; i < jarray.length(); i++) {
JSONObject object = jarray.getJSONObject(i);
Actors actor = new Actors();
actor.setName(object.getString("title"));
actor.setDescription(object.getString("url"));
actor.setImage(object.getString("thumbnail"));
actorsList.add(actor);
}
return true;
}
} catch (ParseException e1) {
e1.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
} catch (JSONException e) {
e.printStackTrace();
}
return false;
}
protected void onPostExecute(Boolean result) {
dialog.cancel();
adapter.notifyDataSetChanged();
if (result) {
try {
dataPath = objectToFile(actorsList);
} catch (IOException e) {
e.printStackTrace();
}
} else {
Toast.makeText(getActivity().getApplicationContext(), "Unable to fetch data from server", Toast.LENGTH_LONG).show();
}
}
}
正如您所看到的，我正在尝试实现与上面链接的问题中提到的相同的内容，但无法弄清楚如何在离线时显示ArrayList。请帮忙
更新： logcat
04-21 14:23:33.161 4870-4870/info.androidhive.slidingmenu E/AndroidRuntime﹕ FATAL EXCEPTION: main
Process: info.androidhive.slidingmenu, PID: 4870
java.lang.RuntimeException: Unable to start activity ComponentInfo{info.androidhive.slidingmenu/info.androidhive.slidingmenu.MainActivity}: java.lang.NullPointerException
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2412)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2470)
at android.app.ActivityThread.access$900(ActivityThread.java:174)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1307)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:146)
at android.app.ActivityThread.main(ActivityThread.java:5593)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:515)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1283)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1099)
at dalvik.system.NativeStart.main(Native Method)
Caused by: java.lang.NullPointerException
at java.io.File.fixSlashes(File.java:185)
at java.io.File.<init>(File.java:134)
at info.androidhive.slidingmenu.ObjectToFileUtil.objectFromFile(ObjectToFileUtil.java:31)
at info.androidhive.slidingmenu.PagesFragment.onCreateView(PagesFragment.java:176)
at android.app.Fragment.performCreateView(Fragment.java:1700)
at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:890)
at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:1062)
at android.app.BackStackRecord.run(BackStackRecord.java:684)
at android.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1453)
at android.app.Activity.performStart(Activity.java:5467)
at
android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2385)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2470)
at android.app.ActivityThread.access$900(ActivityThread.java:174)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1307)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:146)
at android.app.ActivityThread.main(ActivityThread.java:5593)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:515)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1283)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1099)
at dalvik.system.NativeStart.main(Native Method)
这是转换ObjectToFile和ObjectFromFile的类
package info.androidhive.slidingmenu;
import android.os.Environment;
import java.io.*;
public class ObjectToFileUtil {
public static String objectToFile(Object object) throws IOException {
String path = Environment.getExternalStorageDirectory() + File.separator + "cache" + File.separator;
File dir = new File(path);
if (!dir.exists()) {
dir.mkdirs();
}
path += "data";
File data = new File(path);
if (!data.createNewFile()) {
data.delete();
data.createNewFile();
}
ObjectOutputStream objectOutputStream = new ObjectOutputStream(new FileOutputStream(data));
objectOutputStream.writeObject(object);
objectOutputStream.close();
return path;
}
public static Object objectFromFile(String path) throws IOException, ClassNotFoundException {
Object object = null;
File data = new File(path);
if(data.exists()) {
ObjectInputStream objectInputStream = new ObjectInputStream(new FileInputStream(data));
object = objectInputStream.readObject();
objectInputStream.close();
}
return object;
}
}
答案 0 :(得分：0)
根据此更改您的ObjectToFileUtil类 -
public class ObjectToFileUtil {
private static baseFilePath = Environment.getExternalStorageDirectory() + File.separator + "cache" + File.separator+"data";
public static String objectToFile(Object object) throws IOException {
String path = baseFilePath;
File dir = new File(path);
if (!dir.exists()) {
dir.mkdirs();
}
File data = new File(path);
if (!data.createNewFile()) {
data.delete();
data.createNewFile();
}
ObjectOutputStream objectOutputStream = new ObjectOutputStream(new FileOutputStream(data));
objectOutputStream.writeObject(object);
objectOutputStream.close();
return path;
}
public static Object objectFromFile() throws IOException, ClassNotFoundException {
String path = baseFilePath;
Object object = null;
File data = new File(path);
if(data.exists()) {
ObjectInputStream objectInputStream = new ObjectInputStream(new FileInputStream(data));
object = objectInputStream.readObject();
objectInputStream.close();
}
return object;
}
然后更改以下行 -
actorsList = (ArrayList<Actors>)objectFromFile(dataPath);
要 -
actorsList = (ArrayList<Actors>)objectFromFile();
您在网络可用时保存了文件，并将文件路径存储在dataPath成员中。现在，当您在没有网络可用时启动应用程序时，您正在尝试打开该文件，假设该文件已经具有json。到目前为止，每件事都很好。但是存在逻辑错误。查看重新启动应用程序时，dataPath中没有值，但文件可能存在，因为dataPath仅在您刚写入文件时分配。因此，您的路径为null，文件对象未在objectFromFile()
还要确保您的Actors班级正在实施Serializable界面。导致在File＆amp;中保存对象然后正确检索它，对象需要Serializable。
答案 1 :(得分：0)
您可以将对象的arraylist转换为字符串并将其存储在sharedprefrences中，如下所示： -
Gson gson = new Gson();
SharedPreferences sharedPreferences= PreferenceManager.getDefaultSharedPreferences(getActivity());
SharedPreferences.Editor editor = sharedPreferences.edit();
editor.putString("list",gson.toJson(actorsList));
editor.commit();
然后你可以在没有连接的else块中检索这个列表，如下所示： -
SharedPreferences sharedPreferences= PreferenceManager.getDefaultSharedPreferences(getActivity());
String string=sharedPreferences.sharedPreferences.getString("list",null);
List<Actors> list=new List<Actors>();
Type listType = new TypeToken<ArrayList<Actors>>() {
}.getType();
list= new Gson().fromJson(string, listType);
我在这里使用gson来序列化和反序列化对象，并且不要忘记在你的gradle文件中包含gson