将 ArrayList 的内容放入 ArrayAdapter
Put the contents of ArrayList in ArrayAdapter
我有一个存储 JSON 内容的 ArrayList 和一个显示它的 ArrayAdapter。现在我正在尝试保存此 JSON 内容，以便用户即使在离线时也能看到它。
我按照这个 SO 问题来这样做：
上面的问题让我们将 ArrayList 保存在外部存储的文件中
现在我想在用户离线时启动应用程序时显示这个存储的 ArrayList 的内容，即 actorList 在我的 arrayAdapter 中。
到目前为止，这是我的代码，当 运行 离线且 NullPointerException
时应用程序崩溃
@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();
}
}
}
如您所见，我正在尝试实现与上面链接的问题中所问的相同的事情，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.access0(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.access0(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)
这是将 ObjectToFile 和 ObjectFromFile 转换的 Class
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;
}
}
更新代码的更新答案
根据这个更改您的 ObjectToFileUtil class-
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);
到-
actorsList = (ArrayList<Actors>)objectFromFile();
问题的原因
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的情况下启动应用程序时，您正在尝试打开文件，假设该文件已经具有 json。到目前为止一切都很好。但是有一个逻辑错误。当您重新启动您的应用程序时，请查看 dataPath 中没有值，尽管文件可能存在，因为 dataPath 仅在您刚刚写入文件时分配。所以你的路径是 null 并且文件对象没有在 objectFromFile()
中创建
还要确保您的 Actors class 正在实现 Serializable 接口。导致在文件中保存一个对象然后正确地检索它，该对象需要 Serializable.
您可以将对象数组列表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();
然后你可以在连接不可用的 else 块中检索此列表，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，并且不要忘记在您的 gradle 文件中包含 gson
我有一个存储 JSON 内容的 ArrayList 和一个显示它的 ArrayAdapter。现在我正在尝试保存此 JSON 内容，以便用户即使在离线时也能看到它。
我按照这个 SO 问题来这样做：
到目前为止，这是我的代码，当 运行 离线且 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();
}
}
}
如您所见，我正在尝试实现与上面链接的问题中所问的相同的事情，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.access0(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.access0(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)
这是将 ObjectToFile 和 ObjectFromFile 转换的 Class
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;
}
}
更新代码的更新答案
根据这个更改您的 ObjectToFileUtil class-
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);
到-
actorsList = (ArrayList<Actors>)objectFromFile();
问题的原因
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的情况下启动应用程序时，您正在尝试打开文件，假设该文件已经具有 json。到目前为止一切都很好。但是有一个逻辑错误。当您重新启动您的应用程序时，请查看 dataPath 中没有值，尽管文件可能存在，因为 dataPath 仅在您刚刚写入文件时分配。所以你的路径是 null 并且文件对象没有在 objectFromFile()
还要确保您的 Actors class 正在实现 Serializable 接口。导致在文件中保存一个对象然后正确地检索它，该对象需要 Serializable.
您可以将对象数组列表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();
然后你可以在连接不可用的 else 块中检索此列表，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，并且不要忘记在您的 gradle 文件中包含 gson