我有一个应用程序我试图通过ASYNC任务传递数据但在我的onPosteExecute方法中得到一个空指针错误。似乎无论我如何更改我的代码，我仍然会得到同样的致命错误：
10-31 18:43:56.517 1253-1253/com.brianstacks.project1 E/AndroidRuntime﹕ FATAL EXCEPTION: main
Process: com.brianstacks.project1, PID: 1253
java.lang.NullPointerException
at com.brianstacks.project1.MainActivity$MyTask.onPostExecute(MainActivity.java:277)
at com.brianstacks.project1.MainActivity$MyTask.onPostExecute(MainActivity.java:247)
at android.os.AsyncTask.finish(AsyncTask.java:632)
at android.os.AsyncTask.access$600(AsyncTask.java:177)
at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:645)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:136)
at android.app.ActivityThread.main(ActivityThread.java:5001)
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:785)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:601)
at dalvik.system.NativeStart.main(Native Method)
这是我的代码：
MainActivity.java
public class MainActivity extends ListActivity implements MasterFragment.OnListItemClickListener {
final String Tag = "Project 1 test";
EditText myEdit;
Button myButton;
ProgressBar pb;
SerialCustomObject myObject;
// create a reference to the list's needed for data
List<MyTask> tasks;
ArrayList<Places> placeList;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
if ((savedInstanceState != null)&& (savedInstanceState.getSerializable("name") != null)) {
Places name = (Places) savedInstanceState.getSerializable("name");
Log.v("name:",name.getName());
}
//initiate my tasks
tasks = new ArrayList<>();
pb = (ProgressBar)findViewById(R.id.progressBar);
myEdit = (EditText)findViewById(R.id.myEditText);
myButton = (Button)findViewById(R.id.myButton);
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.main, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
// Handle action bar item clicks here. The action bar will
// automatically handle clicks on the Home/Up button, so long
// as you specify a parent activity in AndroidManifest.xml.
int id = item.getItemId();
return id == R.id.action_settings || super.onOptionsItemSelected(item);
}
public void findAndRemoveFragment() {
FragmentManager mgr = getFragmentManager();
DetailFragment frag =
(DetailFragment) mgr.findFragmentByTag(DetailFragment.TAG);
if (frag == null) {
// No fragment found, possibly because the transaction
// hasn't completed yet.
} else {
// Fragment found. You can use it here.
FragmentTransaction trans = mgr.beginTransaction();
trans.remove(frag);
trans.commit();
// When the main thread runs, the fragment will be
// removed from the activity.
}
}
public void deviceStorage() {
// Read in a private file
try {
FileInputStream fis = this.openFileInput("some_file.txt");
} catch (FileNotFoundException e) {
e.printStackTrace();
}
// Create new or open existing private file
try {
FileOutputStream fos = this.openFileOutput("some_other_file.txt", Context.MODE_PRIVATE);
} catch (FileNotFoundException e) {
e.printStackTrace();
}
}
private void writeToFile(Context _c, String _filename, String _data) {
File external = _c.getExternalFilesDir(null);
File file = new File(external, _filename);
try {
FileOutputStream fos = new FileOutputStream(file);
ObjectOutputStream oos = new ObjectOutputStream(fos);
if(myObject == null) {
myObject = new SerialCustomObject();
}
myObject.setData(_data);
oos.writeObject(myObject);
oos.close();
// Write bytes to the stream
fos.write(_data.getBytes());
// Close the stream to save the file.
fos.close();
} catch (IOException e) {
e.printStackTrace();
}
}
private String readFromFile(String _filename) {
File external = getExternalFilesDir(null);
File file = new File(external, _filename);
try {
FileInputStream fin = new FileInputStream(file);
InputStreamReader inReader = new InputStreamReader(fin);
BufferedReader reader = new BufferedReader(inReader);
// Reading data from our file using the reader
// and storing it our string buffer.
StringBuffer buffer = new StringBuffer();
String text = null;
// Make sure a line of text is available to be read.
while((text = reader.readLine()) != null) {
buffer.append(text + "\n");
}
// Close the reader and underlying stream.
reader.close();
// Convert the buffer to a string.
return buffer.toString();
} catch(IOException e) {
e.printStackTrace();
}
return null;
}
@Override
public void displayText(String myText) {
DetailFragment frag = (DetailFragment) getFragmentManager().findFragmentByTag(DetailFragment.TAG);
if (frag == null){
frag = DetailFragment.newInstance(myText);
getFragmentManager().beginTransaction()
.replace(R.id.container2,frag,DetailFragment.TAG)
.commit();
}else {
frag.setDisplayInfo(myText);
}
}
//@Override
public void onClick(View _v){
myEdit = (EditText)findViewById(R.id.myEditText);
String newString = myEdit.getText().toString();
Log.v(Tag,newString);
}
// the method for when the button is clicked
public void myClick(View _v){
// create a string to grab the text of the edit text
String myString = myEdit.getText().toString();
// replace the spaces with + to encode into the url
String encodedString = myString.replace(" ","+");
//check to see if online and if so continue to get the JSON data if not toast a message telling the user no connection
if (isOnline()){
requestData("https://maps.googleapis.com/maps/api/place/textsearch/json?query="+encodedString+"&key=AIzaSyB9iOw6wF4FwbOdUTZYiU_MxsbfWM5iMOI");
}else Toast.makeText(this, "Network isn't available", Toast.LENGTH_SHORT).show();
}
// method to get the data from ASYNC task
private void requestData(String uri) {
MyTask task = new MyTask();
task.execute(uri);
}
// method to check internet connectivity
protected boolean isOnline(){
ConnectivityManager cm =(ConnectivityManager) getSystemService(Context.CONNECTIVITY_SERVICE);
NetworkInfo netInfo = cm.getActiveNetworkInfo();
return netInfo != null && netInfo.isConnectedOrConnecting();
}
protected void updateDisplay(ArrayList<Places> placeList){
// get instance of the Master List fragment then replaces container1 and commits it to the activity
MasterFragment frag = MasterFragment.newInstance(placeList);
getFragmentManager().beginTransaction()
.replace(R.id.container1, frag, MasterFragment.TAG).commit();
}
// Async task method to do network action in
private class MyTask extends AsyncTask<String ,String ,String>{
@Override
protected void onPreExecute() {
// add this to the task
tasks.add(this);
}
@Override
protected String doInBackground(String... params) {
return HttpManager.getData(params[0]);
}
@Override
protected void onPostExecute(String result) {
tasks.remove(this);
if(null != result && !result.isEmpty()) {
placeList = JSONParser.parseFeed(result);
updateDisplay(placeList);
}else {
Toast.makeText(MainActivity.this, "Can't connect to API", Toast.LENGTH_SHORT).show();
}
}
@Override
protected void onProgressUpdate(String... values) {
}
}
}
DetailFragment.java
package com.brianstacks.project1.fragments;
import android.app.Fragment;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;
import com.brianstacks.project1.R;
/**
* Created by Brian Stacks
* on 10/27/14
* for FullSail.edu.
*/
public class DetailFragment extends Fragment {
public static final String TAG = "DetailFragment.TAG";
public static final String ARG_TAG = "DetailFragment.TAG";
public static DetailFragment newInstance(String myString) {
DetailFragment frag = new DetailFragment();
Bundle args = new Bundle();
args.putString(ARG_TAG, myString);
frag.setArguments(args);
return frag;
}
@Override
public View onCreateView(LayoutInflater _inflater, ViewGroup _container,
Bundle _savedInstanceState) {
// Create and return view for this fragment.
return _inflater.inflate(R.layout.detail_layout, _container, false);
}
@Override
public void onActivityCreated(Bundle _savedInstanceState) {
super.onActivityCreated(_savedInstanceState);
Bundle args = getArguments();
if(args != null && args.containsKey(ARG_TAG)){
setDisplayInfo(args.getString(ARG_TAG));
}
}
public void setDisplayInfo(String myText){
getArguments().putString(ARG_TAG,myText);
// Get our TextView and set some text to it.
TextView tv;
tv = (TextView)getView().findViewById(R.id.detailText);
tv.setText(myText);
}
}
MasterFragment.java
package com.brianstacks.project1.fragments;
import android.app.Activity;
import android.app.AlertDialog;
import android.app.ListFragment;
import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.DialogInterface;
import android.content.Intent;
import android.content.IntentFilter;
import android.os.Bundle;
import android.os.Parcelable;
import android.util.Log;
import android.view.View;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import com.brianstacks.project1.JSONParser;
import com.brianstacks.project1.MainActivity;
import com.brianstacks.project1.Places;
import com.brianstacks.project1.PlacesAdapter;
import com.brianstacks.project1.R;
import java.io.Serializable;
import java.util.ArrayList;
import java.util.List;
/**
* Created by Brian Stacks
* on 10/27/14
* for FullSail.edu.
*/
public class MasterFragment extends ListFragment{
public static final String TAG = "MasterFragment.TAG";
public static final String KEY = "places";
private OnListItemClickListener mListener;
private ArrayList<Places> placeList2;
public static MasterFragment newInstance(ArrayList<Places> placeList) {
MasterFragment masterFragment = new MasterFragment();
Bundle args = new Bundle();
args.putSerializable("places", placeList);
masterFragment.setArguments(args);
return masterFragment;
}
public interface OnListItemClickListener{
public void displayText(String myText);
}
@Override
public void onAttach(Activity activity){
super.onAttach(activity);
if (activity instanceof OnListItemClickListener){
mListener = (OnListItemClickListener) activity;
}else {
throw new IllegalArgumentException("Containing Activity must implement the OnListItemClicked");
}
}
@Override
public void onActivityCreated(Bundle _savedInstanceState) {
super.onActivityCreated(_savedInstanceState);
if (_savedInstanceState == null){
Bundle args = getArguments();
String myStrings=args.getString("places");
Log.v("Places",myStrings);
/*String[] presidents = getResources().getStringArray(R.array.presidents);
//ArrayAdapter<String> adapter = new ArrayAdapter<String>(getActivity(), android.R.layout.simple_list_item_1, presidents);
//PlacesAdapter adapter2 = new PlacesAdapter(this.getActivity(),R.layout.item_place,placeList);
//setListAdapter(adapter2);
}
@Override
public void onListItemClick(ListView _l, View _v, int _position, long _id) {
String president = (String)_l.getItemAtPosition(_position);
mListener.displayText(president);
*/
}
}
}
HTTPManager.java
package com.brianstacks.project1;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.URL;
/**
* Created by Brian Stacks
* on 10/20/14
* for FullSail.edu.
*/
public class HttpManager {
public static String getData(String uri){
BufferedReader reader = null;
try {
URL url = new URL(uri);
HttpURLConnection con = (HttpURLConnection) url.openConnection();
StringBuilder sb = new StringBuilder();
reader = new BufferedReader(new InputStreamReader(con.getInputStream()));
String line;
while ((line = reader.readLine()) != null){
sb.append(line).append("");
}
return sb.toString();
}catch (Exception e){
e.printStackTrace();
return null;
}finally {
if (reader!=null) {
try {
reader.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
}
}
JSONParser.java
package com.brianstacks.project1;
import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;
import java.util.ArrayList;
import java.util.List;
/**
* Created by Brian Stacks
* on 10/20/14
* for FullSail.edu.
*/
public class JSONParser {
public static ArrayList<Places> parseFeed(String content) {
JSONObject myObj;
try {
myObj = new JSONObject(content);
JSONArray result = myObj.getJSONArray("results");
ArrayList<Places> placeList = new ArrayList<>();
for (int i = 0; i < result.length(); i++) {
JSONObject obj = result.getJSONObject(i);
Places place = new Places();
place.setName(obj.getString("name"));
place.setFormatted_address(obj.getString("formatted_address"));
place.setTypes(obj.getString("types"));
//place.setPhotos(obj.getString("photos"));
placeList.add(place);
}
return placeList;
} catch (JSONException e) {
e.printStackTrace();
return null;
}
}
}
Places.java
import java.io.Serializable;
/**
* Created by Brian Stacks
* on 10/22/14
* for FullSail.edu.
*/
public class Places implements Serializable {
private String pName;
private String pTypes;
private String pFormatted_address;
public Places(){
pName ="";
pTypes ="";
pFormatted_address = "";
}
public String getName() {
return pName;
}
public void setName(String name) {
pName = name;
}
public String getTypes() {
return pTypes;
}
public void setTypes(String types) {
pTypes= types;
}
public String getFormatted_address() {
return pFormatted_address;
}
public void setFormatted_address(String formatted_address) {
pFormatted_address=formatted_address;
}
}
非常感谢任何帮助
也在阅读这篇文章，但无济于事LINK！
答案 0 :(得分：2)
我找到了解决这个问题的方法，当代码期望Object改变了这一点时，我试图将我的String强制转换为我的MasterFragment.java ArrayList<Places>它的代码和权力，没有错误。
旧代码
Bundle args = getArguments();
String myStrings=args.getString("places");
新代码
Bundle args = getArguments();
ArrayList myStrings = args.getParcelableArrayList("places");