所以我在当前的活动类中有以下代码：
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_chores);
bChore = (Button) findViewById(R.id.addChore);
bChore.setOnClickListener(this);
Bundle extras = getIntent().getExtras();
final String user = extras.getString("username");
fillInBackground();
}
@Override
public void onClick(View v) {
Bundle extras = getIntent().getExtras();
final String user = extras.getString("username");
switch (v.getId()) {
case R.id.addChore:
name = (EditText) findViewById(R.id.choreName);
value = (EditText) findViewById(R.id.points);
String choreTitle = name.getText().toString();
int pointValue = Integer.parseInt(value.getText().toString());
try {
if (choreTitle.length() <= 0)
Toast.makeText(Chores.this, "Please enter a chore", Toast.LENGTH_SHORT).show();
else {
if (pointValue == 0)
Toast.makeText(Chores.this, "Please enter a valid number", Toast.LENGTH_SHORT).show();
else {
Chore chore = new Chore(choreTitle, pointValue, user);
uploadChore(chore);
break;
}
}
} catch (NumberFormatException e) {
Toast.makeText(Chores.this, "Please enter a valid point value", Toast.LENGTH_SHORT).show();
}
}
}
private void fillInBackground() {
new AsyncTask<Void, Void, Wrapper>() {
Bundle extras = getIntent().getExtras();
final String user = extras.getString("username");
@Override
protected Wrapper doInBackground(Void... params) {
String requestURL = SERVER_ADDRESS + "FetchChoreData.php";
ListView choreList = (ListView) findViewById(R.id.choreList);
ArrayList<String> chores = new ArrayList<>();
Wrapper wrapper = new Wrapper();
String headings = "Chore" + "\t" + "Child" + "\t" + "Points";
chores.add(headings);
URL url;
try {
//Opens the connection to the PHP files
//Sets the conditions of the connection
url = new URL(requestURL);
HttpURLConnection conn = (HttpURLConnection) url.openConnection();
conn.setReadTimeout(CONNECTION_TIMEOUT);
conn.setConnectTimeout(CONNECTION_TIMEOUT);
conn.setRequestMethod("POST");
conn.setDoInput(true);
conn.setDoOutput(true);
//Opens an output stream to send the data to be verified
// and then closes the all output streams and flushes the output writer
OutputStream os = conn.getOutputStream();
BufferedWriter writer = new BufferedWriter(new OutputStreamWriter(os, "UTF-8"));
Uri.Builder builder = new Uri.Builder()
.appendQueryParameter("username", user);
String query = builder.build().getEncodedQuery();
writer.write(query);
writer.flush();
writer.close();
os.close();
//saves the response code to ensure connection was succesful
int code = conn.getResponseCode();
Log.d("code", Integer.toString(code));
//Opens an input stream to retrieve verified data back from the server
//Starts a String Builder to read the data off the input
//Closes the BufferedReader once it has finished building the string
InputStream responseStream = new BufferedInputStream(conn.getInputStream());
BufferedReader responseStreamReader = new BufferedReader(new InputStreamReader(responseStream));
String line;
StringBuilder stringBuilder = new StringBuilder();
while ((line = responseStreamReader.readLine()) != null)
stringBuilder.append(line + "/n");
responseStreamReader.close();
String response = stringBuilder.toString();
Log.d("response", response);
JSONObject jsonResponse = new JSONObject(response);
//Creates a JSON object from the string
JSONArray choresArray = jsonResponse.getJSONArray("chore");
for (int i = 0; i < choresArray.length(); i++) {
JSONObject chore = choresArray.getJSONObject(i);
String current_chore = chore.optString("chore_name");
String name = chore.optString("child_username");
String points = chore.optString("point_value");
String currentChore = "";
if (name == null)
currentChore = current_chore + "\t" + "Not Claimed" + "\t" + points;
else
currentChore = current_chore + "\t" + name + "\t" + points;
chores.add(currentChore);
Log.d("Output", currentChore);
}
wrapper.myArrayAdapter = new ArrayAdapter<>(Chores.this, android.R.layout.simple_list_item_1, chores);
wrapper.list = choreList;
} catch (Exception e) {
e.printStackTrace();
}
return wrapper;
}
protected void onPostExecute(Wrapper wrapper) {
//After the previous method executes we end the process dialog
//and we return the user call back and return the verified user
wrapper.list.setAdapter(wrapper.myArrayAdapter);
}
}.execute();
}
private void uploadChore(Chore chore) {
Bundle extras = getIntent().getExtras();
final String user = extras.getString("username");
//Implements the ServerRequest class to register the user details
ServerRequests serverRequests = new ServerRequests(this);
//Checks that the details are valid and what account type it is setting up
serverRequests.storeChoreData(chore,new GetUserCallBack() {
//What is done when the user has successfully registered
@Override
public void complete(User returnedUser) {
Intent chores = new Intent(Chores.this, Chores.class);
chores.putExtra("username", user);
startActivity(chores);
}
});
}
}
包装器类如下所示：
public class Wrapper {
public ListView list;
public ArrayAdapter<String> myArrayAdapter;
}
然而，当我运行应用程序时，它会停止并且我得到以下致命异常：
W/System.err: org.json.JSONException: Value <!DOCTYPE of type java.lang.String cannot be converted to JSONObject
03-14 11:54:17.493 18935-19311/com.jack.pointcollector W/System.err: at org.json.JSON.typeMismatch(JSON.java:111)
03-14 11:54:17.493 18935-19311/com.jack.pointcollector W/System.err: at org.json.JSONObject.<init>(JSONObject.java:160)
03-14 11:54:17.493 18935-19311/com.jack.pointcollector W/System.err: at org.json.JSONObject.<init>(JSONObject.java:173)
03-14 11:54:17.494 18935-19311/com.jack.pointcollector W/System.err: at com.jack.pointcollector.Chores$1.doInBackground(Chores.java:149)
03-14 11:54:17.494 18935-19311/com.jack.pointcollector W/System.err: at com.jack.pointcollector.Chores$1.doInBackground(Chores.java:89)
03-14 11:54:17.494 18935-19311/com.jack.pointcollector W/System.err: at android.os.AsyncTask$2.call(AsyncTask.java:295)
03-14 11:54:17.494 18935-19311/com.jack.pointcollector W/System.err: at java.util.concurrent.FutureTask.run(FutureTask.java:237)
03-14 11:54:17.494 18935-19311/com.jack.pointcollector W/System.err: at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:234)
03-14 11:54:17.494 18935-19311/com.jack.pointcollector W/System.err: at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1113)
03-14 11:54:17.494 18935-19311/com.jack.pointcollector W/System.err: at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:588)
03-14 11:54:17.494 18935-19311/com.jack.pointcollector W/System.err: at java.lang.Thread.run(Thread.java:818)
03-14 11:54:17.581 18935-18935/com.jack.pointcollector D/AndroidRuntime: Shutting down VM
03-14 11:54:17.581 18935-18935/com.jack.pointcollector E/AndroidRuntime: FATAL EXCEPTION: main Process: com.jack.pointcollector, PID: 18935
java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference
at com.jack.pointcollector.Chores$1.onPostExecute(Chores.java:183)
at com.jack.pointcollector.Chores$1.onPostExecute(Chores.java:89)
at android.os.AsyncTask.finish(AsyncTask.java:651)
at android.os.AsyncTask.-wrap1(AsyncTask.java)
at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:668)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5417)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
有人可以告诉我我哪里出错了，因为我只是新手。
答案 0 :(得分：0)
问题只是你在doInBackgroud（）方法中创建了Object Wrapper ........然后你尝试访问onPostExecute（）方法.......
new AsyncTask<Void, Void, Wrapper>() {
@Override
protected Wrapper doInBackground(Void... params) {
ListView choreList = (ListView) findViewById(R.id.choreList);
ArrayList<String> chores = new ArrayList<>();
// change this make them Global.....
Wrapper wrapper = new Wrapper();
wrapper.myArrayAdapter = new ArrayAdapter<>(Chores.this, android.R.layout.simple_list_item_1, chores);
wrapper.list = choreList;
} catch (Exception e) {
e.printStackTrace();
}
return wrapper;
}
protected void onPostExecute(Wrapper wrapper) {
//After the previous method executes we end the process dialog
//and we return the user call back and return the verified user
wrapper.list.setAdapter(wrapper.myArrayAdapter);
}
}.execute();
}
答案 1 :(得分：0)
你应该移动
ListView choreList = (ListView) findViewById(R.id.choreList);
ArrayList<String> chores = new ArrayList<>();
在你的背景之外做，并将它们作为全球变量。
完成提取数据后，可以在onPost执行中设置适配器。您的列表视图从未初始化，因此空指针异常。
答案 2 :(得分：0)
以下是我在您的代码中发现的错误，我也尝试修复它。
您应该在onCreate()初始化所有视图和适配器。
在uploadChore()＆＃39; complete()中，不要将数据传递给其他方法，而是将其作为参数传递。
ListView及其Adapter应在开头初始化。稍后，将数据添加到列表中并调用适配器notifyDataSetChanged()
ArrayList<String> chores;
Wrapper wrapper;
@Override
protected void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_chores);
bChore = (Button) findViewById(R.id.addChore);
name = (EditText) findViewById(R.id.choreName);
value = (EditText) findViewById(R.id.points);
wrapper = new Wrapper();
ListView choreList = (ListView) findViewById(R.id.choreList);
chores = new ArrayList<>();
wrapper.myArrayAdapter = new ArrayAdapter<>(Chores.this, android.R.layout.simple_list_item_1, chores);
wrapper.list = choreList;
wrapper.list.setAdapter(wrapper.myArrayAdapter);
bChore.setOnClickListener(this);
}
@Override
public void onClick(View v)
{
Bundle extras = getIntent().getExtras();
final String user = extras.getString("username");
switch (v.getId())
{
case R.id.addChore:
String choreTitle = name.getText().toString();
int pointValue = Integer.parseInt(value.getText().toString());
try
{
if (choreTitle.length() <= 0)
Toast.makeText(Chores.this, "Please enter a chore", Toast.LENGTH_SHORT).show();
else
{
if (pointValue == 0)
Toast.makeText(Chores.this, "Please enter a valid number", Toast.LENGTH_SHORT).show();
else
{
Chore chore = new Chore(choreTitle, pointValue, user);
uploadChore(chore);
break;
}
}
}
catch (NumberFormatException e)
{
Toast.makeText(Chores.this, "Please enter a valid point value", Toast.LENGTH_SHORT).show();
}
}
}
private void fillInBackground(String user)
{
new AsyncTask<Void, String, Wrapper>()
{
@Override
protected Wrapper doInBackground(Void... params)
{
String requestURL = SERVER_ADDRESS + "FetchChoreData.php";
String headings = "Chore" + "\t" + "Child" + "\t" + "Points";
publishProgress(headings);
URL url;
try
{
//Opens the connection to the PHP files
//Sets the conditions of the connection
url = new URL(requestURL);
HttpURLConnection conn = (HttpURLConnection) url.openConnection();
conn.setReadTimeout(CONNECTION_TIMEOUT);
conn.setConnectTimeout(CONNECTION_TIMEOUT);
conn.setRequestMethod("POST");
conn.setDoInput(true);
conn.setDoOutput(true);
//Opens an output stream to send the data to be verified
// and then closes the all output streams and flushes the output writer
OutputStream os = conn.getOutputStream();
BufferedWriter writer = new BufferedWriter(new OutputStreamWriter(os, "UTF-8"));
Uri.Builder builder = new Uri.Builder()
.appendQueryParameter("username", user);
String query = builder.build().getEncodedQuery();
writer.write(query);
writer.flush();
writer.close();
os.close();
//saves the response code to ensure connection was succesful
int code = conn.getResponseCode();
Log.d("code", Integer.toString(code));
//Opens an input stream to retrieve verified data back from the server
//Starts a String Builder to read the data off the input
//Closes the BufferedReader once it has finished building the string
InputStream responseStream = new BufferedInputStream(conn.getInputStream());
BufferedReader responseStreamReader = new BufferedReader(new InputStreamReader(responseStream));
String line;
StringBuilder stringBuilder = new StringBuilder();
while ((line = responseStreamReader.readLine()) != null)
stringBuilder.append(line + "/n");
responseStreamReader.close();
String response = stringBuilder.toString();
Log.d("response", response);
JSONObject jsonResponse = new JSONObject(response);
//Creates a JSON object from the string
JSONArray choresArray = jsonResponse.getJSONArray("chore");
for (int i = 0; i < choresArray.length(); i++)
{
JSONObject chore = choresArray.getJSONObject(i);
String current_chore = chore.optString("chore_name");
String name = chore.optString("child_username");
String points = chore.optString("point_value");
String currentChore = "";
if (name == null)
currentChore = current_chore + "\t" + "Not Claimed" + "\t" + points;
else
currentChore = current_chore + "\t" + name + "\t" + points;
publishProgress(currentChore);
Log.d("Output", currentChore);
}
}
catch (Exception e)
{
e.printStackTrace();
}
return wrapper;
}
@Override
protected void onProgressUpdate(String... values)
{
super.onProgressUpdate(values);
wrapper.list.add(values[0]);
}
protected void onPostExecute(Wrapper wrapper)
{
//After the previous method executes we end the process dialog
//and we return the user call back and return the verified user
wrapper.myArrayAdapter.notifyDataSetChanged();
}
}.execute();
}
private void uploadChore(Chore chore)
{
Bundle extras = getIntent().getExtras();
final String user = extras.getString("username");
//Implements the ServerRequest class to register the user details
ServerRequests serverRequests = new ServerRequests(this);
//Checks that the details are valid and what account type it is setting up
serverRequests.storeChoreData(chore, new GetUserCallBack()
{
//What is done when the user has successfully registered
@Override
public void complete(User returnedUser)
{
// Simple add data into the list. Don't start same activity again and again.
fillInBackground(user);
}
});
}
答案 3 :(得分：0)
String response = stringBuilder.toString（）; JSONObject jsonResponse = new JSONObject（response）;
检查回复 应该是这样的： { ＆＃34;家务＆＃34;：[{ ＆＃34; chore_name＆＃34;：＆＃34; name＆＃34;， ＆＃34; child_username＆＃34;：＆＃34;用户名＆＃34;， ＆＃34; point_value＆＃34;：＆＃34; value＆＃34; }，{ ＆＃34; chore_name＆＃34;：＆＃34; name＆＃34;， ＆＃34; child_username＆＃34;：＆＃34;用户名＆＃34;， ＆＃34; point_value＆＃34;：＆＃34; value＆＃34; }] }
答案 4 :(得分：0)
<强>原因：
问题不是设置适配器，而是使用JSON解析。根据您的错误日志
W/System.err: org.json.JSONException: Value <!DOCTYPE of type java.lang.String cannot be converted to JSONObject
您收到的字符串无法转换为JSONObject。发生此错误时，正在catch块中捕获并且到那时
wrapper.myArrayAdapter = new ArrayAdapter<>(Chores.this, android.R.layout.simple_list_item_1, chores);
wrapper.list = choreList;
未被调用。这导致onPostExecute接收listview的null对象，最后抛出NPE。
<强>解决方案：
您应该检查您获得表单服务器的JSON并更正它们。