问题如下.我有一个登录活动(在Android Studio中)几天前工作正常.我不记得改变了什么,但是当我在上次运行这个时,应用程序在我点击登录按钮后立即关闭.最后指出的是关于预执行AsyncTask的祝酒词.

我无法理解为什么会出现NullPointerException.

我的注册活动代码几乎相同,而且运行正常.

这是日志：

05-28 16:04:52.395 1218-1232/system_process V/WindowManager﹕ addAppToken: AppWindowToken{5d89eb token=Token{23ccc93a ActivityRecord{2fe54865 u0 utanashati.reminder/.HomepageActivity t17}}} to stack=1 task=17 at 1

05-28 16:04:52.407 19927-19927/utanashati.reminder D/AndroidRuntime﹕ Shutting down VM

05-28 16:04:52.408 19927-19927/utanashati.reminder E/AndroidRuntime﹕ FATAL EXCEPTION: main

Process: utanashati.reminder, PID: 19927

java.lang.RuntimeException: Unable to start activity

ComponentInfo{utanashati.reminder/utanashati.reminder.HomepageActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference

at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2325)

at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2390)

at android.app.ActivityThread.access$800(ActivityThread.java:151)

at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)

at android.os.Handler.dispatchMessage(Handler.java:102)

at android.os.Looper.loop(Looper.java:135)

at android.app.ActivityThread.main(ActivityThread.java:5257)

at java.lang.reflect.Method.invoke(Native Method)

at java.lang.reflect.Method.invoke(Method.java:372)

at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)

at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference

at utanashati.reminder.HomepageActivity.onCreate(HomepageActivity.java:55)

at android.app.Activity.performCreate(Activity.java:5990)

at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1106)

at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2278)

at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2390)

at android.app.ActivityThread.access$800(ActivityThread.java:151)

at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)

at android.os.Handler.dispatchMessage(Handler.java:102)

at android.os.Looper.loop(Looper.java:135)

at android.app.ActivityThread.main(ActivityThread.java:5257)

at java.lang.reflect.Method.invoke(Native Method)

at java.lang.reflect.Method.invoke(Method.java:372)

at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)

at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)

05-28 16:04:52.410 1218-1232/system_process W/ActivityManager﹕ Force finishing activity 1 utanashati.reminder/.HomepageActivity

05-28 16:04:52.411 1218-1232/system_process W/ActivityManager﹕ Force finishing activity 2 utanashati.reminder/.LoginActivity

编辑1

我睁大了眼睛,问题不在于LoginActivity,而在于使用了HomepageActivity.这是代码：

import ...

public class HomepageActivity extends Activity implements AdapterView.OnItemSelectedListener {

protected EditText mAddTaskText;

protected Spinner mPrioritySpinner;

protected Button mAddTaskButton;

protected int intPriority = 0;

protected String taskText;

protected Timestamp taskTimestamp;

protected Task userTask;

protected JsonGenerator taskJSON;

@Override

protected void onCreate(Bundle savedInstanceState) { // Starts activity. The state can be restored from savedInstanceState

super.onCreate(savedInstanceState); // Calls the superclass method (IMPORTANT)

setContentView(R.layout.activity_homepage); // Sets layout from activity_homepage.xml

mPrioritySpinner = (Spinner) findViewById(R.id.prioritySpinner); // Creates an ArrayAdapter using the string array and a default spinner layout

ArrayAdapter adapter = ArrayAdapter.createFromResource(this,

R.array.priorityList, android.R.layout.simple_spinner_item); // Specifies the layout to use when the list of choices appears

adapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item); // Applies the adapter to the spinner

mPrioritySpinner.setAdapter(adapter);

mPrioritySpinner.setOnItemSelectedListener(this);

mAddTaskText = (EditText) findViewById(R.id.addTaskEditText); // Finds View by its id in .xml file

mAddTaskButton.setOnClickListener(new View.OnClickListener() {

@Override

public void onClick(View v) {

Toast.makeText(HomepageActivity.this, "Done!", Toast.LENGTH_LONG).show();

Calendar taskCalendar = Calendar.getInstance(); // Creates new calendar

long taskTime = taskCalendar.getTimeInMillis(); // Gets time in milliseconds

taskTimestamp = new Timestamp(taskTime); // Creates new Timestamp

taskText = mAddTaskText.getText().toString(); // Gets description of the task

userTask.setDate(taskTimestamp); // Sets date

userTask.setText(taskText); // Sets text

/* Creating JsonGenerator */

ObjectMapper mapper = new ObjectMapper();

try {

mapper.writeValue(taskJSON, userTask);

}

catch (IOException e) {

Toast.makeText(HomepageActivity.this, "Could not create JSON", Toast.LENGTH_LONG).show();

}

/* Getting out email and password */

String userPassword = ((EmailPassword) HomepageActivity.this.getApplication()).getPassword();

String userEmail = ((EmailPassword) HomepageActivity.this.getApplication()).getUserEmail();

Toast.makeText(HomepageActivity.this, userEmail + " " + userPassword, Toast.LENGTH_LONG).show();

/* HTTP stuff */

HttpPoster get = new HttpPoster();

get.execute(userEmail, userPassword, taskJSON.toString());

}

});

}

public int getData (String username, String password, String taskJSON) {

try {

HttpPost httpPost = new HttpPost("http://something.com/" + username + "/tasks");

String dataToEncode = username + ":" + password;

String encodedData = Base64.encodeToString(dataToEncode.getBytes(), Base64.NO_WRAP);

httpPost.setHeader("Authorization", encodedData);

try {

StringEntity taskEntity = new StringEntity(taskJSON, "UTF-8");

httpPost.setEntity(taskEntity);

}

catch (UnsupportedEncodingException e) {

Toast.makeText(HomepageActivity.this, "Unsupported encoding", Toast.LENGTH_LONG).show();

}

HttpClient client = new DefaultHttpClient();

HttpResponse response = client.execute(httpPost);

StatusLine statusLine = response.getStatusLine();

int statusCode = statusLine.getStatusCode();

if (statusCode == 200) {

return 1;

}

else if (statusCode == 404) { return 2; }

else if (statusCode == 500) { return 3; }

else if (statusCode == 409) { return 4; }

else { return statusCode; }

}

catch (IOException e) {

e.printStackTrace();

}

return 0;

}

public void onItemSelected(AdapterView> parent, View view,

int pos, long id) {

String priority = parent.getItemAtPosition(pos).toString(); // Gets chosen priority

Toast.makeText(HomepageActivity.this, priority, Toast.LENGTH_LONG).show();

while (!((priority.equals("Low")) || (priority.equals("Medium")) || (priority.equals("High")))) {

Toast.makeText(HomepageActivity.this, "Something bad happened. Try to choose again", Toast.LENGTH_LONG).show();

}

if (priority.equals("Low")) {

intPriority = 0;

}

else if (priority.equals("Medium")) {

intPriority = 1;

}

else if (priority.equals("High")) {

intPriority = 2;

}

userTask.setPriority(intPriority); // Sets chosen priority

}

public void onNothingSelected(AdapterView> parent) {

userTask.setPriority(intPriority); // Sets default priority ("0")

}

public class HttpPoster extends AsyncTask {

@Override

protected void onPreExecute() {

super.onPreExecute();

}

@Override

protected Integer doInBackground(String... params) {

return getData(params[0], params[1], params[3]);

}

@Override

protected void onPostExecute(Integer result) {

super.onPostExecute(result);

if (result == 1) {

Toast.makeText(HomepageActivity.this, "Login successful", Toast.LENGTH_LONG).show();

Intent takeUserHome = new Intent(HomepageActivity.this, HomepageActivity.class);

startActivity(takeUserHome);

}

else if (result == 2) {

Toast.makeText(HomepageActivity.this, "No such user", Toast.LENGTH_LONG).show();

}

else if (result == 3) {

Toast.makeText(HomepageActivity.this, "Internal server error: unable to send email", Toast.LENGTH_LONG).show();

}

else if (result == 4) {

Toast.makeText(HomepageActivity.this, "Task already exists", Toast.LENGTH_LONG).show();

}

else {

Toast.makeText(HomepageActivity.this, result.toString(), Toast.LENGTH_LONG).show();

}

}

}

}

和XML文件：

xmlns:tools="http://schemas.android.com/tools"

android:layout_width="match_parent"

android:layout_height="match_parent"

android:paddingLeft="@dimen/activity_horizontal_margin"

android:paddingRight="@dimen/activity_horizontal_margin"

android:paddingTop="@dimen/activity_vertical_margin"

android:paddingBottom="@dimen/activity_vertical_margin"

tools:context="utanashati.testapp.HomepageActivity">

android:layout_width="match_parent"

android:layout_height="wrap_content"

android:hint="Add a new task..."

android:id="@+id/addTaskEditText"

android:nestedScrollingEnabled="false"

android:minLines="1"

android:maxLines="1" />

android:layout_width="match_parent"

android:layout_height="wrap_content"

android:id="@+id/prioritySpinner"

android:layout_alignRight="@+id/addTaskButton"

android:layout_alignEnd="@+id/addTaskButton"

android:layout_below="@+id/addTaskEditText" />

android:layout_width="match_parent"

android:layout_height="wrap_content"

android:text="Add task"

android:id="@+id/addTaskButton"

android:layout_below="@+id/prioritySpinner"

android:layout_centerHorizontal="true" />