Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference

Question

The problem is as follows. I have a login activity (in Android Studio) which worked fine a few days before. I don't remember changing anything but when I run this one the previous time the app closed right after I clicked the login button. The last thing indicated was the toast about pre-execution of AsyncTask. And I can't understand why there could be a NullPointerException. I have almost the same code for my signup activity and it works fine.

Here is the log:

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 

EDIT 1

I had my eyes opened, the problem is not with LoginActivity, but with HomepageActivity. Here is the code:

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<CharSequence> 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<String, Void, Integer> {          @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();             }         }     } } 

And XML file:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"     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">      <EditText         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" />      <Spinner         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" />      <Button         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" />  </RelativeLayout> 

Answer 1

It seems the button you are invoking is not in the layout you are using in setContentView(R.layout.your_layout) Check it.