title: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
body: <p>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.</p>
  
  <p>Here is the log:</p>
  
  <pre><code>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
  </code></pre>
  
  <p><strong>EDIT 1</strong></p>
  
  <p>I had my eyes opened, the problem is not with LoginActivity, but with HomepageActivity. Here is the code:</p>
  
  <pre><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&lt;CharSequence&gt; 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&lt;?&gt; 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&lt;?&gt; parent) {
          userTask.setPriority(intPriority);                                      // Sets default priority ("0")
      }
  
      public class HttpPoster extends AsyncTask&lt;String, Void, Integer&gt; {
  
          @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();
              }
          }
      }
  }
  </code></pre>
  
  <p>And XML file:</p>
  
  <pre><code>&lt;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"&gt;
  
      &lt;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" /&gt;
  
      &lt;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" /&gt;
  
      &lt;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" /&gt;
  
  &lt;/RelativeLayout&gt;
  </code></pre>