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>

Vitrescent

The stacktrace you posted points to an issue in HomepageActivity not LoginActivity for which you have posted code. – Chamois

Your error in HomepageActivity activity, can you put that code here. – Bruin

@Chamois Wow, you have opened my eyes, thank you :) – Vitrescent

can you specify where is the solution to this problem – Anetteaneurin

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.

Boone

Damn - this answer clued me in to my error. I had accidentally inserted a character to the button name in the xml, but android studio never warned me about it because there was another button in another xml with the same name, so it assumed I was referring to that one in the activity code. – Marshland

Answer 2

mAddTaskButton is null because you never initialize it with:

mAddTaskButton = (Button) findViewById(R.id.addTaskButton);

before you call mAddTaskButton.setOnClickListener().

Chamois

I have the same problem but I have initialized it correctly. – Boone

Casting to Button is redundant – Pappas

Answer 3

Check out this solution. It worked for me..... Check the id of the button for which the error is raised...it may be the same in any one of the other page in your app. If yes, then change the id of them and then the app runs perfectly.

I was having two same button id's in two different XML codes....I changed the id. Now it runs perfectly!! Hope it works

Insincerity

Answer 4

That true,Mustafa....its working..its point to two layout

1. setContentView(R.layout.your_layout)
2. v23(your_layout).

You should take Button both activity layout...
solve this problem successfully

Kingmaker

Answer 5

Check whether you have matching IDs in both Java and XML

Bettencourt

Welcome to SO. This should probably be a comment, rather than an answer... – Haith

Answer 6

Just define the button as lateinit var at top of your class:

 lateinit var buttonOk: Button

When you want to use a button in another layout you should define it in that layout. For example if you want to use button in layout which name is 'dialogview', you should write:

 buttonOk = dialogView.findViewById<Button>(R.id.buttonOk)

After this you can use setonclicklistener for the button and you won't have any error. You can see correct answer of this question: Android Kotlin findViewById must not be null

Brade

Answer 7

Make sure that while using : Button "varName" =findViewById("btID"); you put in the right "btID". I accidentally put in the id of a button from another similar activity and it showed the same error. Hope it helps.

Umbilicus

Answer 8

Got the same error, CHECK THIS : MINOR SILLY MISTAKE

check findviewbyid(R.id.yourID); If you have put the id correct or not.

Babyblueeyes

Answer 9

i had the same problem and it seems like i didn't initiate the button used with click listener, in other words id didn't te

Stephanus

"Didn't te"? Tie, you meant? – Teutonize

Answer 10

Placing setOnClickListener in onStart method solved the problem for me. Checkout "Android Lifecycle concept" for further clarification

Bevbevan

Answer 11

 mAddTaskButton.setOnClickListener(new View.OnClickListener()

you have a click listner but you haven't initialized the mAddTaskButton with your layout binding

Leafy

Answer 12

Please check whether you have two Layout folder (e.g Layout & Layout-V21). The XML file should be same in both folders. That was the case for me.

Herve

Answer 13

This issue also can occur if you define your button before setContentView() function. Like this:

 val button = findViewById<Button>(R.id.button)

 setContentView(R.layout.activity_main)

 button.setOnClickListener {
 //your code
 }

Dusk