NullPointException on OnClickListener Button

Question

I'm making a simple app and want a user to click start to move to the next activity but when implementing the start button I keep getting a NullPointerException when it is pressed at runtime. I'm confused because it seems like the FindViewByID is returning null on the ID of the button (startbutton) even though it is included in the XML file. Been stuck on this for hours and have searched for other solutions and tried cleaning and rebuilding. Using Android Studio 2.2.3 if that helps. Thanks.

XML file activity_start_screen.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout 
xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical" android:layout_width="match_parent"
android:layout_height="match_parent"
android:gravity="top|center"
android:weightSum="1">

<TextView
    android:layout_width="match_parent"
    android:layout_height="0dp"
    android:text="Quiz App"
    android:textSize="36dp"
    android:gravity="center"
    android:layout_weight="0.34" />

<Button
    android:text="Start!"
    android:layout_width="245dp"
    android:layout_height="50dp"
    android:id="@+id/startbutton" />

</LinearLayout>

Main Activity Code StartScreen.java

import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.content.Intent;
import android.app.Activity;

public class StartScreen extends AppCompatActivity
{
    private Button startbutton;



@Override
protected void onCreate(Bundle savedInstanceState)
{
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_start_screen);
    //set start button
    startbutton = (Button) findViewById(R.id.startbutton);
    startbutton.setOnClickListener(new View.OnClickListener()
    {
        @Override
        public void onClick(View v)
        {
            //go to quiz screen
            //change activity
            Intent QuizScreen = new Intent(StartScreen.this, QuizScreen.class);
            startActivity(QuizScreen);

        }
    });
}
}

Stack Trace

E/AndroidRuntime: FATAL EXCEPTION: main
              Process: com.paavan.quizappcoursework, PID: 2305
              java.lang.RuntimeException: Unable to start activity ComponentInfo{com.paavan.quizappcoursework/com.paavan.quizappcoursework.QuizScreen}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
                  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2646)
                  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707)
                  at android.app.ActivityThread.-wrap12(ActivityThread.java)
                  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460)
                  at android.os.Handler.dispatchMessage(Handler.java:102)
                  at android.os.Looper.loop(Looper.java:154)
                  at android.app.ActivityThread.main(ActivityThread.java:6077)
                  at java.lang.reflect.Method.invoke(Native Method)
                  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865)
                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)
               Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
                  at com.paavan.quizappcoursework.QuizScreen.onCreate(QuizScreen.java:63)
                  at android.app.Activity.performCreate(Activity.java:6662)
                  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1118)
                  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2599)
                  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707) 
                  at android.app.ActivityThread.-wrap12(ActivityThread.java) 
                  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460) 
                  at android.os.Handler.dispatchMessage(Handler.java:102) 
                  at android.os.Looper.loop(Looper.java:154) 
                  at android.app.ActivityThread.main(ActivityThread.java:6077) 
                  at java.lang.reflect.Method.invoke(Native Method) 
                  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865) 
                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755) 
Application terminated.

QuizScreen.java

import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;
import android.widget.Toast;


public class QuizScreen extends AppCompatActivity
{
private Button mTrueButton;
private Button mFalseButton;
private Button mNextButton;
private TextView mQuestionTextView;

private Question[] mQuestionBank = new Question[] {
        new Question(R.string.question_oceans, true),
        new Question(R.string.question_mideast, false),
        new Question(R.string.question_africa, false),
        new Question(R.string.question_americas, true),
        new Question(R.string.question_asia, true),
        new Question(R.string.question_uk,false),
        new Question(R.string.question_europe,true),
        new Question(R.string.question_waterfall,false),
        new Question(R.string.question_giraffes,false),
        new Question(R.string.question_penguins,true),
};

private int mCurrentIndex = 0;

private void updateQuestion()
{
    int question = mQuestionBank[mCurrentIndex].getTextResId();
    mQuestionTextView.setText(question);
}

private void checkAnswer(boolean userPressedTrue)
{
    boolean answerIsTrue = mQuestionBank[mCurrentIndex].isAnswerTrue();

    int messageResId = 0;

    if (userPressedTrue == answerIsTrue) {
        messageResId = R.string.correct_toast;
    } else {
        messageResId = R.string.incorrect_toast;
    }

    Toast.makeText(this, messageResId, Toast.LENGTH_SHORT).show();
}



@Override
protected void onCreate(Bundle savedInstanceState)
{
    mQuestionTextView = (TextView) this.findViewById(R.id.question_text_view);


    mTrueButton = (Button) this.findViewById(R.id.true_button);
    mTrueButton.setOnClickListener(new View.OnClickListener()
    {
        @Override
        public void onClick(View v)
        {
            checkAnswer(true);
        }
    });
    mFalseButton = (Button) this.findViewById(R.id.false_button);
    mFalseButton.setOnClickListener(new View.OnClickListener()
    {
        @Override
        public void onClick(View v)
        {
            checkAnswer(false);
        }
    });

    mNextButton = (Button) this.findViewById(R.id.next_button);
    mNextButton.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            mCurrentIndex = (mCurrentIndex + 1) % mQuestionBank.length;
            updateQuestion();
        }
    });

    updateQuestion();

}



}

quizscreen.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:gravity="center"
android:orientation="vertical" >



<TextView
    android:id="@+id/question_text_view"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:padding="24dp"
    />

<LinearLayout
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:orientation="horizontal" >

    <Button
        android:id="@+id/true_button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/true_button" />

    <Button
        android:id="@+id/false_button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/false_button" />

</LinearLayout>


<Button
    android:id="@+id/next_button"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="@string/next_button" />

</LinearLayout>

post your log please,, and are your sure your button is in activity_start_screen.xml? — Charuක, Jan 06 '17 at 03:15
Added the trace and yes that is activity_start_screen.xml. Caused by the setOnClickListener call. — George Coring, Jan 06 '17 at 03:27
There is no problem in the code in the question. Show your QuizScreen.java code... — Daniel Nugent, Jan 06 '17 at 03:55
Added it. Didn't think the error would be in this part because its from a textbook. — George Coring, Jan 06 '17 at 04:01
Have you added code to QuizScreen.java after that stack trace was taken? Line 65 doesn't make sense.... — Daniel Nugent, Jan 06 '17 at 04:04
Also, just to be sure, you have quizscreen.xml in the res folder? Show that as well.... — Daniel Nugent, Jan 06 '17 at 04:06
Sorry yeah reverted it, Line 63 on this and yep quizscreen.xml is in res\layout. — George Coring, Jan 06 '17 at 04:15
What happened to your `setContentView()` call? Looks like it's not there now.... — Daniel Nugent, Jan 06 '17 at 04:24
Added it back in along with super.onCreate(savedInstanceState); and it worked. Would that have been the issue? Thank you for your help anyway. — George Coring, Jan 06 '17 at 04:30

score 1 · Answer 1 · answered Jan 06 '17 at 03:43

1

Try this:

LayoutInflater inflater = (LayoutInflater) getContext().getSystemService(Context.LAYOUT_INFLATER_SERVICE); 
View view = inflater.inflate(R.layout.activity_start_screen, null);

Button startbutton = (Button) view.findViewById(R.id.startbutton);

answered Jan 06 '17 at 03:43

Amey Shirke

574
1
5
16

What? This makes no sense. Why would the code in the question not work as-is? – Daniel Nugent Jan 06 '17 at 03:53
Thanks this seems to have gotten rid of the error! Do you mind explaining it because this has confused me. – George Coring Jan 06 '17 at 03:54
This answer does not make any sense, What u trying to do here? – W4R10CK Jan 06 '17 at 04:02

score 1 · Accepted Answer · answered Jan 06 '17 at 04:28

You need to add the layout to activity.

Before doing any initialization of widgets in onCreate() add the layout using setContentView(R.layout.your_layoutfile);

public class QuizScreen extends AppCompatActivity
   {
        @Override
protected void onCreate(Bundle savedInstanceState) {

    super.onCreate(savedInstanceState);

    setContentView(R.layout.quizscreenlayout);

    //initialize your widgets
}

score -1 · Answer 3 · edited Jan 06 '17 at 04:10

-1

Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener) 
   on a null object reference at 
   com.paavan.quizappcoursework.QuizScreen.onCreate(QuizScreen.java:65)

This null pointer exception is in 65th line in Quizscreen.java. Please check whether all button id's are there in quizscreen.xml.

edited Jan 06 '17 at 04:10

Daniel Nugent

40,780
13
103
126

answered Jan 06 '17 at 04:07

Sangeet Suresh

2,309
15
19

checked quizscreen.xml and seems like they are all there. will post code in main question. – George Coring Jan 06 '17 at 04:18
setContentView(R.layout.quizscreen) is missing – Sangeet Suresh Jan 06 '17 at 18:06

NullPointException on OnClickListener Button

3 Answers3