In a class i am taking we have been learning some android app development and such, and i have managed to run into a problem that I, for the life of me, cannot figure out...
Basic background...
I get a nullpointerexception when I rotate the device... This error is located at Line 100 of QuizActivity.....
this Line is : mCheatButton.setOnClickListener(new View.OnClickListener() {//....
I suspect this has something to do with storing or retrieving information from the bundle...
Anyways here is the rest of The code....
Here is my quiz activity... Sorry for all the code blocks, I dont really know a better way to give you this information, or if you even need all of it.... But right now I just can figure out what is going on... so i have printed the error (stack trace? i think that is the right term...).. here as well...
03-02 09:49:28.858 7921-7921/com.example.ryan.ryans_quiz E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.ryan.ryans_quiz, PID: 7921
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.ryan.ryans_quiz/com.example.ryan.ryans_quiz.QuizActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
at android.app.ActivityThread.-wrap11(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5417)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
at com.example.ryan.ryans_quiz.QuizActivity.onCreate(QuizActivity.java:100)
at android.app.Activity.performCreate(Activity.java:6237)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1107)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2369)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
at android.app.ActivityThread.-wrap11(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5417)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
QuizActivity
package com.example.ryan.ryans_quiz;
import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.ImageButton;
import android.widget.TextView;
import android.widget.Toast;
public class QuizActivity extends AppCompatActivity {
private static final String TAG = "QuizActivity";
private static final String KEY_INDEX = "index";
private static final int REQUEST_CODE_CHEAT = 0;
private Button mTrueButton;
private Button mFalseButton;
private ImageButton mNextButton;
private Button mCheatButton;
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)
};
private int mCurrentIndex = 0;
private boolean mIsCheater;
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 (mIsCheater) {
messageResId = R.string.judgment_toast;
} else {
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) {
if (savedInstanceState != null) {
mCurrentIndex = savedInstanceState.getInt(KEY_INDEX, 0);
}
super.onCreate(savedInstanceState);
Log.d(TAG, "onCreate(Bundle) called");
setContentView(R.layout.activity_quiz);
mQuestionTextView = (TextView) findViewById(R.id.question_text_view);
mTrueButton = (Button) findViewById(R.id.trueButton);
mTrueButton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
checkAnswer(true);
}
});
mFalseButton = (Button) findViewById(R.id.falseButton);
mFalseButton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
checkAnswer(false);
}
});
mNextButton = (ImageButton) findViewById(R.id.nextButton);
mNextButton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
mCurrentIndex = (mCurrentIndex + 1) % mQuestionBank.length;
mIsCheater = false;
updateQuestion();
}
});
mCheatButton = (Button)findViewById(R.id.cheatButton);
mCheatButton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
boolean answerIsTrue = mQuestionBank[mCurrentIndex].isAnswerTrue();
Intent i = CheatActivity.newIntent(QuizActivity.this, answerIsTrue);
startActivityForResult(i, REQUEST_CODE_CHEAT);
}
});
updateQuestion();
}
@Override
protected void onSaveInstanceState(Bundle outState) {
super.onSaveInstanceState(outState);
outState.putInt(KEY_INDEX, mCurrentIndex);
}
@Override
protected void onStart() {
super.onStart();
Log.d(TAG, "onStart() called");
}
@Override
protected void onPause() {
super.onPause();
Log.d(TAG, "onPause() called");
}
@Override
protected void onResume() {
super.onResume();
Log.d(TAG, "onResume() called");
}
@Override
protected void onStop() {
super.onStop();
Log.d(TAG, "onStop() called");
}
@Override
protected void onDestroy() {
super.onDestroy();
Log.d(TAG, "onDestroy() called");
}
@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
if (resultCode != Activity.RESULT_OK) {
return;
}
if (requestCode == REQUEST_CODE_CHEAT) {
if (data == null) {
return;
}
mIsCheater = CheatActivity.wasAnswerShown(data);
}
}
}
My CheatActivity class
package com.example.ryan.ryans_quiz;
import android.animation.Animator;
import android.animation.AnimatorListenerAdapter;
import android.content.Context;
import android.content.Intent;
import android.os.Build;
import android.os.Bundle;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.Snackbar;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewAnimationUtils;
import android.widget.Button;
import android.widget.TextView;
public class
CheatActivity extends AppCompatActivity {
private static final String EXTRA_ANSWER_IS_TRUE =
"com.bignerdranch.android.geoquiz.answer_is_true";
private static final String EXTRA_ANSWER_SHOWN =
"com.bignerdranch.android.geoquiz.answer_shown";
private boolean mAnswerIsTrue;
private TextView mAnswerTextView;
private Button mShowAnswer;
public static Intent newIntent(Context packageContext, boolean answerIsTrue) {
Intent i = new Intent(packageContext, CheatActivity.class);
i.putExtra(EXTRA_ANSWER_IS_TRUE, answerIsTrue);
return i;
}
public static boolean wasAnswerShown(Intent result) {
return result.getBooleanExtra(EXTRA_ANSWER_SHOWN, false);
}
private void setAnswerShownResult(boolean isAnswerShown) {
Intent data = new Intent();
data.putExtra(EXTRA_ANSWER_SHOWN, isAnswerShown);
setResult(RESULT_OK, data);
}
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_cheat);
mAnswerIsTrue = getIntent().getBooleanExtra(EXTRA_ANSWER_IS_TRUE, false);
mAnswerTextView = (TextView)findViewById(R.id.answer_text_view);
mShowAnswer = (Button)findViewById(R.id.show_answer_button);
mShowAnswer.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
if (mAnswerIsTrue) {
mAnswerTextView.setText(R.string.trueButton);
} else {
mAnswerTextView.setText(R.string.falseButton);
}
setAnswerShownResult(true);
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
int cx = mShowAnswer.getWidth() / 2;
int cy = mShowAnswer.getHeight() / 2;
float radius = mShowAnswer.getWidth();
Animator anim = ViewAnimationUtils
.createCircularReveal(mShowAnswer, cx, cy, radius, 0);
anim.addListener(new AnimatorListenerAdapter() {
@Override
public void onAnimationEnd(Animator animation) {
super.onAnimationEnd(animation);
mAnswerTextView.setVisibility(View.VISIBLE);
mShowAnswer.setVisibility(View.INVISIBLE);
}
});
anim.start();
} else {
mAnswerTextView.setVisibility(View.VISIBLE);
mShowAnswer.setVisibility(View.INVISIBLE);
}
}
});
}
my Question Class
package com.example.ryan.ryans_quiz;
/**
* Created by Ryan on 1/20/2016.
*/
public class Question {
private int mTextResId;
private boolean mAnswerTrue;
public Question(int _textResId, boolean _answerTrue) {
mTextResId = _textResId;
mAnswerTrue = _answerTrue;
}
public int getTextResId() {
return mTextResId;
}
public void setTextResId(int textResId) {
mTextResId = textResId;
}
public boolean isAnswerTrue() {
return mAnswerTrue;
}
public void setAnswerTrue(boolean answerTrue) {
mAnswerTrue = answerTrue;
}
}
And finally my xml Files....
activity_cheat
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="center"
android:orientation="vertical"
tools:context="com.example.ryan.ryans_quiz.CheatActivity">
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="center"
android:padding="24dp"
android:text="@string/warning_text"/>
<TextView
android:id="@+id/answer_text_view"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="center"
android:padding="24dp"
tools:text="Answer"/>
<Button
android:id="@+id/show_answer_button"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="center"
android:text="@string/show_answer_button"/>
</LinearLayout>
LandLayout - activity_quiz
<FrameLayout 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:layout_gravity="center_horizontal"
android:padding="24dp"/>
<LinearLayout
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="center_horizontal|center_vertical"
android:orientation="vertical">
<LinearLayout
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="center_horizontal|center_vertical"
android:orientation="horizontal">
<Button
android:id="@+id/trueButton"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@string/trueButton"/>
<Button
android:id="@+id/falseButton"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@string/falseButton"/>
</LinearLayout>
<Button
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="bottom|center"
android:text="@string/cheat_button"/>
<LinearLayout
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="center_horizontal|center_vertical"
android:orientation="horizontal">
<ImageButton
android:id="@+id/previousButton"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:contentDescription="@string/previousButton"
android:src="@drawable/arrow_left"/>
<ImageButton
android:id="@+id/nextButton"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:layout_gravity="center_horizontal|center_vertical"
android:contentDescription="@string/nextButton"
android:src="@drawable/arrow_right"/>
</LinearLayout>
</LinearLayout>
</FrameLayout>
vertical layout- activity_quiz
<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/trueButton"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@string/trueButton"/>
<Button
android:id="@+id/falseButton"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@string/falseButton"/>
</LinearLayout>
<Button
android:id="@+id/cheatButton"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@string/cheat_button"/>
<LinearLayout
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:orientation="horizontal">
<ImageButton
android:id="@+id/previousButton"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:contentDescription="@string/previousButton"
android:src="@drawable/arrow_left"/>
<ImageButton
android:id="@+id/nextButton"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:contentDescription="@string/nextButton"
android:src="@drawable/arrow_right"/>
</LinearLayout>
</LinearLayout>
Strings..
<resources>
<string name="app_name">RyanHull_Quiz</string>
<string name="action_settings">Settings</string>
<string name="trueButton">True</string>
<string name="falseButton">False</string>
<string name="nextButton">Next</string>
<string name="previousButton">Previous</string>
<string name="cheat_button">Cheat!</string>
<string name="correct_toast">Correct! :)</string>
<string name="incorrect_toast">Incorrect! :(</string>"
<string name="question_oceans">The Pacific Ocean is larger than the Atlantic Ocean.</string>
<string name="question_mideast">The Suez Canal connects the Red Sea and the Indian Ocean.</string>
<string name="question_africa">The source of the Nile River is in Egypt.</string>
<string name="question_americas">The Amazon River is the longest river in the Americas.</string>
<string name="question_asia">Lake Baikal is the world\'s oldest and deepest freshwater lake.</string>
<string name="warning_text">Are you sure you want to do this??</string>
<string name="show_answer_button">Show Answer</string>
<string name="judgment_toast">Cheating is Wrong 8/</string>
<string name="title_activity_cheat">Cheat</string>
and finally manifest....
<?xml version="1.0" encoding="utf-8"?>
<manifest package="com.example.ryan.ryans_quiz"
xmlns:android="http://schemas.android.com/apk/res/android">
<application
android:allowBackup="true"
android:icon="@mipmap/ic_launcher"
android:label="@string/app_name"
android:supportsRtl="true"
android:theme="@style/AppTheme">
<activity android:name=".QuizActivity">
<intent-filter>
<action android:name="android.intent.action.MAIN"/>
<category android:name="android.intent.category.LAUNCHER"/>
</intent-filter>
</activity>
<!--
ATTENTION: This was auto-generated to add Google Play services to your project for
App Indexing. See https://g.co/AppIndexing/AndroidStudio for more information.
-->
<meta-data
android:name="com.google.android.gms.version"
android:value="@integer/google_play_services_version"/>
<activity
android:name=".CheatActivity"
android:label="@string/title_activity_cheat"
android:theme="@style/AppTheme.NoActionBar">
</activity>
</application>
EDIT: its not a matter of me know what the exception is.. More of the matter of find where and why...
