Java OnClickListener按钮上的NullPointException
我正在制作一个简单的应用程序,希望用户单击“开始”移动到下一个活动,但在实现“开始”按钮时,当运行时按下该按钮时,我会不断收到NullPointerException。我很困惑,因为FindViewByID似乎在按钮startbutton的ID上返回null,即使它包含在XML文件中。在这个问题上已经坚持了好几个小时,并且一直在寻找其他解决方案,并尝试进行清理和重建。如果有帮助的话,使用Android Studio 2.2.3。谢谢 XML文件activity\u start\u screen.XML 堆栈跟踪 QuizScreen.java quizscreen.xml 试试这个:Java OnClickListener按钮上的NullPointException,java,android,button,nullpointerexception,listener,Java,Android,Button,Nullpointerexception,Listener,我正在制作一个简单的应用程序,希望用户单击“开始”移动到下一个活动,但在实现“开始”按钮时,当运行时按下该按钮时,我会不断收到NullPointerException。我很困惑,因为FindViewByID似乎在按钮startbutton的ID上返回null,即使它包含在XML文件中。在这个问题上已经坚持了好几个小时,并且一直在寻找其他解决方案,并尝试进行清理和重建。如果有帮助的话,使用Android Studio 2.2.3。谢谢 XML文件activity\u start\u screen.
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);
此空指针异常位于Quizscreen.java的第65行。请检查quizscreen.xml中是否有所有按钮id。您需要将布局添加到活动中
在onCreate中初始化小部件之前,请使用
setContentViewR.layout.your_layoutfile
public class QuizScreen extends AppCompatActivity
{
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.quizscreenlayout);
//initialize your widgets
}
请发布您的日志,并确认您的按钮位于activity\u start\u screen.xml中?xml文件是否包含activity\u start\u screen.xml?哪一行导致错误?添加了跟踪,是的,即activity\u start\u screen.xml。由setOnClickListener调用引起。问题出在QuizScreen.javaWhat中的onCreate中?这毫无意义。为什么问题中的代码不能按原样工作?谢谢,这似乎已经消除了错误!你介意解释一下吗,因为这让我很困惑。这个答案没有任何意义,你想在这里做什么?检查quizscreen.xml,似乎它们都在那里。将在main question.setContentViewR.layout.quizscreen中发布代码缺失
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.
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();
}
}
<?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>
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);
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)
public class QuizScreen extends AppCompatActivity
{
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.quizscreenlayout);
//initialize your widgets
}