Question

我正在创建一个简单的应用程序，并希望用户单击“开始”以转到下一个活动但是在实现启动按钮时，在运行时按下它时会不断收到NullPointerException。我很困惑，因为看起来FindViewByID在按钮（开始按钮）的ID上返回null，即使它包含在XML文件中。被困在这几个小时，并寻找其他解决方案，并尝试清洁和重建。如果有帮助，请使用Android Studio 2.2.3。感谢。

XML文件 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>

主要活动代码 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);

        }
    });
}
}

堆栈跟踪

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>

Answer 1

试试这个：

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);

Answer 2

您需要将布局添加到活动中。

在 onCreate（）中进行小部件初始化之前，请使用添加布局的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
}

Answer 3

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)

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。

OnClickListener按钮上的NullPointException

3 个答案: