尝试调用虚拟方法 'void android.widget.Button.setOnClickListener(android.view.View$onClickListener)'

Question

我正在尝试为按钮实现 LayoutInflater，但在实现之后出现此错误。我不确定它是否已正确实施。我针对同一个问题尝试了不同的解决方案，但作为 Android 的新手，我发现很难解决这个问题。

https://www.dropbox.com/s/s2k92n6ss4mtztg/Screenrecorder-2020-11-23-00-34-16-139.mp4?dl=0（很抱歉无法解释工作流程，但请参考此剪辑以便更好地理解，第一个 activity 是 IntroductoryActivity，最后一个有开始按钮的是 onboardingfragment3）

请阐明这个问题。谢谢！

IntroductoryActivity.java:

 @Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_introductory);
     
    ............

    **//The button from fragment_on_boarding3**
    start_l=findViewById(R.id.startb);

    .............

    **//Having issue with this part**

    LayoutInflater inflater = (LayoutInflater) mContext.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
    View layoutScreen = inflater.inflate(R.layout.fragment_on_boarding3,null);

    start_l.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Intent mainActivity = new Intent(getApplicationContext(),MainActivity.class);
            startActivity(mainActivity);

          // I need to save a boolean value to storage so next time when the user runs the app,I could know that he has already checked the intro screen activity
          // I'm going to use shared preferences forthat process
            
            savePrefsData();
            finish();
        }
    });
}

 private boolean restorePrefData() {
        SharedPreferences pref = getApplicationContext().getSharedPreferences("myPrefs",MODE_PRIVATE);
        Boolean isIntroActivityOpnendBefore = pref.getBoolean("isIntroOpnend",false);
        return  isIntroActivityOpnendBefore;
}


private void savePrefsData() {

    SharedPreferences pref = getApplicationContext().getSharedPreferences("myPrefs",MODE_PRIVATE);
    SharedPreferences.Editor editor = pref.edit();
    editor.putBoolean("isIntroOpnend",true);
    editor.commit();

}

}

OnBoardingFragment3.java:

public class OnBoardingFragment3 extends Fragment {

Context mContext ;
Button start;

@Nullable
@Override
public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {

    ViewGroup root=(ViewGroup) inflater.inflate(R.layout.fragment_on_boarding3,container,false);

    return root;
}

fragment_on_boarding3.xml:

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout 
       xmlns:android="http://schemas.android.com/apk/res/android"
       xmlns:app="http://schemas.android.com/apk/res-auto"
       xmlns:tools="http://schemas.android.com/tools"
       android:layout_width="match_parent"
       android:layout_height="match_parent">

         <Button android:id="@+id/startb"
    android:layout_width="157dp"
    android:layout_height="59dp"
    android:fontFamily="@font/bungee"
    android:text="Let's Start"
    android:textSize="15dp"
    app:layout_constraintBottom_toBottomOf="parent"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintHorizontal_bias="0.498"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toTopOf="parent"
    app:layout_constraintVertical_bias="0.754" />
     
    </androidx.constraintlayout.widget.ConstraintLayout>

错误：

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.liq/com.example.liq.IntroductoryActivity}: 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:2946) at....................

Answer 1

findViewById 实际上是这样称呼的：

this.findViewById(someId);

this 指的是 Activity（在你的情况下 IntroductoryActivity）；

来自docs：

Finds a view that was identified by the android:id XML attribute that was processed in onCreate(Bundle).

此 onCreate() 方法是您调用 findViewById 的 Activity 的 onCreate 方法。在您的情况下，该 ID（和视图）属于片段，因此 Activity 无法找到与该 ID 关联的视图，并且 returns null 并且当您想 setOnClickListener 到 start_l按钮。

您可以在片段的 onViewCreated 方法内将 onClickListener 设置为该按钮：

@Override
public void onViewCreated(View view, Bundle savedInstanceState) {
  super.onViewCreated(view, savedInstanceState);
  start_l=view.findViewById(R.id.startb);
  start_l.setOnClickListener.......
  
}

编辑：阿里王子的建议也是一种可行的方法，也许是更好的方法。您可以在 onCreateView:

中初始化视图

public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {

    ViewGroup root=(ViewGroup) inflater.inflate(R.layout.fragment_on_boarding3,container,false);
    start_l=root.findViewById(R.id.startb);
    start_l.setOnClickListener.......
    return root;
}

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尝试调用虚拟方法 'void android.widget.Button.setOnClickListener(android.view.View$onClickListener)'

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