Java findViewById（）上的Android NullPointerException

我正在启动一个活动，并试图通过

findviewbyd（）获取视图（在本例中为视频视图）
在大多数情况下，这很好，但有时当我试图访问我先前试图查找的视图时，它会抛出NullPointerException

这就是我所从事的活动：

public class Video extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_video);
        Intent intent = getIntent();
        VideoView videoView = this.findViewById(R.id.VideoViewVideo);
        videoView.setVideoURI(someURI);
        videoView.start();
    }
}

异常在
videoView.setVideoURI（someURI）行中抛出并如下所示：

java.lang.RuntimeException: Unable to start activity ComponentInfo{Video}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.VideoView.setVideoURI(java.lang.String)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2678)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2743)
at android.app.ActivityThread.-wrap12(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1490)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6165)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:888)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:778)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.VideoView.setVideoUri(java.lang.String)' on a null object reference
at Video.onCreate(Video.java:29)
at android.app.Activity.performCreate(Activity.java:6687)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1140)    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2631)

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.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"
    tools:context="Video">

    <VideoView
        android:id="@+id/VideoViewVideo"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="8dp"
        android:layout_marginEnd="8dp"
        android:layout_marginStart="8dp"
        android:layout_marginTop="8dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />
</android.support.constraint.ConstraintLayout>

活动的布局文件如下所示：

java.lang.RuntimeException: Unable to start activity ComponentInfo{Video}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.VideoView.setVideoURI(java.lang.String)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2678)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2743)
at android.app.ActivityThread.-wrap12(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1490)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6165)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:888)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:778)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.VideoView.setVideoUri(java.lang.String)' on a null object reference
at Video.onCreate(Video.java:29)
at android.app.Activity.performCreate(Activity.java:6687)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1140)    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2631)

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.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"
    tools:context="Video">

    <VideoView
        android:id="@+id/VideoViewVideo"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="8dp"
        android:layout_marginEnd="8dp"
        android:layout_marginStart="8dp"
        android:layout_marginTop="8dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />
</android.support.constraint.ConstraintLayout>



我想我试图获取视图的方式可能有问题，但我不确定

如果有人能帮助我，我们将不胜感激。
请尝试移动此代码

VideoView videoView = this.findViewById(R.id.VideoViewVideo);
videoView.setVideoURI(someURI);
videoView.start();

到

有时，如果要创建视图，必须在onViewCreated而不是onCreate中定义它

让我知道它是否有效
您为什么要写作

VideoView-VideoView=this.findviewbyd（R.id.VideoViewVideo）
你可以不写上面的内容

VideoView VideoView=（VideoView）findViewById（R.id.VideoViewVideo）
也许您有特定于配置的布局文件夹和多个activity\u video.xml布局文件？如果是这样，请确保他们都有一个具有预期id的视频视图。不幸的是，这似乎不是问题所在。我没有任何其他名为activity_video.xml的文件。我唯一能想到的是，您有一些陈旧的构建工件。试着做一个干净的构建。@TedHopp干净的构建修复了它。是我做错了什么，还是这只是一个常见的Gradle/AndroidStudio问题？这种情况偶尔会发生。至于你是否做错了什么，我怀疑。可能是您在Gradle构建过程中编辑了源代码，而构建与自身不同步。字面上没有区别，知道了吗？在这里，我可以看到VideoView是一个自定义视图检查，如果它是从JavaCode初始化的，它不是自定义视图。问题中给出了xml。