Android：Findviewbyid（）返回null

Question

I am trying to make an app with three RelativeLayouts nested in one RelativeLayout. I want to programmatically change the visibility of the RelativeLayouts. It works fine on my smartphone (Android 4.4.2), but not on tablet (Android 5.1.1)

When findViewById() is called on the tablet, it returns null.

What am I doing wrong?

MainActivity.java:

RelativeLayout mainactivity, multiplewifi, nowifi;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    if (savedInstanceState != null) //if switched from landscape to portrait mode or vice versa
        ((WebView) findViewById(R.id.webvieew)).restoreState(savedInstanceState); //restore webview
    else {
        Initialize();
        wifiManager.startScan(); //start a scan for available wifi networks
    }
}

public void Initialize() {
    mainactivity = (RelativeLayout) findViewById(R.id.MainActivity);
    multiplewifi = (RelativeLayout) findViewById(R.id.MultipleWifi);
    nowifi = (RelativeLayout) findViewById(R.id.NoWifi);
}

activity_main.xml:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent">

<RelativeLayout
    android:id="@+id/MultipleWifi">

    <Button/>

    <TextView/>

    <Spinner/>

</RelativeLayout>

<RelativeLayout
    android:id="@+id/MainActivity">

    <WebView
    android:id="@+id/webvieew"/>

    <ImageButton/>
</RelativeLayout>

<RelativeLayout
    android:id="@+id/NoWifi">

    <TextView/>
</RelativeLayout>

Log:

E/AndroidRuntime: FATAL EXCEPTION: main
              Process: com.example.wdt, PID: 5243
              java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.wdt/com.example.wdt.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.RelativeLayout.setVisibility(int)' on a null object reference
                  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2335)
                  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2397)
                  at android.app.ActivityThread.access$800(ActivityThread.java:151)
                  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1310)
                  at android.os.Handler.dispatchMessage(Handler.java:102)
                  at android.os.Looper.loop(Looper.java:135)
                  at android.app.ActivityThread.main(ActivityThread.java:5268)
                  at java.lang.reflect.Method.invoke(Native Method)
                  at java.lang.reflect.Method.invoke(Method.java:372)
                  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:902)
                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:697)
               Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.RelativeLayout.setVisibility(int)' on a null object reference
                  at com.example.wdt.MainActivity.Initialize(MainActivity.java:95)
                  at com.example.wdt.MainActivity.onCreate(MainActivity.java:75)
                  at android.app.Activity.performCreate(Activity.java:6033)
                  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1106)
                  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2288)
                  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2397) 
                  at android.app.ActivityThread.access$800(ActivityThread.java:151) 
                  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1310) 
                  at android.os.Handler.dispatchMessage(Handler.java:102) 
                  at android.os.Looper.loop(Looper.java:135) 
                  at android.app.ActivityThread.main(ActivityThread.java:5268) 
                  at java.lang.reflect.Method.invoke(Native Method) 
                  at java.lang.reflect.Method.invoke(Method.java:372) 
                  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:902) 
                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:697) 

Answer 1

Your application is working on phones but not on tablets, this usually means a problem of resources.

The resources ( layout , values , ...) used by Android depends on the device used. If there is a specific folder that match the device configuration (like large screen), the resource redefine in this folder will be used instead of the general folder.

There is multiple difference between phone/tablet (screen size, ratio, orientation, ...).

In your case, you might need to update or remove the layout main_activity.xml in a layout folder that match your tablet.