Android will crash depending on layout positioning

Question

Here's the layout that works :

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/mainlayout"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".MainActivity" >

<LinearLayout
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:orientation="vertical" >

    <com.astuetz.viewpager.extensions.PagerSlidingTabStrip
        android:id="@+id/tabs"
        android:layout_width="match_parent"
        android:layout_height="48dip" >
    </com.astuetz.viewpager.extensions.PagerSlidingTabStrip>

    <android.support.v4.view.ViewPager
        android:id="@+id/pager"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_weight="1"
        tools:context=".MainActivity" >
    </android.support.v4.view.ViewPager>

</LinearLayout>

and here my Activity OnCreate method:

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    pager = (ViewPager) findViewById(R.id.pager);

    adapter = new MyPagerAdapter(getSupportFragmentManager());

    pager.setAdapter(adapter);

    tabs = (PagerSlidingTabStrip) findViewById(R.id.tabs);
    tabs.setViewPager(pager);
    tabs.setDividerColor(Color.BLUE);
    tabs.setBackgroundColor(Color.DKGRAY);
    tabs.setTextColor(Color.WHITE);
    tabs.setIndicatorColor(Color.GREEN);

}

If in the layout I inverse the PagerSlidingTabStrip and the ViewPager, the project will crash at startup, while in the first implementation, it will work fine.

Here's the error :

05-19 16:50:11.636: E/AndroidRuntime(305): FATAL EXCEPTION: main
05-19 16:50:11.636: E/AndroidRuntime(305): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.hsware.peeem/com.hsware.peeem.MainActivity}: java.lang.ClassCastException: android.support.v4.view.ViewPager
05-19 16:50:11.636: E/AndroidRuntime(305):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2663)
05-19 16:50:11.636: E/AndroidRuntime(305):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2679)
05-19 16:50:11.636: E/AndroidRuntime(305):  at android.app.ActivityThread.access$2300(ActivityThread.java:125)
05-19 16:50:11.636: E/AndroidRuntime(305):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2033)
05-19 16:50:11.636: E/AndroidRuntime(305):  at android.os.Handler.dispatchMessage(Handler.java:99)
05-19 16:50:11.636: E/AndroidRuntime(305):  at android.os.Looper.loop(Looper.java:123)
05-19 16:50:11.636: E/AndroidRuntime(305):  at android.app.ActivityThread.main(ActivityThread.java:4627)
05-19 16:50:11.636: E/AndroidRuntime(305):  at java.lang.reflect.Method.invokeNative(Native Method)
05-19 16:50:11.636: E/AndroidRuntime(305):  at java.lang.reflect.Method.invoke(Method.java:521)
05-19 16:50:11.636: E/AndroidRuntime(305):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)
05-19 16:50:11.636: E/AndroidRuntime(305):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
05-19 16:50:11.636: E/AndroidRuntime(305):  at dalvik.system.NativeStart.main(Native Method)
05-19 16:50:11.636: E/AndroidRuntime(305): Caused by: java.lang.ClassCastException: android.support.v4.view.ViewPager
05-19 16:50:11.636: E/AndroidRuntime(305):  at com.hsware.peeem.MainActivity.onCreate(MainActivity.java:41)
05-19 16:50:11.636: E/AndroidRuntime(305):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
05-19 16:50:11.636: E/AndroidRuntime(305):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2627)

It appears that it can't retrieve de R.id.pager, I'm not sure.

Answer 1

I've noticed that when making small ordering changes, the compiler will not pick it up. A ClassCastException is occurring because findViewById is actually returning the wrong view given R.id.pager, or more accurately - R.id.pager is not referring to the correct view.

The way to fix this is to just clean and re-build your project.

Answer 2

You aren't showing your includes. There are two definitions for ViewPager, one in android.support.v4.view.ViewPager and one in android.view.ViewPager. I'll bet that you have included the wrong one in your code.