Android : Layout misleadings

I can't completely understand how layouts work. As I'm coding at the ADB, everything looks fine in the graphical layout.

But then, errors like this one appear:

09-18 12:09:48.146: E/AndroidRuntime(29515): FATAL EXCEPTION: main
09-18 12:09:48.146: E/AndroidRuntime(29515): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.eetam.uburent/com.eetam.uburent.BokatasActivity}: java.lang.ClassCastException: android.widget.LinearLayout cannot be cast to android.widget.NumberPicker
09-18 12:09:48.146: E/AndroidRuntime(29515):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java)
09-18 12:09:48.146: E/AndroidRuntime(29515):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java)
09-18 12:09:48.146: E/AndroidRuntime(29515):    at android.app.ActivityThread.access$600(ActivityThread.java)
09-18 12:09:48.146: E/AndroidRuntime(29515):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java)
09-18 12:09:48.146: E/AndroidRuntime(29515):    at android.os.Handler.dispatchMessage(Handler.java)
09-18 12:09:48.146: E/AndroidRuntime(29515):    at android.os.Looper.loop(Looper.java)
09-18 12:09:48.146: E/AndroidRuntime(29515):    at android.app.ActivityThread.main(ActivityThread.java)
09-18 12:09:48.146: E/AndroidRuntime(29515):    at java.lang.reflect.Method.invokeNative(Native Method)
09-18 12:09:48.146: E/AndroidRuntime(29515):    at java.lang.reflect.Method.invoke(Method.java:511)
09-18 12:09:48.146: E/AndroidRuntime(29515):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java)
09-18 12:09:48.146: E/AndroidRuntime(29515):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java)
09-18 12:09:48.146: E/AndroidRuntime(29515):    at dalvik.system.NativeStart.main(Native Method)
09-18 12:09:48.146: E/AndroidRuntime(29515): Caused by: java.lang.ClassCastException: android.widget.LinearLayout cannot be cast to android.widget.NumberPicker
09-18 12:09:48.146: E/AndroidRuntime(29515):    at com.eetam.uburent.BokatasActivity.setCourtPicker(BokatasActivity.java:107)
09-18 12:09:48.146: E/AndroidRuntime(29515):    at com.eetam.uburent.BokatasActivity.onCreate(BokatasActivity.java:55)
09-18 12:09:48.146: E/AndroidRuntime(29515):    at android.app.Activity.performCreate(Activity.java)
09-18 12:09:48.146: E/AndroidRuntime(29515):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java)
09-18 12:09:48.146: E/AndroidRuntime(29515):    ... 12 more

Sometimes I finally achieve avoiding that error playing with wrap_content, fill_parent and wrap_content. Error looks like if I can't overlay layouts and widgets, it's so frustating. I would like to know why I can't code it like this:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:background="#000000"
    android:gravity="clip_vertical"
    android:orientation="vertical"
    tools:context=".BokatasActivity" >

    <LinearLayout
        android:id="@+id/relLay3"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:gravity="center_horizontal"
        android:orientation="horizontal"
        >

        <NumberPicker
            android:id="@+id/pista"
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:layout_weight="0.6"
            android:layout_gravity="center_horizontal" />


        <NumberPicker
            android:id="@+id/numero"
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:layout_weight="0.4"
            android:layout_gravity="center_horizontal" >
        </NumberPicker>

    </LinearLayout>     

    <LinearLayout
        android:id="@+id/relLay4"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@+id/relLay3"
        android:gravity="center_horizontal"
        android:orientation="vertical">

        <Button
            android:id="@+id/buttonToast"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_gravity="center_horizontal"
            android:text="Show Toast" />

        <TextView
            android:id="@+id/tvDate"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_gravity="center_horizontal"
            android:text="" />

        <TextView
            android:id="@+id/lblDate"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Current Date (M-D-YYYY): "
            android:textAppearance="?android:attr/textAppearanceLarge" />

        <Button
            android:id="@+id/btnChangeDate"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_gravity="center_horizontal"
            android:text="Show Toast" />

        <ListView
            android:id="@+id/listViewBokatas"
            android:layout_width="wrap_content"
            android:layout_height="0dp"
            android:layout_weight="1" >
        </ListView>

        <Button
            android:id="@+id/button1"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:onClick="enviar"
            android:text="@string/bidali" />
    </LinearLayout>

</RelativeLayout>

It's very anoying the fact of wasting 3 hours evrytime trying all posible combinations. I have to say that I've been looking so many times documentation about this, but no success.

Thanks and have a nice code!