Comparison of Layout_gravity and gravity. And sometimes Layout_gravity does not work

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:orientation="vertical">

    <!-- The layout_gravity example -->
    <LinearLayout
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:orientation="vertical">
    <!-- notice that the textview's width is as same as its content,
         In this way, we can set "layout_gravity" property 
         to let the textview to "left,right,top,buttom" of its parent
     -->
    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/layout_gravity_test"
        android:layout_gravity="right"
        android:background="#CCCCCC"
        android:textColor="#000000" />
    </LinearLayout>
    
    
    <!-- The gravity example -->
    <LinearLayout
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:orientation="vertical"
    android:layout_marginTop="20dp">
    <!-- notice that the textview's width is match the screen,
         set "gravity property" is set the content of the textview to "left,right,top,bottom" 
     -->
    <TextView
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:text="@string/gravity_test"
        android:gravity="right"
        android:background="#CCCCCC"
        android:textColor="#000000" />
    </LinearLayout>
    
    <!-- The "layout_gravity" property does not work example -->
    <!-- 1.When we set the LinearLayout's orientation to horizontal,
         and set layout_gravity to left or right,in this way,
         "layout_gravity" property will not work.
         
         2.Also,when we set the LinearLayout's orientation to vertical,
         set layout_gravity to top or bottom will not work
          
     -->
    <LinearLayout
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:orientation="horizontal"
    android:layout_marginTop="20dp">
    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/not_work"
        android:layout_gravity="right"
        android:background="#CCCCCC"
        android:textColor="#000000" />
    </LinearLayout>


    <!-- How to solve this problem and not change the LinearLayout's orientation -->
    <!-- wrap the TextView with a LinearLayout -->
    <LinearLayout
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:orientation="horizontal"
    android:layout_marginTop="20dp">
    <LinearLayout
            android:layout_width="fill_parent"
            android:layout_height="wrap_content"
            android:orientation="vertical" >

            <TextView
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_gravity="right"
                android:background="#CCCCCC"
                android:text="@string/solve_problem"
                android:textColor="#000000" />
        </LinearLayout>
    </LinearLayout>
</LinearLayout>

values/strings.xml

<?xml version="1.0" encoding="utf-8"?>
<resources>

    <string name="hello">Hello World!</string>
    <string name="app_name">AndroidLayout</string>
    <string name="not_work">layout gravity does not work!</string>
    <string name="gravity_test">gravity test!</string>
    <string name="layout_gravity_test">layout gravity test!</string>
    <string name="solve_problem">layout gravity work again</string>
</resources>

http://www.waitingfy.com/?p=444