Android之隐式启动通过action跳转到指定的Activity

<LinearLayout xmlns:android="https://schemas.android.com/apk/res/android"
    xmlns:tools="https://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context="com.example.test21.MainActivity" >

    <Button 
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="跳转到第二个Activity"
        android:onClick="btnOnclick"/>
    <Button 
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="跳转到第三个Activity"
        android:onClick="btn2Onclick"/>

</LinearLayout>
<LinearLayout xmlns:android="https://sche