1. Menu

　　Three fundamental types of menus or action presentation on all versions of Android:

　　<1>Option menu and action bar

　　　　Android 2.3 or lower, reveal the options menu panel by pressing MEnu button

　　　　Android 3.0 or higher, options menu are presented by the action bar as a combination of on-screen action items and overflow options

　　<2>Context menu and contextual action mode

　　　　floating menu that appears when user performs a long_click on an element

　　<3>Popup menu

　　　　displays a list of items in a vertical list that‘s anchored to the view that invoked the menu.

2. Defining a Menu in XML

　　define a menu and all its items in an XML menu resource. then inflate the menu resource(load it as a Menu object) in activity or fragment

　　<menu>  a container for menu items.

　　<item>    a single item in a menu

　　　　　　　This element may contain a nested <menu> element in order to create a submenu.

　　<group> invisible container for <item> elements

<?xml version="1.0" encoding="utf-8"?><menu xmlns:android="http://schemas.android.com/apk/res/android">    // File menu    <item android:id="@+id/file"          android:title="@string/file" >        <!-- "file" submenu -->        <menu>            <item android:id="@+id/create_new"                  android:title="@string/create_new" />            <item android:id="@+id/open"                  android:title="@string/open" />        </menu>    </item>       // Edit Menu    <item android:id="@+id/edit"           .......></menu>

3. Creating an options Menu

　　Android 2.3 or lower. the contents of your options menu appear at the bottom of the screen when the user presses the Menu button,

　　Android 3.0 and higher, items from the options menu are available in the action bar.

　　By default, the system places all items in the action overflow, which the user can reveal with the action overflow icon on the right side

　　　　of the action bar

　　To enable quick access to important actions, you can promote a few items to appear in the action bar by adding

　　　　android:showAsAction="ifRoom" to the corresponding <item> elements

5.User Interface/Menu