How to add line divider for menu item Android

All you need to do is define a group with an unique ID, I have checked the implementation if group has different id's it will create a divider.

Example menu, creating the separator:

<menu xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
tools:context=".MainActivity">

<group android:id="@+id/grp1">
    <item
        android:id="@+id/navigation_item_1"
        android:checked="true"
        android:icon="@drawable/ic_home"
        android:title="@string/navigation_item_1" />
</group>

<group android:id="@+id/grp2">
    <item
        android:id="@+id/navigation_item_2"
        android:icon="@drawable/ic_home"
        android:title="@string/navigation_item_2" />
</group>

hope this helps

UPDATE

for menu item may be you can use this

    <menu xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools">
    <item
        android:id="@+id/action_cart"
        android:title="cart"
        android:actionLayout="@layout/cart_update_count"
        android:icon="@drawable/shape_notification"
        app:showAsAction="always"/>
</menu>

and actionLayout file will be

 <LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="wrap_content"
    android:layout_height="match_parent"
    android:orientation="vertical">

    <View
        android:id="@+id/divider"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:background="@drawable/divider"/>

    <TextView
        android:id="@android:id/text"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:background="?android:attr/selectableItemBackground"
        android:gravity="center_vertical"          
        android:textAppearance="?attr/textAppearanceListItemSmall"/>

</LinearLayout>

Make sure to call MenuCompat.setGroupDividerEnabled(menu, true); when you inflate your menu, otherwise groups will not be separated by divider!

Example:

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    getMenuInflater().inflate(R.menu.menu_activity_main, menu);

    MenuCompat.setGroupDividerEnabled(menu, true);

    return true;
}

And make sure to have different groups in your menu xml, e.g.:

    <menu>
        <group android:id="@+id/sorting" >
            <item
                android:id="@+id/action_sorting_new_old"
                android:title="@string/action_sorting_new_old"/>

            <item
                android:id="@+id/action_sorting_a_z"
                android:title="@string/action_sorting_a_z"/>
        </group>

        <group android:id="@+id/settings">
            <item
                android:id="@+id/action_settings"
                android:title="@string/action_settings"/>
        </group>
    </menu>

If you need add vertical lines in the Overflow menu (the ellipses at far right):

Assuming you have added a new menu resource file, do the following.

1. Group menu items and assign a unique ID to each group

   <group android:id="@+id/group1">
     <item
        android:id="@+id/action_about"
        android:orderInCategory="100"
        android:title="@string/about"
        app:showAsAction="never" />
    </group>
   
   <group android:id="@+id/group2">
    <item
        android:id="@+id/action_settings"
        android:orderInCategory="200"
        android:title="@string/settings"
        app:showAsAction="never" />
   </group>
   

2. When you inflate the menu in the Activity's oncreate method:

   getMenuInflater().inflate(R.menu.main, menu);//inflate menu
   
   MenuCompat.setGroupDividerEnabled(menu, true);//add horizontal divider
   

Old question, but the above answers didn't work for me (and I object to adding "groups" for single items). What did work was adding a style element as follows:

    <!-- Base application theme. -->
    <style name="AppTheme" parent="Theme.AppCompat.DayNight.DarkActionBar"> <!--  .Light.DarkActionBar"> -->
        <!-- Customize your theme here. -->
        <item name="colorPrimary">@color/colorPrimary</item>
        <item name="colorPrimaryDark">#17161B</item>
        <item name="colorAccent">@color/colorAccent</item>
        <item name="android:dropDownListViewStyle">@style/PopupMenuListView</item>//<-add this
    </style>

that references

    <style name="PopupMenuListView" parent="@style/Widget.AppCompat.ListView.DropDown">
        <item name="android:divider">#dddddd</item>
        <item name="android:dividerHeight">1dp</item>
    </style>

in the same res/values/styles.xml file. Hope it helps!