How to add dividers between specific menu items?

As of SDK version 28, you can use menu.setGroupDividerEnabled(boolean). If you're using ContextMenu this is only supported on SDK 28+, but MenuCompat offers backwards compatibility when used in onCreateOptionsMenu().

This will add a divider between the actions for each different groupId, shown as 0 and 1 below:

menu.add(0, getAdapterPosition(), action1, R.string.action1);
menu.add(1, getAdapterPosition(), action2, R.string.action2);
menu.setGroupDividerEnabled(true); 

// Or for MenuCompat < SDK 28:
MenuCompat.setGroupDividerEnabled(menu, true);

Documentation here: https://developer.android.com/reference/android/view/Menu#setGroupDividerEnabled(boolean)

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EDIT: Sample code as requested by asker:

Here's the code I am currently using in my app, located in a RecyclerView Adapter. It should work with your menu implementation as well. Since you're defining the menu by XML, the below will also work for you as long as you reference the menu resource. Here's what the result looks like:

Override onCreateContextMenu or your menu's relevant onCreate.. method like so within the:

@Override
public void onCreateContextMenu(ContextMenu menu, View v, ContextMenu.ContextMenuInfo menuInfo) {
    menu.setHeaderTitle(getStr(R.string.actions_title));

    // Groups 0 and 1, first parameter for menu.add()
    menu.add(0, getAdapterPosition(), 0, R.string.homescreen);
    menu.add(0, getAdapterPosition(), 1, R.string.lockscreen);
    menu.add(0, getAdapterPosition(), 2, R.string.wpLocation_both);
    menu.add(1, getAdapterPosition(), 3, R.string.action_download);

    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.P) {
        menu.setGroupDividerEnabled(true);  // This adds the divider between groups 0 and 1, but only supported on Android 9.0 and up.
    }
}

You should use action layout

<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=".LandingActivity">
    <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 then the action layout can have the textview with divider.

<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>

then you can add the click listener in code