res/menu/main_menu.xml code

<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto">

    <item android:id="@+id/menu_settings"
        android:title="@string/setting"
        app:showAsAction="never">

        <!-- The submenu starts here -->
        <menu>
            <item android:id="@+id/submenu_item1"
                android:title="@string/submenu_item1" />
            <item android:id="@+id/submenu_item2"
                android:title="@string/submenu_item2" />
        </menu>
    </item>
</menu>

String Code:

<resources>
    <string name="app_name">Android Inflate Submenu</string>
    <string name="submenu_item1">Item 1</string>
    <string name="submenu_item2">Item 2</string>
    <string name="setting">setting</string>
</resources>


MainActivity.kt

package com.example.androidinflatesubmenu

import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.view.Menu

class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
    }
    override fun onCreateOptionsMenu(menu: Menu): Boolean {
        val inflater = menuInflater
        inflater.inflate(R.menu.main_menu, menu)
        return super.onCreateOptionsMenu(menu)
    }
}