setSupportActionBar removes menu items

Apologies if I am not explaining the issue I am having clearly enough.

I have a project that utilizes a MaterialToolbar, it references a menu.xml for its appearances. The menu holds potential icons I wish to use (favourite, delete etc).

In my code, however when initializing setSupportActionBar(), it overrides the reference I have set, the icons no longer appear aside from the Navigation icon.

I require setSupportActionBar() for my project, is there a way I can retain setSupportActionBar() while keeping the appearance of top_app_bar.xml? Or at the very least the icon initialized in the menu item?

activity_main_menu.xml

<com.google.android.material.appbar.AppBarLayout
    android:layout_width="match_parent"
    android:layout_height="wrap_content">
    <com.google.android.material.appbar.MaterialToolbar
        android:id="@+id/topAppBar"
        android:layout_width="match_parent"
        android:layout_height="?attr/actionBarSize"
        app:menu="@menu/top_app_bar"
        app:navigationIcon="@drawable/ic_menu_24dp"
        style="@style/Widget.MaterialComponents.Toolbar.Primary"/>
</com.google.android.material.appbar.AppBarLayout>

top_app_bar.xml

<?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/delete"
        android:icon="@drawable/ic_delete_24dp"
        android:title="test"
        app:showAsAction="ifRoom"
        />
</menu>

<?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/delete"
        android:icon="@drawable/ic_delete_24dp"
        android:title="test"
        app:showAsAction="ifRoom"
        />
</menu>

MainMenu.java

MaterialToolbar topAppBar = findViewById(R.id.topAppBar);
setSupportActionBar(topAppBar);
getSupportActionBar().setDisplayShowTitleEnabled(false);

It is expected behaviour. When you set an action bar to an Activity it is the role of Activity to decide what menu will be displayed.

Here is the documentation and method declaration of setSupportActionBar from appcompat-1.0.0 dependency. You can read it all but I explicitly selected a single sentence from the java-doc and mentioned it below this snippet.

/**
 * Set a {@link Toolbar} to act as the {@link ActionBar} for this delegate.
 *
 * <p>When set to a non-null value the {@link #getSupportActionBar()} ()} method will return
 * an {@link ActionBar} object that can be used to control the given toolbar as if it were
 * a traditional window decor action bar. The toolbar's menu will be populated with the
 * Activity's options menu and the navigation button will be wired through the standard
 * {@link android.R.id#home home} menu select action.</p>
 *
 * <p>In order to use a Toolbar within the Activity's window content the application
 * must not request the window feature
 * {@link AppCompatDelegate#FEATURE_SUPPORT_ACTION_BAR FEATURE_SUPPORT_ACTION_BAR}.</p>
 *
 * @param toolbar Toolbar to set as the Activity's action bar, or {@code null} to clear it
 */
public abstract void setSupportActionBar(@Nullable Toolbar toolbar);

The emphasis is on this sentence:

 * ... The toolbar's menu will be populated with the
 * Activity's options menu and the navigation button will be wired through the standard
 * {@link android.R.id#home home} menu select action ...

It means you have to override onCreateOptionsMenu method in order to create menu in the activity for the action bar:

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    getMenuInflater().inflate(R.menu.resource_id, menu);
    return true;
}

By default, if you do not override this method, the menu will be populated
> ... with standard system menu items.

Source of the quote (note, I removed explanations from java-doc of onCreateOptionsMenu that are irrelevant to this issue):

/**
 * Initialize the contents of the Activity's standard options menu.  You
 * should place your menu items in to <var>menu</var>.
 *
 * <p>This is only called once, the first time the options menu is
 * displayed.
 *
 * <p>The default implementation populates the menu with standard system
 * menu items. 
 *
 * @param menu The options menu in which you place your items.
 *
 * @return You must return true for the menu to be displayed;
 *         if you return false it will not be shown.
 */
public boolean onCreateOptionsMenu(Menu menu) {
    if (mParent != null) {
        return mParent.onCreateOptionsMenu(menu);
    }
    return true;
}