安卓开发Android Studio新版本menu菜单不显示的问题

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <androidx.appcompat.widget.Toolbar
        android:id="@+id/toolbar"
        android:layout_width="match_parent"
        android:layout_height="?attr/actionBarSize"
        android:background="?attr/colorPrimary"
        android:elevation="4dp"
        android:theme="@style/ThemeOverlay.AppCompat.ActionBar"
        android:popupTheme="@style/ThemeOverlay.AppCompat.Light">

    </androidx.appcompat.widget.Toolbar>

    <TextView
        android:id="@+id/textView"
        android:layout_below="@+id/toolbar"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Hello World!" />

</RelativeLayout>

<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android">
    <item
        android:id="@+id/menu_save"
        android:title="Save"
        android:icon="@drawable/ic_save" />
    <item
        android:id="@+id/menu_delete"
        android:title="Delete"
        android:icon="@drawable/ic_delete" />
</menu>

import androidx.appcompat.app.AppCompatActivity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.TextView;
import androidx.appcompat.widget.Toolbar;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity {

    private Toolbar toolbar;
    private TextView textView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        toolbar = findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        textView = findViewById(R.id.textView);
    }

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

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        int id = item.getItemId();

        if (id == R.id.menu_save) {
            Toast.makeText(this, "Save clicked", Toast.LENGTH_SHORT).show();
            return true;
        } else if (id == R.id.menu_delete) {
            Toast.makeText(this, "Delete clicked", Toast.LENGTH_SHORT).show();
            return true;
        }

        return super.onOptionsItemSelected(item);
    }
}