Android: case:R.id.Action_item1: getting CONSTANT expression required error

Question

I checked the item it is in menu res under res file.
I was trying to build a toolbar.I created menu file under res file which contain item. I was trying to use switch to toogle between items. I was id names as case as i was doing I am constant expression required error.

import androidx.appcompat.app.AppCompatActivity;
import androidx.appcompat.widget.Toolbar;
import android.annotation.SuppressLint;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.Toast;
import com.rabbani.toolbar.R;

    public class MainActivity extends AppCompatActivity {

    @Override
 
protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar =findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        getSupportActionBar().setDisplayHomeAsUpEnabled(true);
    }
    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        getMenuInflater().inflate(R.menu.menu_item_list, menu);
        return true;
    }
    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        switch (item.getItemId()) {
            case R.id.action_item1:
                // Handle item 1 click
                Toast.makeText(this, "You selected muffin", Toast.LENGTH_LONG).show();
                return true;


            // Add more cases for additional menu items
            default:
                return super.onOptionsItemSelected(item);
        }
    }
}

<menu xmlns:android="http://schemas.android.com/apk/res/android"
      xmlns:app="http://schemas.android.com/apk/res-auto">
    <item
        android:id="@+id/action_item1"
        android:title="Muffin"
        android:icon="@drawable/muffin"
        app:showAsAction="ifRoom" />
<menu/>

``