Android Menu not working - Please help

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Java

Mobile

Android

This code brings up an error and I am unsure why:

public boolean onOptionsItemSelected(MenuItem item) {
        switch (item.getItemId()) {
        case R.id.Friends:
            // TODO
            break;

The error says R.id cannot be resolved
It brings up 2 fixes, but they both edit the R.java file which I have been instructed not to mess about with.

I am just wondering what I can do to resolve it.
Please help, thanks.

My Code:

XML file:

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

<item android:id="@+id/friends"
    android:title="Friends"
    android:icon="@drawable/friends" >
</item>

<item android:id="@+id/events"
    android:title="Events"
    android:icon="@drawable/events" >
</item>

<item android:id="@+id/Profile"
    android:title="Profile"
    android:icon="@drawable/profile" >
</item>

<item android:id="@+id/Close"
    android:title="Close"
    android:icon="@drawable/close" >
</item>
</menu>

Java File:

package com.example.helloandroid;
import android.app.Activity;
import android.os.Bundle;
import android.widget.TextView;
import android.view.Menu;
import android.view.MenuInflater;
import android.view.MenuItem;
public class HelloAndroid extends Activity {
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
    }


    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        MenuInflater inflater = getMenuInflater();
        inflater.inflate(R.menu.app_menu, menu);
        return true;
    }
    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        switch (item.getItemId()) {
        case R.id.Friends:
            // TODO
            break;
        case R.id.Events:
            // TODO
            break;
            case R.id.Profile:
            // TODO
            break;
        case R.id.Close:
            // TODO
            break;
        default:
            return super.onOptionsItemSelected(item);
        }
        return true;
    }
}