Eclipse not finding Main_Activity

Question

For some reason when I design this Android app, the .java file is not finding the main_activity and will not run the app.
Where it says: R.activity.main_activity there is a red line under "Main_Activity".
Does anyone know why, how can I fix this?
Thanks!

package com.test.helloworld;

import android.R;
import android.os.Bundle;
import android.app.Activity;
import android.view.Menu;
import android.widget.EditText;

public class MainActivity extends Activity {

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

        EditText num = (EditText)findViewById(R.id.num);

    }

Answer 1

you imported android.R; , the resource-id file for the android SDK. you will probably want to import your own R.java that was generated for you.

Answer 2

Import as

import android.com.test.helloworld.R;

instead of

import android.R;

Also clean your project and also run it again.