R cannot be resolved to a variable

import java.util.HashSet;
import java.util.Iterator;
import java.util.Map;
import java.util.Map.Entry;
import java.util.Set;
import java.util.TreeMap;

public class MainClass {
	public static void main(String[] arg) {

		// english;germany dictionary
		String[] arrayOfString = { "one;eine", "two;zwei", "two sets of;zwei"
				, "three;drei", "four;vier" };

		Map<String, String> map = new TreeMap<String, String>();

		for (String s : arrayOfString) {
			String[] array = s.split(";");
			String sKey = "", sValue = "";
			if (array.length > 1) {
				sKey = array[0];
				sValue = array[1];
				map.put(sKey, sValue);
			}
		}

		if (map.containsValue("zwei")) {
			Set<String> references = getKeysByValue(map, "zwei");
			 Iterator<String> it = references.iterator();
			    while (it.hasNext()) {
			      String key = (String) it.next();
			      String value = map.get(key);
			      System.out.println(key + " = " + value);
			    }
		}

	}

	public static <T, E> Set<T> getKeysByValue(Map<T, E> map, E value) {
		Set<T> keys = new HashSet<T>();
		for (Entry<T, E> entry : map.entrySet()) {
			if (entry.getValue().equals(value)) {
				keys.add(entry.getKey());
			}
		}
		return keys;
	}
}
/*
two = zwei
two sets of = zwei
 */

// start_dark.png is stored in path	/package_name/res/drawable/start_dark.png
Drawable dw = getApplicationContext().getResources().getDrawable(R.drawable.start_dark);
Button hButtonStart = (Button)findViewById(R.id.buttonStart);
hButtonStart.setCompoundDrawablesWithIntrinsicBounds(dw, null, null, null);

public class MainActivity extends ListActivity {
    private class Sample {
        private CharSequence title;
        private Class<? extends Activity> activityClass;

        public Sample(int titleResId, Class<? extends Activity> activityClass) {
            this.activityClass = activityClass;
            this.title = getResources().getString(titleResId);
        }

        @Override
        public String toString() {
            return title.toString();
        }
    }

    private static Sample[] mSamples;

    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        // Instantiate the list of samples.
        mSamples = new Sample[]{
                new Sample(R.string.title_first_app, FirstActivity.class),
                new Sample(R.string.title_second_app, SecondActivity.class),
                new Sample(R.string.title_third_app, ThirdActivity.class),
        };

        setListAdapter(new ArrayAdapter<Sample>(this,
                android.R.layout.simple_list_item_1,
                android.R.id.text1,
                mSamples));
    }

    @Override
    protected void onListItemClick(ListView listView, View view, int position, long id) {
        // Launch the sample associated with this list position.
        startActivity(new Intent(MainActivity.this, mSamples[position].activityClass));
    }
}