"class extends activity{}" throws "unfortunately app has stopped" while prefixing with "public" doesn't

I have a small app like this

package com.ibuk.app;

import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

public class HelloActivity extends Activity
{

    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);

        setContentView(R.layout.main);        
    }
}

That one works. But if I don't prefix the Activity class with public it throws the error Unfortunately app has stopped. Shouldn't a class be public by default? What's going on?