Android helloWorld with button
<p>I'm new to Android. I've download some code to run but there are problems:</p>

<pre><code>package t.t.t4;

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

public class Test4Activity extends Activity {
    /** Called when the activity is first created. */
    OnClickListener listener1 = null;
    Button button1;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        listener1 = new OnClickListener() {
            public void onClick(View v) {
                TextView text_view = (TextView) findViewById(R.id.TextView01);
                CharSequence text_view_old = text_view.getText();
                text_view.setText("Before: "+ text_view_old +"\nAdded information: Hello World again !");
            }
        };

        setContentView(R.layout.main);
        button1 = (Button) findViewById(R.id.Button01);
        button1.setOnClickListener(listener1);
    }
} 
</code></pre>

<p><code>R.id.TextView01</code> is the line that had an error. What is the meaning of R and R.id?</p>
