title: Android helloWorld with button
body: <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>