Set the default button in Java

/*from   w w w .  j av a  2  s .  c  om*/
    
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JRootPane;

public class Main {
  public static void main(String args[]) {
    JFrame frame = new JFrame("DefaultButton");
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    
    JButton button4 = new JButton("AAA");
    frame.add(button4);
    frame.add(new JButton("BBB"));
    
    JRootPane rootPane = frame.getRootPane();
    rootPane.setDefaultButton(button4);
    frame.setSize(300, 200);
    frame.setVisible(true);
  }
}