Java Swing - 如何确保索引可见的JList

import java.awt.GridLayout;

import javax.swing.JFrame;
import javax.swing.JList;
import javax.swing.JScrollPane;
import javax.swing.event.ListSelectionEvent;
import javax.swing.event.ListSelectionListener;

public class Main extends JFrame implements ListSelectionListener {
  public Main() {
    setLayout(new GridLayout(1, 2));
    String[] numbers = { "one", "two", "three", "four", "five", "six", "seven" };
    JList<String> list = new JList<>(numbers);
    list.setVisibleRowCount(1);
    list.addListSelectionListener(this);
    JScrollPane s = new JScrollPane(list);
    s.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_NEVER);
    s.setHorizontalScrollBarPolicy(JScrollPane.HORIZONTAL_SCROLLBAR_NEVER);
    add(s);
    list.setSelectedIndex(3);
  }

  @Override
  public void valueChanged(final ListSelectionEvent e) {
    JList list = (JList) e.getSource();
    list.ensureIndexIsVisible(list.getSelectedIndex());
  }

  public static void main(String[] args) {
    Main frame = new Main();
    frame.setDefaultCloseOperation(EXIT_ON_CLOSE);
    frame.setLocationRelativeTo(null);
    frame.pack();
    //frame.setBounds(20,20,300,300);
    frame.setVisible(true);
  }
}