对于下列代码,哪个叙述是正确的? import java.awt.*; import javax.swing.*; import java.awt.event.*; public class Win extends JFrame { JButton b; public Win() { setLayout(new FlowLayout()); b = new JButton("yes"); add(b); b.addActionListener(new ActionListener(){ public void actionPerformed(ActionEvent e) { b.setText("ok"); } }); setVisible(true); setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); } public static void main(String args[]){ new Win(); } }