问JTable工作，但JTable子类不工作

EN

package com.grantbroadwater.signInAssistant.view;

import java.awt.BorderLayout;
import java.awt.Dimension;
import java.awt.GridLayout;
import java.awt.FlowLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JTable;
import javax.swing.JTextField;

import org.junit.Test;

import com.grantbroadwater.school.Student;

public class SignInSheetTableTest implements ActionListener{

static SignInSheetTableModel model;
static JTextField tfFirst, tfLast;
static JTable table;

public SignInSheetTableTest() {
    // TODO Auto-generated constructor stub
}

private static JPanel createContentPanel(){
    JPanel panel = new JPanel(new GridLayout(1, 0));

    table = new JTable(new SignInSheetTableModel());
    model = (SignInSheetTableModel)table.getModel();
    table.setPreferredScrollableViewportSize(new Dimension(500, 80));
    table.setFillsViewportHeight(true);

    JScrollPane scrollPane = new JScrollPane(table);

    panel.add(scrollPane);

    return panel;
}

private static void createAndShowGUI(){
    JFrame frame = new JFrame("Table Test");
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    JPanel panel = SignInSheetTableTest.createContentPanel();
    panel.setOpaque(true);

    JPanel entryPanel = new JPanel(new FlowLayout(FlowLayout.CENTER));
    tfFirst = new JTextField(8);
    tfLast = new JTextField(8);
    JButton btnAdd = new JButton("Add");
    btnAdd.addActionListener(new SignInSheetTableTest());
    JButton btnDelete = new JButton("Delete");
    btnDelete.addActionListener(new ActionListener() {

        @Override
        public void actionPerformed(ActionEvent e) {
            int rowIndex = table.getSelectedRow();
            model.deleteRow(rowIndex);
        }
    });

    entryPanel.add(tfFirst);
    entryPanel.add(tfLast);
    entryPanel.add(btnAdd);
    entryPanel.add(btnDelete);

    JPanel housingPanel = new JPanel(new BorderLayout());
    housingPanel.add(entryPanel, BorderLayout.NORTH);
    housingPanel.add(panel, BorderLayout.CENTER);

    frame.setContentPane(housingPanel);

    frame.pack();
    frame.setVisible(true);
}

public static void main(String[] args) {
    javax.swing.SwingUtilities.invokeLater(new Runnable() {
        @Override
        public void run() {
            createAndShowGUI();
        }
    });
}

@Override
public void actionPerformed(ActionEvent e) {
    String first = tfFirst.getText();
    String last = tfLast.getText();

    Student s = new Student(first, last, "11111");

    model.addStudent(s);

    table.scrollRectToVisible(table.getCellRect(table.getRowCount() - 1, 0, true));
}

@Test
public void test(){
    main(null);

    try {
        Thread.sleep(15000);
    } catch (InterruptedException e) {}

}

}

package com.grantbroadwater.signInAssistant.view;

import javax.swing.JTable;

import com.grantbroadwater.school.Student;
import com.grantbroadwater.util.Log;
import com.grantbroadwater.util.Log.LogType;

public class SignInSheetTable extends JTable{

private static final long serialVersionUID = 1L;
private SignInSheetTableModel model;

public SignInSheetTable() {
    super(new SignInSheetTableModel());
    model = (SignInSheetTableModel) super.getModel();

    this.setFillsViewportHeight(true);
    new Log(LogType.DEBUG, "new Sign In sheet");
}

public SignInSheetTableModel getModel() {
    return model;
}

public void addStudent(Student student) {
    model.addStudent(student);
}

public void deleteRow(int rowIndex) {
    model.deleteRow(rowIndex);
}

}