设置表格行数出错

我在设置表格行数时,下面相应的一部分内容不见了,请教!
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.swing.event.*;
class Win extends JFrame implements ActionListener
{
    JTable ta;
    JButton row,count;
    JLabel jb;
    Object[][] a;
    Object[] name={"语文","数学","英语","总成绩"};
    JTextField jtf;
    int r=1;
    JPanel p;
    Win()
    {
        super("成绩表");
        a=new Object[r][4];
        row=new JButton("确定");
        count=new JButton("总成绩");
        jb=new JLabel("请输入行数");
        jtf=new JTextField(8);
        count.addActionListener(this);
        row.addActionListener(this);
        p=new JPanel();
        p.add(jb);
        p.add(jtf);
        p.add(row);
        p.add(count);
        ta=new JTable(a,name);
        add(p,BorderLayout.SOUTH);
        add(new JScrollPane(ta),BorderLayout.NORTH);
        setBounds(200,200,600,260);
        setVisible(true);
        validate();
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }
    
    public void actionPerformed(ActionEvent e)
    {
        if(e.getSource()==row)
        {
            r=Integer.parseInt(jtf.getText());
            System.out.println(r);
            a=new Object[r][4];
            ta=new JTable(a,name);
            getContentPane().removeAll();
            add(new JScrollPane(ta),BorderLayout.NORTH);
            add(p,BorderLayout.SOUTH);
            validate();
        }
    }
}

public class Exam820
{
    public static void main(String[] ar)
    {
        new Win();
    }
}
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