JAVA GUI : Coba coba Jtable

// paket yang diperlukan
import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;
import javax.swing.table.DefaultTableModel;

class CobaTable extends JFrame{
    // attribut
    //membuat tombol
    JButton tombol1 = new JButton("Tambah baris");
    JButton tombol2 = new JButton("Hapus semua");
    // membuat kolom dan masukan kedalam table
    String[] kolom = {"no","nama","umur","keterangan"};
    JTable jta = new JTable();
    DefaultTableModel model = new DefaultTableModel(null,kolom);
    // menambahkan scroll ke text area
    JScrollPane sp;
    int index = 0;

    //konstruktor
    public CobaTable(){
        //frame seting---------------------------------------------------------------------
        //memanggil konstruktor kelas induk (JFrame)
        super("Contoh Table");
        //seting besar frame 400 x 400 px
        this.setSize(350,250);
        //seting agar bisa ditutup
        this.setDefaultCloseOperation(EXIT_ON_CLOSE);
        //seting kemunculan frame di tengah window
        this.setLocationRelativeTo(null);
        //seting layout frame
        this.getContentPane().setLayout(null);
        //frame seting---------------------------------------------------------------------
        // setbounds(x,y,lebar,tinggi)
        tombol1.setBounds(10,10,150,25);
        tombol2.setBounds(10,40,150,25);
        jta.setModel(model);
        sp  = new JScrollPane(jta);
        sp.setSize(300,100);
        sp.setLocation(10,110);
        //menambahkan komponen ke frame dan seting letak
        this.add(tombol1);
        this.add(tombol2);
        this.add(sp);
        //seting aksi tombol
        tombol1.addActionListener(new ActionListener( ) {
            public void actionPerformed(ActionEvent e){
                // kode disini; cek keadaan check box
                String[] data = {""+(index+1),"halo",""+(index+10),"tambah"};
                model.addRow(data);
                index++;
            }
        });
        tombol2.addActionListener(new ActionListener( ) {
            public void actionPerformed(ActionEvent e){
                // kode disini; cek keadaan check box ganti keadaan chek box
                if(model.getRowCount()>1){
                    model.removeRow(model.getRowCount()-1);
                    index -=1;
                }
                // kalau mau hapus semua
                // model.getDataVector().removeAllElements();
            }
        });
        // menampilkan frame
        show();

    }

    public static void main(String[] args){
        new CobaTable();
    }
}