¿La tabla de clasificación es incorrecta cuando se presiona el botón de clasificación más de una vez?

Tengo un problema aquí. Creo un programa para agregar datos a la tabla y lo ordeno cuando presiono el botón. cuando presiono el botón de ordenar una vez, no está mal. Pero cuando presiono de nuevo está mal. ¿entonces por qué? por favor, ayúdame. este es el codigo

Clase nama

public class Nama  {
    private String nama;
    private int index;

    public Nama(String n,int i){
        nama=n;index=i;
    }
    void setData(String n,int i){
        nama=n;index=i;
    }
    String nama(){
        return nama;
    }
    int ind(){
        return index;
    }


    public String toString(){
        return(String.format("%s %d", nama,index));
    }


}

MergeSort Class

    import java.util.*;
    public class MergeSortS{


        public void merge_sort(int low,int high,Nama [] a){
            int mid;
            if(low<high) {
                   mid=(low+high)/2;
                   merge_sort(low,mid,a);
                   merge_sort(mid+1,high, a);
                   merge(low,mid,high,a);
            }
        }
        public void merge(int low,int mid,int high,Nama [] a){
            int h,i,j,k;
            Nama b[]=new Nama[50];
            h=low;
            i=low;
            j=mid+1;

            while((h<=mid)&&(j<=high)){
                if(a[h].nama().compareToIgnoreCase(a[j].nama())<0){
                    b[i]=a[h];
                    h++;
                }

                else{
                    b[i]=a[j];
                    j++;
                }

                i++;
            }
            if(h>mid){
                for(k=j;k<=high;k++){
                    b[i]=a[k];
                    i++;
                }
            }
            else{
                for(k=h;k<=mid;k++){
                    b[i]=a[k];
                    i++;
                }
            }
            for(k=low;k<=high;k++) a[k]=b[k];
        }
        public MergeSortS() {
        }

    }

Clase de panel

/*
 * To change this template, choose Tools | Templates
 * and open the template in the editor.
 */


import java.awt.Dimension;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.Vector;
import javax.swing.*;
import javax.swing.table.DefaultTableModel;

/**
 *
 * @author Kareem
 */
public class Panel extends JPanel implements ActionListener{
    JTable table;
    JTextField tf1,tf2,tf3,tf4;
    JButton b1,b2,b3,b4,b5,b6,b7;
    Vector rows,columns,temp;
    DefaultTableModel tabModel;
    String[]data = new String[3];
    String [] column = {"Nama", "Nim", "IP"};
    Nama[] n,nim,ip;
    MergeSortS mS;
    int c=0,index=0;

    public Panel(){
        this.setBounds(0,0,1280, 800);
        this.setLayout(null);
        inaTf();
        inaTab();
        inaBut();

        n= new Nama[50];
        nim= new Nama[50];
        ip= new Nama[50];
        mS=  new MergeSortS();
        this.setVisible(true);
    }

    public void inaTab(){
                        rows=new Vector();
                columns= new Vector();
                temp= new Vector();


                addColumns(column);

            tabModel=new DefaultTableModel();
            tabModel.setDataVector(rows,columns);

            table = new JTable(tabModel);
            table.setPreferredScrollableViewportSize(new 
               Dimension(500, 70));
            table.setFillsViewportHeight(true);
            table.setBounds(100,100,200,200);
            JScrollPane scroll = new JScrollPane(table);
            scroll.setBounds(50,50,400,400);
            add(scroll);

    }
    public void inaBut(){
        b1=new JButton("add Row");
        b1.setBounds(50,600,90,25);
        add(b1);
        b1.addActionListener(this);

        b2=new JButton("Delete Row");
        b2.setBounds(170,600,90,25);
        add(b2);
        b2.addActionListener(this);

        b3=new JButton("SortName");
        b3.setBounds(290,600,120,25);
        add(b3);
        b3.addActionListener(this);
        b5=new JButton("SortNim");
        b5.setBounds(290,650,120,25);
        add(b5);
        b5.addActionListener(this);
        b6=new JButton("SortIP");
        b6.setBounds(290,700,120,25);
        add(b6);
        b6.addActionListener(this);

        b4=new JButton("RESET");
        b4.setBounds(170,650,90,25);
        add(b4);
        b4.addActionListener(this);
    }

    public void inaTf(){
        tf1=new JTextField();
        tf1.setBounds(640,50,90,25);
        add(tf1);

        JLabel l1= new JLabel("Nama \t: ");
        l1.setBounds(530,50,90,25);
        add(l1);

        tf2=new JTextField();
        tf2.setBounds(640,80,90,25);
        add(tf2);

        JLabel l2= new JLabel("Nim     : ");
        l2.setBounds(530,80,90,25);
        add(l2);

        tf3=new JTextField();
        tf3.setBounds(640,110,90,25);
        add(tf3);

        JLabel l3= new JLabel("IPK      : ");
        l3.setBounds(530,110,90,25);
        add(l3);

        tf4=new JTextField();
        tf4.setBounds(640,140,90,25);
        add(tf4);

        JLabel l4= new JLabel("Hapus Baris ke ");
        l4.setBounds(530,140,120,25);
        add(l4);
    }

        public void addRow() 
        {
                Vector r;
                r = createBlankElement();
                rows.addElement(r);

                table.addNotify();
        }
        public void addRow(String [] data) 
        {       
                Vector r=new Vector();
                r = isi(data);
                rows.addElement(r);

                table.addNotify();
        }

        public Vector createBlankElement() 
        {
                Vector t = new Vector();
                t.addElement((String) " ");
                t.addElement((String) " ");
                t.addElement((String) " ");

                return t;
        }
        public Vector isi(String[] data) {
                Vector t = new Vector();

                    for(int j=0;j<3;j++){
                        t.addElement((String) data[j]);
                    }


                return t;
        }
        public void addColumns(String[] colName) {
                for(int i=0;i<colName.length;i++)
                columns.addElement((String) colName[i]);
        }
        void deleteRow(int index) {
             if(index!=-1) { 
                rows.removeElementAt(index);
                table.addNotify();
               }


        }



    @Override
    public void actionPerformed(ActionEvent e) {
        try{
        if(e.getSource()==b1){
            data[0]=tf1.getText()+" "+index;
            n[index]=new Nama(data[0],index);
            data[1]=tf2.getText();
            nim[index]=new Nama(data[1],index);
            data[2]=tf3.getText()+rows.size();
            ip[index]=new Nama (data[2],index);
            c=c+1;
            index=index+1;
            addRow(data);
        }
        if(e.getSource()==b2){
            int i;
            i=Integer.parseInt(tf4.getText());

            deleteRow(i);

//            for(;i<rows.size();i++){
//                n[i]=n[i+1];
//            }

        }
        if(e.getSource()==b3){

          mS.merge_sort(0, rows.size()-1, n);
          temp.setSize(rows.size());
          for(int i=0;i<index;i++){
                temp.set(i, rows.get(n[i].ind()));

           }
           for(int i=0;i<index;i++){
                rows.set(i, temp.get(i));
           }
        }

        if(e.getSource()==b4){
            rows.setSize(0);
            temp.setSize(0);
            for(int i=0;i<index;i++)n[i]=null;
            index=0;
        }
        if(e.getSource()==b5){
             mS.merge_sort(0, rows.size()-1, nim);
             temp.setSize(rows.size());
           for(int i=0;i<rows.size();i++){
                temp.set(i, rows.get(nim[i].ind()));

           }

           for(int i=0;i<rows.size();i++){
                rows.set(i, temp.get(i));
           }
        }
        if(e.getSource()==b6){
             mS.merge_sort(0, rows.size()-1, ip);
             temp.setSize(rows.size());
           for(int i=0;i<rows.size();i++){
                temp.add(i, rows.get(ip[i].ind()));

           }

           for(int i=0;i<rows.size();i++){
                rows.set(i, temp.get(i));
           }
        }
           repaint();


        }
        catch (Throwable t){
         JOptionPane.showMessageDialog(null, "AAAAAA");   
        }

    }

}

Clase de marco

/*
 * To change this template, choose Tools | Templates
 * and open the template in the editor.
 */

import javax.sound.sampled.AudioInputStream;
import javax.sound.sampled.AudioSystem;
import javax.sound.sampled.Clip;
import javax.swing.JFrame;
import javax.swing.JOptionPane;
import javax.swing.JScrollPane;
import javax.swing.JTable;
import javax.swing.table.DefaultTableModel;

/**
 *
 * @author Kareem
 */
public class Frame extends JFrame {

    public Frame(){
        super("Penghitung Gaji");
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        this.setLayout(null);
        this.setSize(1280, 800);
        this.getContentPane().add(new Panel());
        this.setVisible(true);

    }

    public static void main(String[] args) {
        Frame frame = new Frame();
    }
}

Gracias. Y lo siento por mi mal inglés.

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