Button in a jpanel

HELLO I WANT TO ADD A BUTTON UNDER THE TABLE I HAVE DONE!!!HOW CAN I DO THIS ANY HELPS PLEASE...I HAVE STUCK HERE!!
HERE IS MY CODE, I DIPLAY A TXT FILE IN A TABLE AND I WANT TO ADD A BUTTON UNDER IT!

Code:

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import javax.swing.table.*;
import java.awt.*;
import java.io.*;
import java.util.*;
import javax.swing.*;
import javax.swing.table.DefaultTableModel;
/**
 *
 * @author AcerLaptop
 */
public class Buy extends Items {



    public int countLines(String filename){
        int lines = 0; //mporei na metrhsei mexri "int" grammes (~2.1 dis grammes)
        try {
            // Open the file that is the first command line parameter
            FileInputStream fstream = new FileInputStream(filename);
            // Get the object of DataInputStream
            DataInputStream in = new DataInputStream(fstream);
            BufferedReader br = new BufferedReader(new InputStreamReader(in));
            String strLine;
            //Read File Line By Line
            while ((strLine = br.readLine()) != null) {
                lines++;//metrhths grammwn/eggrafwn
            }
            //close input stream
        } catch (Exception e) {//Catch exception if any
            System.err.println("Error: " + e.getMessage());
        }

    return lines;}



        public String[] showAllRegisteredItems(String filename, int size) {

        String[] temp = new String[size+1]; //mexri "size" kataxwrhseis ma8hmatwn dld (mege8os "int")

        try {
            int x = 0;
            // Open the file that is the first command line parameter
            FileInputStream fstream = new FileInputStream(filename);
            // Get the object of DataInputStream
            DataInputStream in = new DataInputStream(fstream);
            BufferedReader br = new BufferedReader(new InputStreamReader(in));
            String strLine;
            //Read File Line By Line

            while ((strLine = br.readLine()) != null) {
                //System.out.println(strLine.replace("#", " "));
                temp[x] = strLine;
                x++;
            }
            //close input stream
        } catch (Exception e) {//Catch exception if any
            System.err.println("Error: " + e.getMessage());
        }
        return temp;
    }






public JPanel pinakas(String[] pinaka){
        int sr = 0;
        //int ari8mos  =0;

        String[] COLUMN_NAMES = {"Κωδικός", "Ποσότητα", "Τιμή", "Περιγραφή", "Μέγεθος", "Ράτσα"};
//pio panw mporoume na pros8esoume ws prwto column to "#", wste na deixnei ton ari8mo ths ka8e kataxwrhshs
        DefaultTableModel modelM = new DefaultTableModel(COLUMN_NAMES, 0);
        JTable tableM = new JTable(modelM);
        JPanel mainPanel = new JPanel(new BorderLayout());
        JButton button1 = new JButton("Button Text");
        mainPanel.add(new JScrollPane(tableM),BorderLayout.CENTER);
        mainPanel.add(button1);

        // Install a button renderer in the last column


        Display disp = new Display();
        while (pinaka[sr] != null) // !!!!tha ektupwsei kai mia parapanw "/n" logo ths kataxwrhshs prwtou h teleytaiou mahmatos
        {
            String[] temp5 = disp.lineDelimiter(pinaka[sr],6, "#");
            Object[] doge = { temp5[0], temp5[1], temp5[2], temp5[3], temp5[4], temp5[5]};//edw mporoume sthn arxh na valoume to ari8mos gia na fainetai o ari8mos twn kataxwrhsewn
            modelM.addRow(doge);
            sr++;
            //ari8mos++;
        }

        return mainPanel;
    }




      public static void main(String[] args) {


 
 Display disp= new Display();
      Dogs dog=new Dogs();
      int numberofline=disp.countLines("Dogss.txt");
      String[] tempΜ1 = disp.showAllRegisteredItems("Dogss.txt",numberofline);

      JOptionPane.showMessageDialog(null, dog.pinakas(tempΜ1), "Καταχωρημένα Kατοικίδια", JOptionPane.PLAIN_MESSAGE);

 
      }

}

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Suggestions:

• First and foremost, read/study the layout section of the Oracle Java Swing tutorials. Your problem (I believe) is one of not understanding how to use layouts properly, in this situation the BorderLayout since you appear to be adding more than one component to the BorderLayout.CENTER position of a JPanel that uses Borderlayout, and that won't work.
• Please avoid using all-uppercase in your posts here because that is equivalent to SHOUTING.

Much luck!

I say follow Fubarable's suggestions and study the layout and swing tutorials, however if you are desperate to create a working gui (sigh) then i would recommend using a tool like googles window builder :
WindowBuilder User Guide - Google Web Toolkit - Google Code

and if you choose to use this method you should keep watch of the code generated by it and use it as a learning experience. However you will still need a basic grasp of layouts, since it demands you to choose a layout for each component. But at least you can play around with it and visually see the differences.

Other links :
Using Layout Managers (The Java™ Tutorials > Creating a GUI With JFC/Swing > Laying Out Components Within a Container)
A Visual Guide to Layout Managers (The Java™ Tutorials > Creating a GUI With JFC/Swing > Laying Out Components Within a Container)

Working examples:
Layout Example : Layout*«*SWT JFace Eclipse*«*Java
JavaTechniques » GridBagLayout Example: A Simple Form Layout