import java.awt.event.*;
import javax.swing.*;
import java.awt.*;

public class CenterJPanel extends JPanel 
{

// Ecran en attribut
    NorthJPanel ecran;

    String str[] = {"1","2","3","4","5","6","7","8","9",".","0","bs"};
    JButton numbers[] = new JButton[str.length];
    private boolean update = false;
    private JLabel topLabel = new JLabel();
    
    public CenterJPanel()
    {
        initComposants();

    }
    
    public CenterJPanel(JLabel topLabel)
    {
        initComposants();
    }

// Constructeur permettant le passage de l'écran en paramètre
    public CenterJPanel(JLabel topLabel, NorthJPanel ecran)
    {
        initComposants();
        this.ecran = ecran;
    }
    
    private void initComposants()
    {
        
   ..................

import javax.swing.*;
import java.awt.*;

public class CalculatorJFrame extends JFrame 
{
   public static void main(String[] args)
   {
       new CalculatorJFrame();
   }
   
   public CalculatorJFrame()
   {
      setTitle("La calculatrice de Meryama");     
      setLayout(new BorderLayout());
      setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      setLocationRelativeTo(null);
      setResizable(true);
      
      CalculatorType calculator = new CalculatorType();

      NorthJPanel northJPanel = new NorthJPanel();
      JLabel topLabel = northJPanel.getTopLabel();     
      
      getContentPane().add( northJPanel, BorderLayout.NORTH);
// Ajout de l'écran à la création du panel
      getContentPane().add( new CenterJPanel( topLabel, northJPanel ), BorderLayout.CENTER); 
      getContentPane().add( new EastJPanel( topLabel, calculator),  BorderLayout.EAST); 
      getContentPane().add( new SouthJPanel(topLabel, calculator),  BorderLayout.SOUTH); 

      pack();
      setVisible(true);
   }
   
}

import javax.swing.*;
import java.awt.*;

public class CalculatorJFrame extends JFrame 
{
    public static void main(String[] args){
        new CalculatorJFrame();
    }

    public CalculatorJFrame(){
        setTitle("La calculatrice de Meryama"); 
        setLayout(new BorderLayout());
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setLocationRelativeTo(null);
        setResizable(true);

        CalculatorType calculator = new CalculatorType();

        NorthJPanel northJPanel = new NorthJPanel();
        JLabel topLabel = northJPanel.getTopLabel(); 

        getContentPane().add( northJPanel, BorderLayout.NORTH);
        getContentPane().add( new CenterJPanel( northJPanel ), BorderLayout.CENTER); 
        getContentPane().add( new EastJPanel( topLabel, calculator), BorderLayout.EAST); 
        getContentPane().add( new SouthJPanel(topLabel, calculator), BorderLayout.SOUTH); 

        pack();
        setVisible(true);
    }

}

import javax.swing.*;
import java.awt.*;
import java.util.regex.Pattern;

public class NorthJPanel extends JPanel{
    
    private JLabel topLabel = new JLabel("0");

    public NorthJPanel()
    {
        initComposants();
    }

    private void initComposants()
    {
        topLabel.setPreferredSize(new Dimension(340,35));
        this.add(topLabel);
        topLabel.setHorizontalAlignment(JLabel.RIGHT);
        setBorder(BorderFactory.createLineBorder(Color.BLUE, 5)); 
    }

    public JLabel getTopLabel()
    {
        return topLabel ;
    }

// Méthode utilitaire de l'affichage.    
    
    /** Renvoi ce qui est affiché */
    public String getAffichage(){
        return topLabel.getText();
    }
    
    /** Renvoi ce qui est affiché au format numérique */
    public Double getNumericAffichage(){
        return Double.parseDouble(topLabel.getText());
    }
    
    /** Affiche le paramètre */
    public void setAffichage(String str){
        topLabel.setText(str);
    }
    /** Affiche le paramètre */
    public void setAffichage(int num){
        topLabel.setText(Integer.toString(num));
    }
    
    /** Efface l'affichage */
    public void clearAffichage(){
        topLabel.setText("0");
    }
    
    /** Ajoute à la fin de l'affichage */
    public void appendAffichage(String str){
        topLabel.setText(topLabel.getText()+str);
    }
    /** Ajoute à la fin de l'affichage */
    public void appendAffichage(int num){
        topLabel.setText(topLabel.getText()+num);
    }
    
    /** Vérifie si l'affichage est à 0 */
    public boolean isClear(){
        return topLabel.getText().equals("0");
    }
    
    /** Vérifie si l'affichage est un nombre décimal */
    public boolean isDecimal(){
        Pattern p = Pattern.compile(".*\\..*");
        return p.matcher(topLabel.getText()).matches();
    }
    
}

import java.awt.event.*;
import javax.swing.*;
import java.awt.*;

public class CenterJPanel extends JPanel {
    
    String str[] = {"1","2","3","4","5","6","7","8","9",".","0","bs"};
    JButton numbers[] = new JButton[str.length];
    private boolean update = false;
    
    /** Notre écran 
     * @see NorthJPanel*/
    private NorthJPanel screen;

    public CenterJPanel(){
        initComposants();
    }

    public CenterJPanel(NorthJPanel screen){
        initComposants();
        this.screen = screen;
    }

    private void initComposants(){

        setLayout(new GridLayout (4, 3, 5, 5));
        setBackground(Color.CYAN);
        setBorder(BorderFactory.createLineBorder(Color.BLUE, 5)); 

        for (int i = 0 ; i < str.length ; i++){
            numbers[i] = new JButton(str[i]);
            numbers[i].setPreferredSize(new Dimension(65,35)); 
            this.add(numbers[i]);
            if(i == 9) numbers[i].addActionListener(new DecimalListener());
            else if(i == 11) numbers[i].addActionListener(new BSListener());
            else numbers[i].addActionListener(new NumberListener());
        }
    }

    class NumberListener implements ActionListener{
        
        public void actionPerformed (ActionEvent e){
            JButton source = (JButton)e.getSource();
            int number = Integer.parseInt(source.getText());
            switch(number){
                case 0:
                    if(!screen.isClear()) screen.appendAffichage(0);
                    break;
                default:
                    if(screen.isClear()) screen.setAffichage(number);
                    else screen.appendAffichage(number);
                    break;
            }
        }
    } 

    class DecimalListener implements ActionListener{
        public void actionPerformed(ActionEvent e)
        {
            if(!screen.isDecimal()){
                screen.appendAffichage(".");
            }
        }
    }

    class BSListener implements ActionListener{
        
        public void actionPerformed(ActionEvent e)
        {
            screen.clearAffichage();
        }
    }

}