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


public class JConcentration
{
    private JFrame frame;
    private JPanel panel;

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

    public JConcentration()
    {
        this.frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        this.panel = new JPanel(new GridLayout(4,5));
        addButtons();
        frame.getContentPane().add(panel);
        frame.setSize(600,600);
        frame.setVisible(true);
    }

    public void addButtons()
    {
        for(int i = 0; i < 20; i++)
        {
            JButton button = new Card();
            Card.button.setBackground(Color.red);
            Card.button.setActionListener(e ->
            { button.setText("5");
            });
            this.panel.add(button);
        }
    }

    class Card{
         private int number;
                private JButton button;

                public Card(int number)
                {
                    this.number = number;
                    this.button = new JButton();
                    this.button.setBackground(Color.red);
                }

                public final int getNumber()
                {
                    return number;
                }

                public final JButton getButton()
                {
                    return button;
                }

        }
}