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

public class Concentration extends JFrame implements ActionListener {

    private JButton buttons[][]=new JButton[4][4];
    int i,j,n;      

    public Concentration() {            
        super ("Concentration");    
        JFrame frame=new JFrame();
        setSize(1000,1000);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        JPanel panel=new JPanel(new GridLayout(4,4));
        panel.setSize(400, 400);            
        for( i=0; i