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

public class Calculator {
    private JLabel screen;
    private boolean start;
    private String operator;
    private double result;
    public Calculator() {
        operator = "=";
        start = true;
        JFrame frame = new JFrame();
        screen = new JLabel("0",JLabel.RIGHT);
        screen.setFont(new Font("Open Sans",Font.BOLD, 20));
        frame.add(screen, BorderLayout.NORTH);
        frame.add(buttonsPanel(),BorderLayout.CENTER);
        frame.setSize(300, 300);
        frame.setLocationRelativeTo(null);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
    }
    private JPanel buttonsPanel(){
        JPanel panel = new JPanel();
        panel.setLayout(new GridLayout(4,4));
        String buttons[] = {
                "7", "8", "9", "/",
                "4", "5", "6", "*",
                "1", "2", "3", "-",
                "0", ".", "=", "+"
        };
        for (int i = 0; i < buttons.length; i++) {
            panel.add(addButton(buttons[i]));
        }
        return panel;
    }
    private JButton addButton(String btnName){
        JButton btn = new JButton(btnName);
        btn.setFont(new Font("Open Sans", Font.BOLD, 16));
        btn.addActionListener(calcListener);
        return btn;
    }
    private ActionListener calcListener = new ActionListener() {
        @Override
        public void actionPerformed(ActionEvent e) {
            String v = ((JButton)e.getSource()).getText();
            System.out.println(v);
            if("/*+-=".indexOf(v) >= 0){
                calculate(v);
            }else{
                insertNumber(v);
            }
        }
    };
    public void calculate(String s){
//        System.out.println("calculate: " + s);
        double num = Double.parseDouble(screen.getText());
        if (operator.equals("=")) result = num;
        if (operator.equals("=")) result += num;
        if (operator.equals("=")) result -= num;
        if (operator.equals("=")) result *= num;
        if (operator.equals("=")) result /= num;


        screen.setText("" + result);
        operator = s;
        start = true;
    }
    public void insertNumber(String s){
//        System.out.println("insertNumber: " + s);
        if(start){
            screen.setText("");
            start = false;
        }
        screen.setText(screen.getText() + s);
    }

    public static void main(String[] args) {
        Calculator calc = new Calculator();
    }
}