import javax.swing.JFrame;
import javax.swing.JTextField;
import javax.swing.JPanel;
import javax.swing.JLabel;
import javax.swing.JButton;
import java.awt.Container;
import java.awt.BorderLayout;
import java.awt.FlowLayout;
import java.awt.Color;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;

/**
 A simplified calculator.
 The only operations are addition and subtraction.
*/
public class Calculator extends JFrame
                        implements ActionListener
{
    public static final int WIDTH = 400;
    public static final int HEIGHT = 200;
    public static final int NUMBER_OF_DIGITS = 30;

    private JTextField ioField;
    private double result = 0.0;

    public static void main(String[] args)
    {
        Calculator aCalculator = new Calculator( );
        aCalculator.setVisible(true);
    }

    public Calculator( )
    {
        setTitle("Simplified Calculator");
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setSize(WIDTH, HEIGHT);
        Container contentPane = getContentPane( );
        contentPane.setLayout(new BorderLayout( ));

        JPanel textPanel = new JPanel( );
        textPanel.setLayout(new FlowLayout( ));
        ioField =
             new JTextField("Enter numbers here.", NUMBER_OF_DIGITS);
        ioField.setBackground(Color.WHITE);
        textPanel.add(ioField);
        contentPane.add(textPanel, BorderLayout.NORTH);

        JPanel buttonPanel = new JPanel( );
        buttonPanel.setBackground(Color.BLUE);
        buttonPanel.setLayout(new FlowLayout( ));

        JButton addButton = new JButton("+");
        addButton.addActionListener(this);
        buttonPanel.add(addButton);
        JButton subtractButton = new JButton("-");
        subtractButton.addActionListener(this);
        buttonPanel.add(subtractButton);
        JButton resetButton = new JButton("Reset");
        resetButton.addActionListener(this);
        buttonPanel.add(resetButton);

        contentPane.add(buttonPanel, BorderLayout.CENTER);
    }


    public void actionPerformed(ActionEvent e)
    {
        try
        {
            assumingCorrectNumberFormats(e);
        }
        catch (NumberFormatException e2)
        {
            ioField.setText("Error: Reenter Number.");
        }
    }


    //Throws NumberFormatException.
    public void assumingCorrectNumberFormats(ActionEvent e)
    {
        String actionCommand = e.getActionCommand( );

        if (actionCommand.equals("+"))
        {
            result = result + stringToDouble(ioField.getText( ));
            ioField.setText(Double.toString(result));
        }
        else if (actionCommand.equals("-"))
        {
            result = result - stringToDouble(ioField.getText( ));
            ioField.setText(Double.toString(result));
        }
        else if (actionCommand.equals("Reset"))
        {
            result = 0.0;
            ioField.setText("0.0");
        }
        else
            ioField.setText("Unexpected error.");
     }


    //Throws NumberFormatException.
    private static double stringToDouble(String stringObject)
    {
        return Double.parseDouble(stringObject.trim( ));
    }

}