import java.awt.BorderLayout;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.*;

public class GrowthGUI extends JFrame
{
    // fields
    JLabel labelBirth = new JLabel("Birth rate: ");
    JLabel labelDeath = new JLabel("Death rate: ");
    JLabel labelYears = new JLabel("Number of years: ");
    JLabel labelPop = new JLabel("Initial Population: ");
    JLabel labelNewPop = new JLabel("New Population: ");
    
    JTextField fieldBirth = new JTextField();
    JTextField fieldDeath = new JTextField();
    JTextField fieldYears = new JTextField();
    JTextField fieldPop = new JTextField();
    JTextField fieldNewPop = new JTextField();
    
    JButton go = new JButton("Go");
    // inner class
    class HandlerGo implements ActionListener
    {

        public void actionPerformed(ActionEvent arg0)
        {
            try
            {
                double birth = Double.parseDouble(fieldBirth.getText());
                double death = Double.parseDouble(fieldDeath.getText());
                double oldPop = Double.parseDouble(fieldPop.getText());
                int years = Integer.parseInt(fieldYears.getText());
            
                // compute new population
                double newPop = Growth.getNewPop(birth, death, oldPop, years);
            
                // produce output
                fieldNewPop.setText(String.valueOf(newPop));
            }
            catch(Exception e)
            {
                JOptionPane.showMessageDialog(null, "Error: " + e.getMessage());
            }
        }        
    }
    
    // constructor
    public GrowthGUI()
    {
        super("Population Predictor");
        JPanel data = new JPanel(new GridLayout(5, 2));
        data.add(labelBirth);
        data.add(fieldBirth);
        data.add(labelDeath);
        data.add(fieldDeath);
        data.add(labelYears);
        data.add(fieldYears);
        data.add(labelPop);
        data.add(fieldPop);
        data.add(labelNewPop);
        data.add(fieldNewPop);
        
        getContentPane().setLayout(new BorderLayout());
        getContentPane().add("Center", data);
        getContentPane().add("South", go);
     
        go.addActionListener(new HandlerGo());
        
        pack();
        setVisible(true);
        setDefaultCloseOperation(EXIT_ON_CLOSE);
    }
    // methods
    
    // main
    public static void main(String args[])
    {
        GrowthGUI g = new GrowthGUI();
    }
}