/*
 * ExceptionsDemo1.java - demonstration of throwing and catching exceptions
 *
 * This program asks the user to enter a number, the outputs its square root.
 * It handles appropriately two kinds of invalid data - input that is not a 
 * number at all, and input that is negative
 *
 * Copyright (c) 2000, 2004, 2008 - Russell C. Bjork
 *
 */
 
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class ExceptionsDemo1 extends JFrame implements ActionListener
{
    /** Main method for the program
     */
    public static void main(String [] args)
    {
        new ExceptionsDemo1().setVisible(true);
    }
    
    /** Constructor - create the GUI 
     */    
    public ExceptionsDemo1()
    {
        getContentPane().setLayout(new GridBagLayout());
        GridBagConstraints constraints = new GridBagConstraints(
            0, GridBagConstraints.RELATIVE, 1, 1, 0, 0,
            GridBagConstraints.CENTER, GridBagConstraints.NONE,
            new Insets(10, 10, 10, 10), 0, 0);

        numberPrompt = new JLabel("Please enter a positive number");
        getContentPane().add(numberPrompt, constraints);
        
        numberInput = new JTextField(20);
        getContentPane().add(numberInput, constraints);
        
        computeButton = new JButton("Compute square root");
        getContentPane().add(computeButton, constraints);
        
        result = new JLabel(" ");
        getContentPane().add(result, constraints);
        
        // Prepare to handle mouse click on the compute button, or return
        // in the text field
        
        numberInput.addActionListener(this);
        computeButton.addActionListener(this);
        
        pack();
    }
      
    /** Method invoked when the user clicks the compute button */
    
    public void actionPerformed(ActionEvent e)
    {
        // Get the number the user typed:
        
        String numberTyped = numberInput.getText();
        
        // Convert it to a double
        
        double value;
        
        try
        {
            value = Double.parseDouble(numberTyped);
            if (value < 0) 
                throw new ArithmeticException();
	        double sqrt = Math.sqrt(value);
     	   	result.setText("Square root is " + sqrt);
        	result.setForeground(Color.black);
        }
        catch(NumberFormatException exception)
        {
            result.setText("Please enter a number");
            result.setForeground(Color.red);
            return;
        }
        catch(ArithmeticException exception)
        {
            result.setText("Not a real number");
            result.setForeground(Color.red);
            return;
        }
    }
      
    // Instance variables - components of the GUI
    
    private JLabel numberPrompt;
    private JTextField numberInput;
    private JButton computeButton;
    private JLabel result;
}