Java: Example - Roman Numerals

// File   : gui/componenents/calculators/Dec2Rom.java
// Description: Converts decimal to roman numerals.
// Shows  : Simple GUI, anonymous inner class listener.
//          Separate logic.
// Author : Fred Swartz - 2006-12-29 - Placed in public domain.

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

//////////////////////////////////////////////////////////////// Dec2Rom
public class Dec2Rom extends JFrame {
    
    //=============================================== instance variables
    private JTextField _decimalTf = new JTextField(6);
    private JTextField _romanTf   = new JTextField(8);
    
    //============================================================= main
    public static void main(String[] args) {
        Dec2Rom win = new Dec2Rom();
    }
    
    //====================================================== constructor
    public Dec2Rom() {
        JButton convertButton = new JButton("Convert");
        
        //... Create a listener with anonymous inner class syntax.
        ActionListener doIt = new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                try {
                    int val = Integer.parseInt(_decimalTf.getText());
                    String roman = Roman.int2roman(val);
                    _romanTf.setText("" + roman);
                } catch (NumberFormatException ex) {
                    _romanTf.setText("Error");
                }
            }};
        
        //... Add this same listener to two components.
        convertButton.addActionListener(doIt);
        _decimalTf.addActionListener(doIt);
        
        //... Create panel, set layout, add components.
        JPanel content = new JPanel();
        content.setLayout(new FlowLayout());
        content.add(_decimalTf); 
        content.add(convertButton);
        content.add(_romanTf); 
        
        //... Set characteristics of this window.
        this.setContentPane(content);
        this.setTitle("Decimal to Roman");
        this.pack();                       // Do layout.
        this.setLocationRelativeTo(null);  // Center window.
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        this.setVisible(true);
    }
}

// File   : gui/componenents/calculators/Roman.java
// Description: A static method for converting binary integers to Roman numbers.
// Illustrates: Static inner value class, StringBuffer, throw exceptions.
// Author : Fred Swartz - 2006-12-29 - Placed in public domain.

/////////////////////////////////////////////////////////////////// class Roman
public class Roman {
    //================================================================ constant
    // This could be alternatively be done with parallel arrays.
    // Another alternative would be Pair<Integer, String>
    final static RomanValue[] ROMAN_VALUE_TABLE = {
        new RomanValue(1000, "M"),
        new RomanValue( 900, "CM"),
        new RomanValue( 500, "D"),
        new RomanValue( 400, "CD"),
        new RomanValue( 100, "C"),
        new RomanValue(  90, "XC"),
        new RomanValue(  50, "L"),
        new RomanValue(  40, "XL"),
        new RomanValue(  10, "X"),
        new RomanValue(   9, "IX"),
        new RomanValue(   5, "V"),
        new RomanValue(   4, "IV"),
        new RomanValue(   1, "I")
    };
    
    //============================================================== int2roman
    public static String int2roman(int n) {
        if (n >= 4000  || n < 1) {
            throw new NumberFormatException("Numbers must be in range 1-3999");
        }
        StringBuffer result = new StringBuffer(10);
        
        //... Start with largest value, and work toward smallest.
        for (RomanValue equiv : ROMAN_VALUE_TABLE) {
            //... Remove as many of this value as possible (maybe none).
            while (n >= equiv.intVal) {
                n -= equiv.intVal;            // Subtract value.
                result.append(equiv.romVal);  // Add roman equivalent.
            }
        }
        return result.toString();
    }
    
    ///////////////////////////////////////////////////////// inner value class
    private static class RomanValue {
        //============================================================== fields
        //... No need to make this fields private because they are
        //    used only in this private value class.
        int    intVal;     // Integer value.
        String romVal;     // Equivalent roman numeral.
        
        //========================================================= constructor
        RomanValue(int dec, String rom) {
            this.intVal = dec;
            this.romVal = rom;
        }
    }
}