diff -r c38391fb9b38 -r 58ec6da75f6f task4/solution06/src/org/apidesign/apifest08/currency/Convertor.java
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/task4/solution06/src/org/apidesign/apifest08/currency/Convertor.java	Sat Oct 11 23:38:46 2008 +0200
@@ -0,0 +1,176 @@
+package org.apidesign.apifest08.currency;
+
+import static org.apidesign.apifest08.currency.Assert.notNull;
+
+import java.math.BigDecimal;
+import java.math.RoundingMode;
+import java.util.ArrayList;
+import java.util.Currency;
+import java.util.List;
+
+/**
+ * Currency covertor.
+ */
+public final class Convertor {
+	
+	private List<ConvertorDelegate> convertorDelegates = new ArrayList<ConvertorDelegate>();
+	
+	
+	/**
+	 * Create new instance of the converter for the given currencies and its rate.
+	 * 
+	 * @param rateValue the rate between the first and the second currency
+	 * @param currencyFirst the first currency
+	 * @param currencySecond the second currency
+	 */
+	public Convertor(BigDecimal rateValue, Currency currencyFirst, Currency currencySecond) {
+		notNull(currencyFirst, "currencyFirst");
+		notNull(currencySecond, "currencySecond");		
+		notNull(rateValue, "rateValue");	
+		convertorDelegates.add(new ConvertorDelegate(new StaticRateProvider(rateValue), currencyFirst, currencySecond));
+	}
+	
+	/**
+	 * Create new instance of the converter for the given currencies and its rate. 
+	 * A rate value is provided by {@link RateProvider}.
+	 * 
+	 * @param rateProvider the rate provider
+	 * @param currencyFirst the first currency
+	 * @param currencySecond the second currency
+	 */
+	public Convertor(RateProvider rateProvider, Currency currencyFirst, Currency currencySecond) {
+		notNull(currencyFirst, "currencyFirst");
+		notNull(currencySecond, "currencySecond");		
+		notNull(rateProvider, "rateProvider");
+		convertorDelegates.add(new ConvertorDelegate(rateProvider, currencyFirst, currencySecond));
+	}
+	
+	/**
+	 * Create new instance of the convertor from the given convertors. 
+	 * @param convertors the convertors
+	 */
+	public Convertor(Convertor... convertors) {
+		notNull(convertors, "convertors");
+		if(convertors.length == 0) {
+			throw new IllegalArgumentException("There must be at least one converter.");
+		}
+		
+		for(Convertor convertor: convertors) {
+			if(convertor != null) {
+				convertorDelegates.addAll(convertor.convertorDelegates);
+			}
+		}
+	}
+    	
+	/**
+	 * Converts an amount value between the two currencies of this converter.
+	 *  
+	 * @param amount an amount
+	 * @param fromCurrency an amount currency
+	 * @param toCurrency to a target currency
+	 * @return a converted amount value
+	 * 
+	 * @throws ConversionException if the conversion fails
+	 * @throws UnsupportedConversionException if the conversion between a given currencies is not supported.
+	 */
+	public Amount convert(BigDecimal amount, Currency fromCurrency, Currency toCurrency) throws ConversionException {
+		notNull(amount, "amount");
+		notNull(fromCurrency, "fromCurrency");
+		notNull(toCurrency, "toCurrency");
+		ConvertorDelegate appropriateDelegate = null;
+		
+		//try find an appropriate delegate for conversion
+		for(ConvertorDelegate delegate : convertorDelegates) {
+			if(delegate.isConversionSupported(fromCurrency, toCurrency)) {
+				appropriateDelegate = delegate;
+				break;
+			}
+		}
+		
+		if(appropriateDelegate == null) {
+			throw new UnsupportedConversionException(fromCurrency, toCurrency);
+		}
+		
+		return appropriateDelegate.convert(amount, fromCurrency, toCurrency);	
+	}
+	
+	/**
+	 * Internal delegate implements a logic for conversion between two currencies
+	 * and vice versa.
+	 * @see #isConversionSupported(Currency, Currency)
+	 */
+	private static class ConvertorDelegate {
+		private final Currency first;
+		private final Currency second;
+		private final RateProvider rateProvider;
+		public static final BigDecimal one = new BigDecimal(1);
+		
+		
+		private ConvertorDelegate(RateProvider rateProvider, Currency currencyFirst, Currency currencySecond) {
+			this.rateProvider = rateProvider;
+			this.first = currencyFirst;
+			this.second = currencySecond;
+		}
+		
+		private Amount convert(BigDecimal amount, Currency fromCurrency, Currency toCurrency) throws ConversionException {
+			BigDecimal rateValue = getRateValue(fromCurrency, toCurrency);
+			BigDecimal result = rateValue.multiply(amount);			
+			return new Amount(result, toCurrency);	
+		}
+		
+		private BigDecimal getRateValue(Currency fromCurrency, Currency toCurrency) {		
+			
+			BigDecimal retVal;
+			
+			if(first == fromCurrency) {
+				BigDecimal rateValue = rateProvider.getRate();
+				if(rateValue == null) {
+					throw new NullPointerException("Rate cannot be null!");
+				}
+				retVal = rateValue;
+			} else {	
+				BigDecimal rateValue = rateProvider.getRate();
+				if(rateValue == null) {
+					throw new NullPointerException("Rate cannot be null!");
+				}
+				//reverse rate	
+				retVal = one.divide(rateValue, 10 ,RoundingMode.HALF_UP);
+			}
+			
+			return retVal;
+		}
+		
+		/**
+		 * @return <code>true</code> if the delegate is able to convert from the given currency
+		 * to the given currency and vice versa otherwise <code>false</code>.
+		 */
+		private boolean isConversionSupported(Currency fromCurrency, Currency toCurrency) {
+			return ((fromCurrency == first || fromCurrency == second) && (toCurrency == first || toCurrency == second));
+		}
+	}
+	
+	/**
+	 * A rate provider. This class represents a way how could be "static" convertor
+	 * extended in order converts according to current rate.
+	 */
+	public static abstract class RateProvider { 
+		
+		/**
+		 * @return a rate between the from currency and the to currency associated with
+		 * a given convertor.
+		 */
+		public abstract BigDecimal getRate();
+	}
+	
+	private static class StaticRateProvider extends RateProvider{
+		private final BigDecimal rateValue;
+		
+		private StaticRateProvider(BigDecimal rateValue){
+			this.rateValue = rateValue;
+		}
+		
+		public BigDecimal getRate() {
+			return this.rateValue;
+		}
+	}
+}