diff -r 14e78f48ac2b -r 58ec6da75f6f task4/solution04/src/org/apidesign/apifest08/currency/ExchangeRate.java
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/task4/solution04/src/org/apidesign/apifest08/currency/ExchangeRate.java	Sat Oct 11 23:38:46 2008 +0200
@@ -0,0 +1,125 @@
+package org.apidesign.apifest08.currency;
+
+
+import java.math.BigDecimal;
+import java.util.Currency;
+
+
+/**
+ * The exchange rate between two currencies.
+ *
+ * @author D'Arcy Smith
+ * @version 1.0
+ */
+public final class ExchangeRate
+{
+    /**
+     * 
+     */
+    private final Currency currencyA;
+
+    /**
+     * 
+     */
+    private final Currency currencyB;
+
+    /**
+     * 
+     */
+    private final BigDecimal rateAtoB;
+
+    /**
+     * 
+     */
+    private final BigDecimal rateBtoA;
+
+    /**
+     * Construct an ExchangeRate with the specified values.
+     * 
+     * @param a the first currency
+     * @param b the second currency
+     * @param ra the rate to convert a to b
+     * @param rb the rate to covertt b to a
+     * @throws IllegalArgumentException if any parameter is null.
+     */
+    public ExchangeRate(final Currency   a,
+                        final Currency   b,
+                        final BigDecimal ra,
+                        final BigDecimal rb)
+    {
+        if(a == null)
+        {
+            throw new IllegalArgumentException("a cannot be null");
+        }
+        
+        if(b == null)
+        {
+            throw new IllegalArgumentException("b cannot be null");
+        }
+        
+        if(ra == null)
+        {
+            throw new IllegalArgumentException("ra cannot be null");
+        }
+        
+        if(rb == null)
+        {
+            throw new IllegalArgumentException("rb cannot be null");
+        }
+        
+        if(ra.compareTo(BigDecimal.ZERO) <= 0)
+        {
+            throw new IllegalArgumentException("ra cannot be <= 0, was: " + ra);
+        }
+        
+        if(rb.compareTo(BigDecimal.ZERO) <= 0)
+        {
+            throw new IllegalArgumentException("rb cannot be <= 0, was: " + ra);
+        }
+        
+        currencyA = a;
+        currencyB = b;
+        rateAtoB  = ra;
+        rateBtoA  = rb;
+    }
+
+    /**
+     * Get the first currency.
+     * 
+     * @return the first currency.
+     */
+    public Currency getCurrencyA()
+    {
+        return currencyA;
+    }
+
+    /**
+     * Get the second currency.
+     * 
+     * @return the second currency.
+     */
+    public Currency getCurrencyB()
+    {
+        return currencyB;
+    }
+
+    /**
+     * Get the conversion rate from currencyA to currencyB.
+     * 
+     * @return the conversion rate from currencyA to currencyB.
+     */
+    public BigDecimal getRateAtoB()
+    {
+        return rateAtoB;
+    }
+
+    /**
+     * Get the conversion rate from currencyB to currencyA.
+     * 
+     * @return the conversion rate from currencyB to currencyA.
+     */
+    public BigDecimal getRateBtoA()
+    {
+        return rateBtoA;
+    }
+}