diff -r 251d0ed461fb -r 58ec6da75f6f task4/solution11/src/org/apidesign/apifest08/currency/ExchangeRateValue.java
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/task4/solution11/src/org/apidesign/apifest08/currency/ExchangeRateValue.java	Sat Oct 11 23:38:46 2008 +0200
@@ -0,0 +1,78 @@
+package org.apidesign.apifest08.currency;
+
+import java.io.Serializable;
+
+/**
+ * Value class, holding an exchange rate between two currencies.
+ * Designed to be an immutable.
+ * 
+ * @author ked
+ */
+public final class ExchangeRateValue<AmountType, IdentifierType> implements Serializable {
+
+    private final CurrencyValue<AmountType, IdentifierType> currencyA;
+    private final CurrencyValue<AmountType, IdentifierType> currencyB;
+
+    private ExchangeRateValue(
+            CurrencyValue<AmountType, IdentifierType> currencyA,
+            CurrencyValue<AmountType, IdentifierType> currencyB) {
+        if (currencyA.getIdentifier() == null ||
+            currencyB.getIdentifier() == null ||
+            currencyA.getIdentifier().equals(currencyB)) {
+                throw new IllegalArgumentException("Inappropriate exchange rates' identifiers!");
+        }
+
+        this.currencyA = currencyA;
+        this.currencyB = currencyB;
+    }
+
+    public CurrencyValue<AmountType, IdentifierType> getCurrencyA() {
+        return currencyA;
+    }
+
+    public CurrencyValue<AmountType, IdentifierType> getCurrencyB() {
+        return currencyB;
+    }
+
+    @Override
+    public boolean equals(Object obj) {
+        if (obj == null) {
+            return false;
+        }
+        if (getClass() != obj.getClass()) {
+            return false;
+        }
+        final ExchangeRateValue other = (ExchangeRateValue) obj;
+        if (this.currencyA != other.currencyA && (this.currencyA == null || !this.currencyA.equals(other.currencyA))) {
+            return false;
+        }
+        if (this.currencyB != other.currencyB && (this.currencyB == null || !this.currencyB.equals(other.currencyB))) {
+            return false;
+        }
+        return true;
+    }
+
+    @Override
+    public int hashCode() {
+        int hash = 3;
+        hash = 71 * hash + (this.currencyA != null ? this.currencyA.hashCode() : 0);
+        hash = 71 * hash + (this.currencyB != null ? this.currencyB.hashCode() : 0);
+        return hash;
+    }
+
+    /**
+     * Creates new instance.
+     * Generic types of the new instance are derived from types of the parameters.
+     * 
+     * @param <AmountType> type of the currency amount
+     * @param <IdentifierType> type of the currency identifier
+     * @param currencyA one currency of the exchange rate
+     * @param currencyB another currency of the exchange rate
+     * @return new instance
+     */
+    public static <AmountType, IdentifierType> ExchangeRateValue<AmountType, IdentifierType> getExchangeRate(
+            CurrencyValue<AmountType, IdentifierType> currencyA,
+            CurrencyValue<AmountType, IdentifierType> currencyB) {
+        return new ExchangeRateValue<AmountType, IdentifierType>(currencyA, currencyB);
+    }
+}