diff -r 09d690bb97f6 -r 58ec6da75f6f task4/solution11/src/org/apidesign/apifest08/currency/ExchangeRateDataSource.java
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/task4/solution11/src/org/apidesign/apifest08/currency/ExchangeRateDataSource.java	Sat Oct 11 23:38:46 2008 +0200
@@ -0,0 +1,103 @@
+package org.apidesign.apifest08.currency;
+
+import org.apidesign.apifest08.currency.ExchangeRateProvider.ExchangeRateRequest;
+import org.apidesign.apifest08.currency.ExchangeRateProvider.ExchangeRateResponse;
+
+/**
+ * Exchange rate data source.
+ * 
+ * @author ked
+ */
+public final class ExchangeRateDataSource<AmountType, IdentifierType> {
+
+  private final IdentifierType currencyAIdentifier;
+
+  private final IdentifierType currencyBIdentifier;
+
+  private final ExchangeRateProvider<AmountType, IdentifierType> exchangeRateProvider;
+
+  private ExchangeRateDataSource(
+      IdentifierType currencyAIdentifier,
+      IdentifierType currencyBIdentifier,
+      ExchangeRateProvider<AmountType, IdentifierType> exchangeRateProvider) {
+        if (currencyAIdentifier == null ||
+            currencyBIdentifier == null ||
+            currencyAIdentifier.equals(currencyBIdentifier)) {
+                throw new IllegalArgumentException("Inappropriate exchange rates' identifiers!");
+        }
+  this.currencyAIdentifier = currencyAIdentifier;
+    this.currencyBIdentifier = currencyBIdentifier;
+    this.exchangeRateProvider = exchangeRateProvider;
+  }
+
+  public IdentifierType getCurrencyAIdentifier() {
+    return currencyAIdentifier;
+  }
+
+  public IdentifierType getCurrencyBIdentifier() {
+    return currencyBIdentifier;
+  }
+
+  public ExchangeRateValue<AmountType, IdentifierType> getExchangeRate() {
+    ExchangeRateRequest<AmountType, IdentifierType> request =
+        new ExchangeRateRequest<AmountType, IdentifierType>();
+    ExchangeRateResponse<AmountType, IdentifierType> response =
+        new ExchangeRateResponse<AmountType, IdentifierType>();
+
+    request.setCurrencyAIdentifier(currencyAIdentifier);
+    request.setCurrencyBIdentifier(currencyBIdentifier);
+
+    exchangeRateProvider.getExchangeRate(request, response);
+
+    if (response.getExchangeRate().getCurrencyA().getIdentifier().equals(currencyAIdentifier) &&
+        response.getExchangeRate().getCurrencyB().getIdentifier().equals(currencyBIdentifier)) {
+      return response.getExchangeRate();
+    } else {
+      throw new IllegalStateException("Data source's provider returned inappropriate exchange rate!");
+    }
+  }
+
+  public static <AmountType, IdentifierType> ExchangeRateDataSource<AmountType, IdentifierType> getExchangeRateDataSource(
+      IdentifierType currencyAIdentifier,
+      IdentifierType currencyBIdentifier,
+      ExchangeRateProvider<AmountType, IdentifierType> exchangeRateProvider) {
+    return new ExchangeRateDataSource<AmountType, IdentifierType>(
+        currencyAIdentifier,
+        currencyBIdentifier,
+        exchangeRateProvider);
+  }
+
+  @Override
+  public boolean equals(Object obj) {
+    if (obj == null) {
+      return false;
+    }
+    if (getClass() != obj.getClass()) {
+      return false;
+    }
+    final ExchangeRateDataSource other =
+        (ExchangeRateDataSource) obj;
+    if (this.currencyAIdentifier != other.currencyAIdentifier &&
+        (this.currencyAIdentifier == null || !this.currencyAIdentifier.equals(other.currencyAIdentifier))) {
+      return false;
+    }
+    if (this.currencyBIdentifier != other.currencyBIdentifier &&
+        (this.currencyBIdentifier == null || !this.currencyBIdentifier.equals(other.currencyBIdentifier))) {
+      return false;
+    }
+    if (this.exchangeRateProvider != other.exchangeRateProvider &&
+        (this.exchangeRateProvider == null || !this.exchangeRateProvider.equals(other.exchangeRateProvider))) {
+      return false;
+    }
+    return true;
+  }
+
+  @Override
+  public int hashCode() {
+    int hash = 7;
+    hash = 83 * hash + (this.currencyAIdentifier != null ? this.currencyAIdentifier.hashCode() : 0);
+    hash = 83 * hash + (this.currencyBIdentifier != null ? this.currencyBIdentifier.hashCode() : 0);
+    hash = 83 * hash + (this.exchangeRateProvider != null ? this.exchangeRateProvider.hashCode() : 0);
+    return hash;
+  }
+}