diff -r 251d0ed461fb -r 58ec6da75f6f task4/solution07/src/org/apidesign/apifest08/currency/Convertor.java
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/task4/solution07/src/org/apidesign/apifest08/currency/Convertor.java	Sat Oct 11 23:38:46 2008 +0200
@@ -0,0 +1,109 @@
+package org.apidesign.apifest08.currency;
+
+import java.util.Currency;
+
+/** This is the skeleton class for your API. You need to make it public, so
+ * it is accessible to your client code (currently in Task1Test.java) file.
+ * <p>
+ * Feel free to create additional classes or rename this one, just keep all
+ * the API and its implementation in this package. Do not spread it outside
+ * to other packages.
+ */
+public interface Convertor {
+    
+    /**
+     * Converts by taking a request and producing a response.
+     * If a convertor finds it cannot perform the requested conversion,
+     * it should return a non-null {@link ConversionResult} that has null {@link ConversionResult#getNetAmount()}.
+     * A convertor must not convert to a different currency than the one specified in the request.
+     * <p>
+     * When the need comes to extend the semantics, one subclasses the ConversionRequest and/or ConversionResult classes.
+     * <p>
+     * This method can be called as many times as you like.
+     * A {@link Convertor} shall be considered immutable wrt calls to {@link #convert(org.apidesign.apifest08.currency.Convertor.ConversionRequest).
+     * This method of a single {@link Convertor} can be called from many threads concurrently.
+     * @param req the conversion request; mustn't be null
+     * @return the result of carrying out the conversion request; never null
+     * @throws IllegalRequestSubtypeException when the particular implementation cannot handle a specific ConversionRequest type
+     */
+    public ConversionResult convert( final ConversionRequest req ) throws IllegalRequestSubtypeException;
+
+    /**
+     * The request for converting a monetary amout into another currency.
+     * Immutable.
+     */
+    public class ConversionRequest {
+        
+        private final MonetaryAmount srcAmount;
+        private final Currency tgtCurrency;
+
+        /**
+         * A request to convert srcAmount into tgtCurrency.
+         * @param srcAmount the source amount; must not be null
+         * @param tgtCurrency the currency we want it in afterwards; must not be null
+         */
+        public ConversionRequest( final MonetaryAmount srcAmount, final Currency tgtCurrency ) {
+            this.srcAmount = srcAmount;
+            this.tgtCurrency = tgtCurrency;
+            if ( srcAmount == null ) {
+                throw new NullPointerException( "The source amount" );
+            }
+            if ( tgtCurrency == null ) {
+                throw new NullPointerException( "The target currency" );
+            }
+            if ( srcAmount.getCurrency().equals( tgtCurrency ) ) {
+                throw new IllegalArgumentException( "Cannot request conversion from " + srcAmount.getCurrency() + " to " + tgtCurrency );
+            }
+        }
+
+        /**
+         * The source amount.
+         */
+        public MonetaryAmount getSrcAmount() {
+            return srcAmount;
+        }
+
+        /**
+         * The target currency.
+         */
+        public Currency getTgtCurrency() {
+            return tgtCurrency;
+        }
+        
+    }
+    
+    /**
+     * The result of converting a monetary amount into another currency.
+     * For now it records just the net amount one recieves from the conversion.
+     * Immutable.
+     * <p>
+     * <b>Extension note:</b> 
+     * Other items can be added further down the road, as the need for them arises.
+     * These items might provide info on other aspects of the conversion,
+     * such as the fee or a reason why the conversion might not be admissible.
+     */
+    public class ConversionResult {
+        
+        private final MonetaryAmount netAmount;
+        
+        /**
+         * A new conversion result.
+         * @param netAmount the amount one recieves from the conversion; 
+         * null means the conversion was not admissible
+         */
+        public ConversionResult( final MonetaryAmount netAmount ) {
+            this.netAmount = netAmount;
+        }
+        
+        /**
+         * The amount one recieves from the conversion.
+         * If null, the conversion is not admissible.
+         * @return the amount
+         */
+        public MonetaryAmount getNetAmount() {
+            return netAmount;
+        }
+        
+    }
+    
+}