diff -r 221f1930cbfb -r 58ec6da75f6f task4/solution12/src/org/apidesign/apifest08/currency/Convertor.java
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/task4/solution12/src/org/apidesign/apifest08/currency/Convertor.java	Sat Oct 11 23:38:46 2008 +0200
@@ -0,0 +1,183 @@
+package org.apidesign.apifest08.currency;
+
+import java.util.ArrayList;
+import java.util.Currency;
+import java.util.Hashtable;
+import java.util.List;
+
+import org.apidesign.apifest08.currency.exceptions.ConvertorException;
+import org.apidesign.apifest08.currency.exceptions.InvalidCurrencyException;
+import org.apidesign.apifest08.currency.exceptions.UnknownConvertorException;
+
+/**
+ * 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 class Convertor {
+
+  private static Hashtable<String, ExchangeRate> exchangeRates;
+
+  private List<String> currencyConvertors; 
+
+  /**
+   * Constructor. Checks if all selected currencies are not null and has defined exchange rates for both
+   * directions.
+   * @param currencies currencies for new instance of convertor
+   * @throws UnknownConvertorException if there are not defined exchange rates for both directions for each
+   * pair of currencies
+   */
+  private Convertor(Currency[] currencies) throws UnknownConvertorException {
+    currencyConvertors = new ArrayList<String>();
+
+    for (Currency currency1 : currencies) {
+      for (Currency currency2 : currencies) {
+        if(currency1 == null || currency2 == null) {
+          throw new ConvertorException("None of the currencies should be null!!!");
+        }
+          
+        if(!currency1.getCurrencyCode().equals(currency2.getCurrencyCode())) {
+          String key = currency1.getCurrencyCode() + currency2.getCurrencyCode();
+          if (!exchangeRates.containsKey(key)) {
+            throw new UnknownConvertorException("Selected convertor (" + currency1.getCurrencyCode() + "->"
+                + currency2.getCurrencyCode() + ") has not defined exchange rates!!!");
+          } 
+          
+          currencyConvertors.add(key);            
+        }
+      }
+    }
+  }
+
+  /**
+   * Sets convertor rate for selected currencies.
+   * @param currency1
+   *          one of the currencies we want to convert to/from
+   * @param currency2
+   *          the other currency
+   * @param rate
+   *          exchange rate from currency1 to currency2
+   * @param unit
+   *          unit of exchangeRate (USD->CZK - unit=1, you exchange one dollar, SKK->CZK unit=100, exchange rate is for
+   *          100SKK)
+   */
+  public static void setConvertorRates(Currency currency1, Currency currency2, double rate, double unit) {
+    if (currency1 == null || currency2 == null) {
+      throw new ConvertorException("None of the currencies should be null!!!");
+    }
+
+    if (rate <= 0 || unit <= 0) {
+      throw new ConvertorException("Rate(" + rate + ") and unit(" + unit + ") has to be grater then zero!!!");
+    }
+
+    if (exchangeRates == null) {
+      exchangeRates = new Hashtable<String, ExchangeRate>();
+    }
+
+    String key12 = currency1.getCurrencyCode() + currency2.getCurrencyCode();
+    String key21 = currency2.getCurrencyCode() + currency1.getCurrencyCode();
+    double recountedRate = (unit / rate) * unit;
+
+    exchangeRates.put(key12, new ExchangeRate(currency1, currency2, rate, unit));
+    exchangeRates.put(key21, new ExchangeRate(currency2, currency1, recountedRate, unit));
+
+  }
+  
+  /**
+   * Merge exchange rates of actual convertor with exchange rates from selected 
+   * convertor. If there are same currencies in both convertors, these from selected
+   * convertor rewrites these in actual convertor.
+   * @param convertor convertor to merge with actual one
+   * @return convertor with merged exchange rates 
+   */
+  public Convertor merge(Convertor convertor) {
+    if(convertor == null) {
+      throw new ConvertorException("It's impossible to merge with null convertor!!!");
+    }
+    
+    currencyConvertors.addAll(convertor.getCurrencyConvertors());
+	  return this;
+  }
+
+  /**
+   * Creates new instance of convertor.
+   * @param currency1
+   *          one of the currencies we want to convert to/from
+   * @param currency2
+   *          the other currency
+   * @return new instance of convertor
+   * @throws UnknownConvertorException
+   *           thrown if convertor for selected currencies has not been defined
+   */
+  public static Convertor getConvertorInstance(Currency... currencies) throws UnknownConvertorException {
+   	if(currencies.length < 2) {
+   		throw new ConvertorException("To get convertor instance, you have to select at least two currencies!!!");
+   	}
+    
+    return new Convertor(currencies);
+  }
+
+  /**
+   * Converts selected amout of selected currency to other currency of this convertor instance.
+   * @param amount
+   *          amount to convert
+   * @param originalCurrency
+   *          currency of this amount
+   * @param newCurrency
+   *          currency to which we want convert
+   * @return converted amount
+   * @throws InvalidCurrencyException
+   *           while one or both currencies doesn't fit for this convertor
+   */
+  public double convert(double amount, Currency originalCurrency, Currency newCurrency) throws InvalidCurrencyException {
+    ExchangeRate actualyUsedExchangeRate = null;
+
+    if (originalCurrency == null) {
+      throw new ConvertorException("Original currency is null!!!");
+    }
+
+    if (newCurrency == null) {
+      throw new ConvertorException("Destination currency is null!!!");
+    }
+
+    actualyUsedExchangeRate = getExchangeRate(originalCurrency, newCurrency);
+
+    return countResult(actualyUsedExchangeRate, amount);
+  }
+
+  private double countResult(ExchangeRate actualyUsedExchangeRate, double amount) {
+    return amount * actualyUsedExchangeRate.getRate() / actualyUsedExchangeRate.getUnit();
+  }
+
+  /**
+   * Decides the direction of conversion and returns instance of actual exchange rate.
+   * @param actualCurrency
+   *          actual currency we want to convert
+   * @return actual exchange rate of this convertor for selected currency
+   */
+  private ExchangeRate getExchangeRate(Currency originalCurrency, Currency newCurrency) throws InvalidCurrencyException {
+    ExchangeRate actualyUsedExchangeRate = null;
+    
+    String key = originalCurrency.getCurrencyCode() + newCurrency.getCurrencyCode();
+
+    if(currencyConvertors.contains(key)) {
+      actualyUsedExchangeRate = exchangeRates.get(key);
+    } else {
+      throw new InvalidCurrencyException("This convertor could not be used for converting selected currencies (" + originalCurrency.getCurrencyCode() + "->"
+          + newCurrency.getCurrencyCode() + ") !!!");
+    }
+
+    return actualyUsedExchangeRate;
+  }
+  
+  /**
+   * Returns currency convertors for actual instance of convertor.
+   * @return currency convertors for actual instance of convertor
+   */
+  List<String> getCurrencyConvertors() {
+	  return currencyConvertors;
+  }
+  
+}