Wrote OfferIdentifier and updated ProductRetriever

src/main/java/de/rwu/easydrop/service/retriever/OfferRetriever.java

@@ -0,0 +1,14 @@
+package de.rwu.easydrop.service.retriever;
+
+import de.rwu.easydrop.model.Offer;
+
+import java.util.ArrayList;
+import java.util.List;
+
+public class OfferRetriever {
+
+    public List<Offer> loadOffers() {
+        return new ArrayList<>();
+    }
+
+}

src/main/java/de/rwu/easydrop/service/retriever/ProductRetriever.java

@@ -81,4 +81,28 @@ public class ProductRetriever {
 
         return product;
     }
+
+    /**
+     * Retrieves a product from an API by name of the API so that high-level components do not need to be extended
+     * with exact function call names if we extend the list of webshops.
+     *
+     * @param dataSourceName Data source name, e.g. amazon
+     * @param productName Product name, translated to the correct product ID for the data source
+     * @return Product from that data source or null if data source not available
+     */
+    public Product getProductFromDataSource(final String dataSourceName, final String productName) {
+        switch(dataSourceName.toLowerCase()) {
+        case "amazon":
+            // TODO: Translation from productName to productId (Amazon) needed
+            return getProductFromAmazon(productName);
+        case "ebay":
+            // TODO: Translation from productName to productId (eBay) needed
+            return getProductFromEbay(productName);
+        case "persistence":
+            // TODO: Translation from productName to productId (persistence layer) needed
+            return getProductFromPersistence(productName);
+        default:
+            return null;
+        }
+    }
 }