Changing call order, adding null checks

src/main/java/millions/Exchange.java

@@ -28,17 +28,23 @@ public Exchange(String name, List<Stock> stockList) {
     }
   }
 
-  public void buy(Player player, Stock stock, BigDecimal quantity) {
+  public void buy(String symbol, Player player, BigDecimal quantity) {
+
+    Stock stock = this.stocks.get(symbol);
+    if (stock == null) {
+      throw new IllegalArgumentException("Stock not found");
+    }
+
     Share shareToBuy = new Share(stock, quantity, stock.getSalesPrice());
     Purchase purchase = new Purchase(shareToBuy, this.weekNumber);
     purchase.commit(player);
   }
 
-  public void buy(Player player, Stock stock, int quantity) {
-    this.buy(player, stock, BigDecimal.valueOf(quantity));
+  public void buy(String symbol, Player player, int quantity) {
+    this.buy(symbol, player, BigDecimal.valueOf(quantity));
   }
 
-  public void sell(Player player, Share share) {
+  public void sell(Share share, Player player) {
     Sale sale = new Sale(share, weekNumber);
     sale.commit(player);
   }