Add PurchaseCalculator

src/main/java/PurchaseCalculator.java

@@ -0,0 +1,29 @@
+import java.math.BigDecimal;
+
+public class PurchaseCalculator {
+    private BigDecimal purchasePrice;
+    private BigDecimal quantity;
+
+    public PurchaseCalculator(Share share) {
+        this.purchasePrice = share.getPurchasePrice();
+        this.quantity = share.getQuantity();
+    }
+
+    public BigDecimal calculateGross() {
+        return this.purchasePrice.multiply(this.quantity);
+    }
+
+    public BigDecimal calculateCommission() {
+        BigDecimal rate = new BigDecimal("0.005");
+        return calculateGross().multiply(rate);
+    }
+
+    public BigDecimal calculateTax() {
+        BigDecimal tax = new BigDecimal("0");
+        return tax;
+    }
+
+    public BigDecimal calculateTotal() {
+        return calculateGross().add(calculateCommission()).add(calculateTax());
+    }
+}