whole PurchaseCalculator class

this is the whole purchase class

src/main/java/edu/ntnu/idi/idatt2003/g40/mappe/PurchaseCalculator.java

@@ -1,5 +1,67 @@
 package edu.ntnu.idi.idatt2003.g40.mappe;
 
-public class PurchaseCalculator {
-
+import java.math.BigDecimal;
+
+
+/**
+ * Calculator for purchase transactions.
+ * <p>
+ * Uses the share's purchase price and quantity to calculate:
+ * gross value, commission, tax and total cost.
+ * </p>
+ */
+public class PurchaseCalculator implements TransactionCalculator {
+
+    private static final BigDecimal COMMISSION_RATE = new BigDecimal("0.005"); // 0.5%
+
+    private final BigDecimal purchasePrice;
+    private final BigDecimal quantity;
+
+    /**
+     * Creates a new {@code PurchaseCalculator} based on a share.
+     *
+     * @param share the share to base calculations on
+     */
+    public PurchaseCalculator(Share share) {
+        this.purchasePrice = share.getPurchasePrice();
+        this.quantity = share.getQuantity();
+    }
+
+    /**
+     * {@inheritDoc}
+     * Gross value = purchasePrice * quantity.
+     */
+    @Override
+    public BigDecimal calculateGross() {
+        return purchasePrice.multiply(quantity);
+    }
+
+    /**
+     * {@inheritDoc}
+     * Commission = 0.5% of gross value.
+     */
+    @Override
+    public BigDecimal calculateCommission() {
+        return calculateGross().multiply(COMMISSION_RATE);
+    }
+
+    /**
+     * {@inheritDoc}
+     * No tax on purchases.
+     */
+    @Override
+    public BigDecimal calculateTax() {
+        return BigDecimal.ZERO;
+    }
+
+    /**
+     * {@inheritDoc}
+     * Total cost = gross + commission + tax.
+     */
+    @Override
+    public BigDecimal calculateTotal() {
+        return calculateGross()
+                .add(calculateCommission())
+                .add(calculateTax());
+    }
 }