Feat: Updated validation for share

src/main/java/edu/ntnu/idi/idatt2003/g40/mappe/model/Share.java

@@ -32,13 +32,25 @@ public final class Share {
    * @param quantity      the quantity purchased
    * @param purchasePrice the price per unit at purchase time
    *
-   * @throws IllegalArgumentException if stock is null.
+   * @throws IllegalArgumentException if parameters are null or invalid.
    */
   public Share(final Stock stock,
                final BigDecimal quantity,
                final BigDecimal purchasePrice) throws IllegalArgumentException {
-    if (stock == null) {
-      throw new IllegalArgumentException("Invalid stock!");
+    if (stock == null
+        || quantity == null
+        || purchasePrice == null) {
+      throw new IllegalArgumentException("Invalid share configuration!");
+    }
+    if (quantity.compareTo(BigDecimal.ZERO) <= 0) {
+      throw new IllegalArgumentException(
+              "Quantity cannot be negative or zero!"
+      );
+    }
+    if (purchasePrice.compareTo(BigDecimal.ZERO) <= 0) {
+      throw new IllegalArgumentException(
+              "Purchase price cannot be negative or zero!"
+      );
     }
     this.stock = stock;
     this.quantity = quantity;