Convert ItemDate to abstract. Implement hashcode in both dates.

src/main/java/seedu/foodrem/model/item/Item.java

@@ -52,7 +52,7 @@ public Item(ItemName name,
                 Set<Tag> tagSet) {
         requireAllNonNull(name, quantity, unit, boughtDate, expiryDate, price, remarks, tagSet);
 
-        // Same date is accepted
+        // Same Bought date and Expiry date is accepted
         if (!boughtDate.isNotSet() && !expiryDate.isNotSet() && boughtDate.isAfterDate(expiryDate)) {
             throw new IllegalArgumentException("The item bought date should not be after the item expiry date.");
         }

src/main/java/seedu/foodrem/model/item/ItemBoughtDate.java

@@ -58,6 +58,14 @@ public boolean equals(Object other) {
                 && getDate().equals(((ItemBoughtDate) other).getDate()));
     }
 
+    /**
+     * {@inheritDoc}
+     */
+    @Override
+    public int hashCode() {
+        return super.hashCode();
+    }
+
     /**
      * {@inheritDoc}
      */

src/main/java/seedu/foodrem/model/item/ItemDate.java

@@ -7,7 +7,7 @@
  * Represents an item date in an {@link Item}.
  * Guarantees: details are present and not null, immutable.
  */
-public class ItemDate {
+public abstract class ItemDate {
     // Remember to change relevant messages displayed to users when changing the regex.
     public static final String DATE_PATTERN_REGEX = "dd-MM-uuuu";
     public static final DateTimeFormatter DATE_FORMATTER = DateTimeFormatter

src/main/java/seedu/foodrem/model/item/ItemExpiryDate.java

@@ -46,6 +46,14 @@ public boolean isNotSet() {
         return this == NOT_SET_EXPIRY_DATE;
     }
 
+    /**
+     * {@inheritDoc}
+     */
+    @Override
+    public int hashCode() {
+        return super.hashCode();
+    }
+
     /**
      * Returns {@code true} if both {@link ItemExpiryDate} have the same date by
      * {@link LocalDate#equals(Object)}.