Feat: Implemented TransactionCalculator interface

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

@@ -0,0 +1,22 @@
+package edu.ntnu.idi.idatt2003.g40.mappe;
+
+import java.math.BigDecimal;
+
+/**
+ * Interface implemented by various calculators.
+ *
+ * <p>Has methods for calculating gross, commission fee, tax and total</p>*/
+public interface TransactionCalculator {
+
+  /** Method for calculating gross transaction amount. */
+  BigDecimal calculateGross();
+
+  /** Method for calculating commission fee for transaction amount. */
+  BigDecimal calculateCommission();
+
+  /** Method for calculating tax fee for transaction amount. */
+  BigDecimal calculateTax();
+
+  /** Method for calculating total amount for transaction. */
+  BigDecimal calculateTotal();
+}