diff --git a/src/main/java/TransactionCalculator.java b/src/main/java/TransactionCalculator.java
new file mode 100644
index 0000000..dd17eef
--- /dev/null
+++ b/src/main/java/TransactionCalculator.java
@@ -0,0 +1,9 @@
+import java.math.BigDecimal;
+
+public interface TransactionCalculator {
+    // Methods in interface is automatically public abstract.
+    BigDecimal calculateGross();
+    BigDecimal calculateCommission();
+    BigDecimal calculateTax();
+    BigDecimal calculateTotal();
+}