src/main/java/com/example/baisgate/PaymentService.java

@@ -0,0 +1,10 @@
+package com.example.baisgate;
+
+public class PaymentService {
+    public String approve(String transactionId) {
+        if (transactionId == null || transactionId.trim().isEmpty()) {
+            throw new IllegalArgumentException("Transaction ID cannot be blank");
+        }
+        return "APPROVED:" + transactionId;
+    }
+}