TCalculatorService.patch

 import com.facebook.swift.service.ThriftMethod;
 import com.facebook.swift.service.ThriftService;
+import com.google.common.util.concurrent.ListenableFuture;
 
 @ThriftService
 public interface TCalculatorService {
 
     @ThriftMethod
-    int calculate(int num1, int num2, TOperation op) throws TDivisionByZeroException;
+    ListenableFuture<Integer> calculate(int num1, int num2, TOperation op) throws TDivisionByZeroException;
 }