In UML sequence diagrams, given two objects A and B, if Object A sends a message to Object B. Does this mean the message is a method in Object B which is invoked from Object A?

2 Answers 2


UML diagrams are an abstraction. What a message is exactly can be different depending on what concrete entities your diagram shows. The lifelines are not necessarily objects, and the diagram might also be at a higher level of abstraction where a message represents something more complex.

Another factor is that the original concept of "object oriented programming" actually used the concept of messages, not methods - methods are merely a limited (but efficient) way to implement message handlers.

So if the lifelines in your diagrams represent objects in a language that uses only methods to implement message handles, and if the person who draws the program wants it to show low-level concepts then your UML messages will represent method calls.


Sending a message from Object A to Object B is any form of communication initiated by Object A. One of possible forms is calling a method of Object B. But it can be also a signal captured by Object B or even something more complex/totally different. Since UML is not restricted to programming only it can even be one person (Object A) telling something the other person (Object B).

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.