Question

Draw a sequence diagram indicating that object A calls the method bb() in object B, B performs the requested action and returns control to \(A\), and then A calls the method \(c c()\) in object B.

Short answer

Draw lifelines for A and B, arrows for the `bb()` and `cc()` calls, and return arrows.

Step-by-step solution

Identify Objects and Methods

Begin by identifying the objects and the methods being called. In this exercise, we have two objects: A and B. Object A calls two methods on Object B: `bb()` and `cc()`. The task involves showing these method calls and returns on a sequence diagram.

Draw Lifelines

Draw vertical dashed lines to represent the lifelines of Object A and Object B. The lifeline is the time axis in a sequence diagram, indicating instances of the involved objects.

Draw Method Call `bb()`

Draw a horizontal arrow from the lifeline of Object A to the lifeline of Object B indicating that Object A calls the method `bb()` on Object B. Label this arrow with the method name `bb()`.

Draw Action in Object B

Below the `bb()` method call arrow, draw a filled rectangle (often called an activation or execution specification) on the lifeline of Object B. This rectangle shows the time during which Object B is executing the method `bb()`.

Return Control to A

Draw a dashed horizontal arrow from Object B back to Object A. This represents the return of control from Object B to Object A after performing the `bb()` method. The arrow can be labeled with 'return' or left unlabeled, depending on the detail level desired.

Draw Method Call `cc()`

Underneath the return arrow, draw another horizontal arrow from Object A’s lifeline to Object B’s lifeline to indicate that Object A now calls the `cc()` method on Object B. Label this arrow with the method name `cc()`.

Final Review and Fixes

Review the sequence diagram to ensure it correctly represents the sequence of method calls and returns as described: A calls `bb()`, B processes and returns, then A calls `cc()`. Confirm all lifelines, activations, and arrows are correctly placed.

Key concepts

Objects and Methods

When constructing a UML Sequence Diagram, one of the first steps is identifying the objects involved and the methods they will interact with. Objects in a sequence diagram represent the entities that communicate with each other. In our example, we have two objects: A and B.
Object A is responsible for initiating actions by calling methods on object B, specifically the methods `bb()` and `cc()`. Methods in UML serve as the operations that objects can perform. They encapsulate the behaviors or functionalities that the object can execute or trigger in another object.
Therefore, recognizing which objects are present and which methods they interact with forms the foundation of creating a meaningful sequence diagram. This structured approach helps in visually depicting the flow of messages and responses between objects, ensuring clarity in the communication pathways of the system.

Lifelines

Lifelines play a crucial role in UML Sequence Diagrams. They visually represent the existence of an object over a period as they participate in the interaction. Lifelines are depicted as vertical dashed lines that stretch down the diagram.
In our diagram, Object A and Object B each have their own lifelines. The placement of these lifelines is essential as they form the timeline along which events like method calls and returns occur. Each point on a lifeline signifies a specific point in time, and events are organized chronologically from top to bottom.
The lifeline allows us to see when an object is active and when it is simply waiting. Understanding the concept of lifelines is key to accurately capturing the timing and sequence of interactions between different objects in the system.

Method Calls

Method calls in a UML Sequence Diagram are the arrows that travel horizontally between lifelines, showcasing the interaction between objects. These arrows are unidirectional, representing that one object is invoking a method on another object.
In the exercise, Object A calls the method `bb()` on Object B. This interaction starts with a solid line with an arrowhead pointing from A's lifeline to B's lifeline, labeled `bb()`. This visual cue signifies that A is requesting B to perform an action.
Following the `bb()` method call, Object A makes a subsequent call to `cc()` on Object B. Drawing these calls accurately not only shows the flow of interaction but also ensures that the sequence of operations being modeled adheres to what happens in the real system.
Method calls are significant because they define what actions are initiated, and the way they are drawn indicates their chronological order in the diagram.

Activation Rectangles

Activation rectangles, also known as execution specifications, are visual elements in UML Sequence Diagrams that represent the time span during which an object is executing a method. These are drawn as narrow rectangles on top of a lifeline.
For Object B, when method `bb()` is called by Object A, an activation rectangle is placed on B's lifeline right where the `bb()` method call arrow ends. It visually denotes the duration Object B is active and processing the method `bb()`. This execution period continues until control has returned back to Object A.
The presence of activation rectangles is vital for understanding the "busy" period of an object and helps in illustrating the processing times and delays within the system. By doing so, they provide an additional layer of clarity to the sequence diagram, making the interactions and their timing more explicit.