2.1.4 Switch Statements in Activity Diagrams

Show a Multiway Choice

An airline displays directions for boarding groups 1 and 2 and a fallback for other group numbers. A switch activity diagram shows one selector with several labeled alternatives. It describes which action is chosen and where the alternatives rejoin.

Distinguish a Selector from a Yes/No Question

A condition such as group == 1 asks a Boolean question. A switch selector such as group supplies the value whose case will be chosen. Label the outgoing alternatives with the matching values and a default alternative instead of labeling every edge Yes or No.

In this example, assume the application already has an integer group number. The diagram documents the directions for that value; it does not introduce a new input form or validation system.

Connect the Alternative Routes

Read the Boarding Model

Switch on group. Group 1 selects the forward lane, group 2 selects the rear lane, and default selects the service desk. The alternatives merge, then the chosen direction is displayed. — Boarding group routes

The start node reaches the selector. Exactly one outgoing alternative is taken for a particular group value. Its action stores a direction, and the path rejoins the other alternatives before the direction is displayed.

In a UML editor, the decision and merge may be shown as separate diamonds. What matters is their role: the first chooses a path; the later merge allows any selected path to continue. A merge does not wait for all alternatives to execute.

Compare the Source Code

The source represented by the diagram

string direction;
switch (group)
{
    case 1:
        direction = "Forward lane";
        break;
    case 2:
        direction = "Rear lane";
        break;
    default:
        direction = "Service desk";
        break;
}

Console.WriteLine(direction);

The break statements are reflected by paths that leave their selected section and rejoin after the switch. They do not need separate action boxes. Display direction is outside the switch, matching its location after the merge.

Avoid a Misleading Chain of Actions

Do not draw Forward lane followed by Rear lane on the same path. That would describe two assignments during one selection. Do not return the display action to the selector either: this is one choice, so there is no loop-back edge.

The default path is also meaningful. A diagram that omits it would fail to show the behavior for a group such as 7, even though the code supplies a direction for that value.

Test Every Outgoing Route

Use a Route Checklist

Inputs to trace
Group	Expected action	Shared continuation
1	Choose Forward lane	Display direction
2	Choose Rear lane	Display direction
7	Choose Service desk	Display direction

Trace each row with a finger or the diagram editor's selection tool. Confirm that each route selects one direction and reaches the display once. Then decide which route group 0 takes and explain why it follows the same fallback as group 7.