What is a state diagram and when to use it?

Question

Accepted Answer

**State Diagram** — UML diagram showing states and transitions of an object.

**When to use:**
- Objects with complex lifecycle.
- Order, ticket, document workflows.
- To validate state transitions.
- To identify invalid states.

**Components:**
```
States: rounded rectangles
Initial State: filled circle (●)
Final State: bullseye (◎)
Transitions: arrows with trigger [guard] / action
Guard Conditions: [condition]
Actions: / action
```

**Order Example:**
```
● → Created → [Payment received] / Paid → Processing
Processing → [Shipped] / SentNotification → Shipped
Shipped → [Delivered] → Delivered → ●
Created → [Cancelled] → Cancelled → ●
```

**Benefits:** Visualizes lifecycle, catches missing transitions.

Systems Analyst: What is a state diagram and when to use it?