What is a State Diagram?
A State Diagram (also called a State Machine Diagram) models the different states an object can be in and the transitions between those states. It shows how an object responds to events throughout its lifecycle — from creation to destruction.
State diagrams are perfect for objects that have distinct behavioral modes. An order can be "pending," "confirmed," "shipped," or "delivered." A TCP connection can be "listening," "syn-sent," "established," or "closed." Each state has different valid operations and behaviors.
State Diagram Notation
Basic Elements:
1. Initial State (solid circle)
●
2. Final State (circle with X)
⊗
3. State (rounded rectangle)
┌────────────────────┐
│ State Name │
├────────────────────┤
│ entry / action │ ← entry action
│ do / activity │ ← ongoing activity
│ exit / action │ ← exit action
└────────────────────┘
4. Transition (solid arrow)
──────▶
Label format: event [guard] / action
5. Decision Node (diamond)
◇Example: Order Lifecycle
Order State Diagram:
●
│
▼
┌─────────────────┐
│ Pending │ entry / initializeOrder()
│ │ do / validateItems()
└────────┬────────┘
│
│ [all items in stock] / reserveInventory()
▼
┌─────────────────┐
│ Confirmed │ entry / sendConfirmation()
│ │ do / processPayment()
└────────┬────────┘
│
┌────┴────┐
│ │
▼ ▼
┌────────┐ ┌──────────┐
│ Shipped│ │ Cancelled│
│ │ │ │
└───┬────┘ └──────────┘
│ ╱
│ [delivered] ╱
│ / updateStatus ╱
▼ ╱
┌──────────────┐ ╱
│ Delivered │ ╱
│ │ ╱
└──────────────┘
⊗
States:
• Pending — order created, waiting for confirmation
• Confirmed — payment processed, waiting for shipment
• Shipped — in transit to customer
• Delivered — received by customer
• Cancelled — order was cancelledGuard Conditions and Actions
Transitions can include conditions (guards) that must be true for the transition to occur, and actions that execute when the transition happens:
Transition Label Format:
event [guard condition] / action
Examples:
──────────────────────────────────────────────────────
paymentReceived [amount >= total] / confirmOrder()
──────────────────────────────────────────────────────
│ │ │ │
│ │ │ └─ action to execute
│ │ └─ condition that must be true
│ └─ the event that triggers the transition
└─ name of the event
More Examples:
click [itemInCart] / checkout()
timeout [retries < 3] / retry()
signal [signalStrength > 80] / establishConnection()
do / processQueue() ← internal activity, not event-triggeredComposite States
When states themselves have internal states, you can use composite states (hierarchical state machines):
Connection State Diagram with Composite States:
●
│
▼
┌────────────────────────────────────┐
│ Disconnected │
└────────────────┬───────────────────┘
│ connect()
▼
┌────────────────────────────────────┐
│ Connecting │
│ ┌──────────────────────────────┐ │
│ │ TCP Handshake │ │
│ └──────────────────────────────┘ │
└────────────────┬───────────────────┘
│ connected
▼
┌────────────────────────────────────┐
│ Connected │
│ ┌─────────┐ ┌──────────────┐ │
│ │ Idle │───▶│ Transferring│ │
│ └─────────┘ └──────────────┘ │
│ ▲ │ │
│ └───────────────┘ │
└────────────────┬───────────────────┘
│ disconnect()
▼
┌────────────────────────────────────┐
│ Disconnected │
└────────────────────────────────────┘
│
⊗
Inside Connected, the object can be Idle or Transferring.
Events to Connected are routed to the current sub-state.Entry, Do, and Exit Actions
States can have three types of internal actions:
┌──────────────────────────────┐
│ Processing │
├──────────────────────────────┤
│ entry / startTimer() │ ← runs when entering the state
│ do / processItems() │ ← runs while in the state
│ exit / stopTimer() │ ← runs when leaving the state
└──────────────────────────────┘
entry — Executes once when the state is entered.
Good for setup: start timers, log messages, acquire resources.
do — Executes continuously (or repeatedly) while in the state.
Good for ongoing work: processing, monitoring, waiting.
exit — Executes once when leaving the state.
Good for cleanup: stop timers, release resources, save data.Real Example — Download State:
┌─────────────────────────────────┐
│ Downloading │
├─────────────────────────────────┤
│ entry / startDownload() │ ← connects to server
│ do / receiveData() │ ← receives chunks
│ exit / saveFile() │ ← writes to disk
└─────────────────────────────────┘
Events:
start → (from Idle)
complete → (to Completed)
error → (to Error)
cancel → (to Cancelled)