State Machines and Diagrams

📘 State Machines and State Diagrams (OOAD)

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🔹 1. Definition

A State Machine (Finite State Machine) is a behavioral model that describes how an object changes its state in response to events over time.

👉 Simple idea: It shows what an object is doing at any point in time and how it moves from one condition to another.

A State Diagram (UML State Machine Diagram) is the graphical representation of this behavior.

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🔹 2. Key Idea

• Objects do not behave the same all the time
• They exist in different states
• Events cause transitions between states

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🔹 3. Real-Life Example

👉 Example: ATM Machine

• Idle → Card Inserted → PIN Entered → Transaction → Idle

Each step is a state, and actions move it forward.

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🔹 4. Components of State Machine Diagram ⭐

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🔸 4.1 State

A condition or situation of an object.

Example:

• Idle
• Processing
• Active
• Locked

📌 Represented as rounded rectangles.

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🔸 4.2 Initial State

• Starting point of the system
• Shown as a black dot (●)

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🔸 4.3 Final State

• End of process
• Shown as a bullseye (◎)

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🔸 4.4 Transition

• Movement from one state to another
• Triggered by an event

Example:

Idle → (Insert Card) → Active

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🔸 4.5 Event

• Something that causes a change

Example:

• Button press
• Login request
• Timeout

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🔸 4.6 Action

• Activity performed during transition

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🔹 5. State Diagram Notation

● → Initial State
○ → Final State
[State] → Rounded rectangle
→ → Transition arrow

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🔹 6. Example: ATM System ⭐

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🔹 States:

• Idle
• Card Inserted
• PIN Verification
• Transaction
• Exit

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🔹 Transitions:

Idle → Insert Card → Card Inserted  
Card Inserted → Enter PIN → PIN Verification  
PIN Verification → Success → Transaction  
Transaction → Finish → Idle  

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🔹 7. Example Diagram Description

• Start at Idle (●)
• Card inserted → move to next state
• PIN entered → system verifies
• Transaction happens
• Ends or returns to Idle

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🔹 8. Types of States

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🔸 8.1 Simple State

• No internal structure

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🔸 8.2 Composite State

• Contains sub-states

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🔸 8.3 Concurrent State

• Multiple states active at the same time

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🔹 9. State vs Activity Diagram

Feature	State Diagram	Activity Diagram
Focus	Object states	Workflow/process
Behavior	Event-driven	Action-driven
Level	Object level	System level

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🔹 10. Where State Diagrams are Used

• ATM systems
• Traffic lights
• Online shopping carts
• Vending machines
• User login systems

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🔹 11. Advantages of State Diagrams

• Shows dynamic behavior clearly
• Helps in system understanding
• Useful for real-time systems
• Detects invalid states
• Improves design accuracy

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🔹 12. Important Rules / Guidelines

• Identify all possible states first
• Define events carefully
• Avoid unnecessary states
• Ensure all transitions are valid
• Always define initial and final state

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🔹 13. Likely Exam Questions

1. Define state machine in OOAD.
2. What is a state diagram?
3. Explain components of state machine diagram.
4. Draw state diagram of ATM system.
5. Difference between state and activity diagram.
6. What is a transition?
7. Explain initial and final state.
8. What are events in state diagrams?
9. Types of states in UML.
10. Advantages of state diagrams.

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🔹 14. Quick Revision Summary 🧠

• State Machine = Behavior of object over time
• State = Condition of object
• Event = Trigger for change
• Transition = Movement between states

👉 Trick to Remember: "SET" → State, Event, Transition

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