Interaction Diagrams

📘 Interaction Diagrams (OOAD)

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🔹 1. Definition

Interaction Diagrams are UML diagrams that show how objects communicate with each other by sending messages to perform a task.

👉 Simple idea: They describe how the system behaves (dynamic view) — who talks to whom and in what order.

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🔹 2. Purpose of Interaction Diagrams

• Show object collaboration
• Describe flow of messages
• Help understand system behavior
• Validate system design

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🔹 3. Types of Interaction Diagrams ⭐

There are mainly two important types:

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🔸 3.1 Sequence Diagram

✅ Definition:

A Sequence Diagram shows how objects interact in a time sequence (step-by-step order).

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🔹 Key Elements:

Element	Description
Object	Participant in interaction
Lifeline	Vertical dashed line showing object life
Message	Communication between objects
Activation	Time an object is active

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🔹 Diagram Description:

• Objects are placed horizontally
• Time flows top to bottom
• Arrows show messages

User → Login System → Database

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🔹 Example: ATM System

1. User inserts card
2. System asks for PIN
3. User enters PIN
4. System verifies
5. Cash is dispensed

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🔹 Types of Messages:

• Synchronous (wait for response)
• Asynchronous (no wait)
• Return message

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🔸 3.2 Communication Diagram (Collaboration Diagram)

✅ Definition:

A Communication Diagram shows interaction between objects focusing on relationships rather than time.

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🔹 Key Elements:

Element	Description
Objects	Connected with links
Messages	Labeled arrows
Sequence Numbers	Show order of messages

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🔹 Diagram Description:

• Objects are connected like a network
• Messages are numbered (1, 2, 3…)

1: request()
2: process()
3: response()

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🔹 4. Sequence vs Communication Diagram

Feature	Sequence Diagram	Communication Diagram
Focus	Time sequence	Object relationships
Layout	Vertical	Network-like
Message Order	Top → Bottom	Numbered

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🔹 5. Steps to Create Interaction Diagrams

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🔹 Step 1: Identify Scenario

• Choose a use case

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🔹 Step 2: Identify Objects

• List participating objects

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🔹 Step 3: Determine Messages

• Define communication steps

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🔹 Step 4: Arrange Order

• Show sequence or numbering

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🔹 Step 5: Draw Diagram

• Use UML notation

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🔹 6. Example (Library System)

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🔹 Scenario: Borrow Book

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🔹 Sequence Diagram:

Member → Librarian → Book

Steps:

1. Member requests book
2. Librarian checks availability
3. Book is issued

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🔹 Communication Diagram:

1: requestBook()
2: checkAvailability()
3: issueBook()

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🔹 7. Advantages of Interaction Diagrams

• Clear understanding of system flow
• Helps in debugging design
• Improves communication
• Shows real-time interactions

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🔹 8. Important Rules / Guidelines

• Keep diagrams simple and clear
• Use meaningful object names
• Show correct message order
• Avoid unnecessary complexity

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🔹 9. Likely Exam Questions

1. Define interaction diagram.
2. Explain sequence diagram with example.
3. What is a communication diagram?
4. Difference between sequence and communication diagrams.
5. Explain elements of sequence diagram.
6. Describe types of messages.
7. Steps to create interaction diagrams.
8. Draw sequence diagram for ATM system.
9. Advantages of interaction diagrams.
10. Role of interaction diagrams in OOAD.

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🔹 10. Quick Revision Summary 🧠

• Interaction Diagrams = Object communication

• Types:

  • Sequence Diagram → Time-based
  • Communication Diagram → Relationship-based

👉 Key Points:

• Objects send messages
• Show dynamic behavior

👉 Trick to Remember: "SC → Sequence & Communication"

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