Behavioral patterns (Observer, Strategy, Command) in Software Engineering - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main purpose of the Observer pattern?

The Observer pattern lets one object (called the subject) notify many other objects (called observers) automatically when its state changes. It helps keep objects updated without tight coupling.

beginner

How does the Strategy pattern help in software design?

The Strategy pattern allows selecting an algorithm’s behavior at runtime by defining a family of algorithms and making them interchangeable. It helps change how something works without changing the code that uses it.

beginner

Explain the Command pattern in simple terms.

The Command pattern turns a request or action into an object. This lets you store, queue, or undo actions easily by treating commands as objects.

intermediate

Which behavioral pattern helps in decoupling the sender of a request from its receiver by using objects to represent actions?

The Command pattern helps decouple the sender and receiver by encapsulating requests as objects, allowing flexible command management like undo, queue, or logging.

intermediate

Why is the Observer pattern useful in user interface design?

Because it allows UI elements to automatically update when the data they display changes, without the UI needing to constantly check for updates. This keeps the interface responsive and consistent.

Which pattern allows an object to notify multiple other objects about changes?

A Observer

B Strategy

C Command

D Singleton

The Observer pattern is designed to notify multiple observers when the subject changes.

What does the Strategy pattern primarily help with?

A Creating only one instance of a class

B Notifying objects of changes

C Changing algorithms at runtime

D Encapsulating requests as objects

Strategy pattern allows selecting and changing algorithms dynamically.

Which pattern encapsulates a request as an object to support undo operations?

A Observer

B Strategy

C Factory

D Command

The Command pattern encapsulates requests as objects, enabling undo and other features.

In the Observer pattern, what are the objects that receive updates called?

A Observers

B Subjects

C Commands

D Strategies

Observers are the objects that get notified when the subject changes.

Which pattern would you use to switch between different ways of performing a task without changing the task's code?

A Observer

B Strategy

C Command

D Adapter

Strategy pattern allows changing the algorithm or behavior without modifying the client code.

Describe the Observer pattern and give a real-life example where it might be used.

Think about how a news app updates you when new articles arrive.

Explain how the Command pattern can help implement undo functionality in software.

Consider how a text editor remembers your actions to undo them.

Practice

1. Which behavioral pattern is used when an object needs to notify multiple other objects about a change in its state?

easy

A. Command

B. Strategy

C. Observer

D. Singleton

Solution

1. Step 1: Understand the role of Observer pattern

   The Observer pattern allows one object (the subject) to notify many dependent objects (observers) automatically when its state changes.

2. Step 2: Compare with other patterns

   Strategy selects algorithms at runtime, Command encapsulates requests, and Singleton restricts instance creation. Only Observer fits the notification role.

3. Final Answer:

   Observer -> Option C

4. Quick Check:

   Notification to many = Observer [OK]

Hint: Notification to many objects? Think Observer pattern [OK]

Common Mistakes:

• Confusing Strategy with Observer
• Thinking Command handles notifications
• Mixing Singleton with behavioral patterns

2. Which of the following is the correct way to define a Command pattern interface in object-oriented design?

easy

A. An interface with a method like execute() to perform the action

B. A class that holds multiple algorithms to choose from

C. A subject that notifies observers about changes

D. A singleton class that manages global state

Solution

1. Step 1: Identify Command pattern structure

   The Command pattern encapsulates a request as an object, typically by defining an interface with an execute() method to perform the action.

2. Step 2: Eliminate other options

   A class that holds multiple algorithms to choose from describes Strategy, C describes Observer, and D describes Singleton, none of which define a command interface.

3. Final Answer:

   An interface with a method like execute() to perform the action -> Option A

4. Quick Check:

   Command interface = execute() method [OK]

Hint: Command pattern always has an execute() method [OK]

Common Mistakes:

• Confusing Command with Strategy interface
• Thinking Command notifies observers
• Mixing Singleton with Command

3. Consider this code snippet using the Strategy pattern:

class Context:
    def __init__(self, strategy):
        self.strategy = strategy
    def execute(self, data):
        return self.strategy.do_algorithm(data)

class StrategyA:
    def do_algorithm(self, data):
        return sorted(data)

class StrategyB:
    def do_algorithm(self, data):
        return sorted(data, reverse=True)

context = Context(StrategyA())
result = context.execute([3, 1, 2])
print(result)

What will be the output?

medium

A. [3, 1, 2]

B. [1, 2, 3]

C. [3, 2, 1]

D. TypeError

Solution

1. Step 1: Understand the Strategy pattern usage

   The Context uses StrategyA, whose do_algorithm method sorts the list in ascending order.

2. Step 2: Trace the execution

   Calling context.execute([3, 1, 2]) calls StrategyA.do_algorithm which returns [1, 2, 3].

3. Final Answer:

   [1, 2, 3] -> Option B

4. Quick Check:

   StrategyA sorts ascending = [1, 2, 3] [OK]

Hint: StrategyA sorts ascending, so output is sorted list [OK]

Common Mistakes:

• Assuming StrategyB is used
• Confusing sorting order
• Expecting original list unchanged

4. In the Observer pattern, what is the main issue if observers are not properly detached when no longer needed?

medium

A. Memory leaks due to lingering references

B. Strategy selection becomes unpredictable

C. Commands fail to execute

D. Singleton instance is duplicated

Solution

1. Step 1: Understand observer detachment importance

   If observers are not detached, the subject keeps references to them, preventing garbage collection.

2. Step 2: Identify consequence

   This causes memory leaks because unused observers remain in memory unnecessarily.

3. Final Answer:

   Memory leaks due to lingering references -> Option A

4. Quick Check:

   Undetached observers cause memory leaks [OK]

Hint: Undetached observers cause memory leaks [OK]

Common Mistakes:

• Confusing with Strategy or Command issues
• Thinking it causes command execution failure
• Assuming Singleton duplication

5. You want to design a system where users can perform different operations (like save, load, delete) on files, and you want to queue these operations to execute later or undo them if needed. Which behavioral pattern best fits this requirement and why?

hard

A. Singleton pattern, because it ensures only one file manager exists

B. Observer pattern, because it notifies users about file changes

C. Strategy pattern, because it selects the best file operation algorithm

D. Command pattern, because it encapsulates requests as objects allowing queuing and undo

Solution

1. Step 1: Analyze the requirement for queuing and undo

   Queuing operations and supporting undo requires encapsulating each request as an object to store and manage them flexibly.

2. Step 2: Match with behavioral patterns

   The Command pattern encapsulates requests as objects, enabling queuing, logging, and undo functionality. Observer and Strategy do not support queuing commands, and Singleton only controls instance creation.

3. Final Answer:

   Command pattern, because it encapsulates requests as objects allowing queuing and undo -> Option D

4. Quick Check:

   Queue and undo operations = Command pattern [OK]

Hint: Queue and undo operations? Use Command pattern [OK]

Common Mistakes:

• Choosing Observer for queuing commands
• Confusing Strategy with Command
• Selecting Singleton for operation management