Why Behavioral patterns (Observer, Strategy, Command) in Software Engineering? - Purpose & Use Cases

The Big Idea

Discover how simple patterns can save you from tangled, hard-to-change code!

The Scenario

Imagine you are building a software system where many parts need to react to changes in one part, or where you want to switch how a task is done without changing the whole program. Doing this by hand means writing lots of repeated code and tangled logic everywhere.

The Problem

Manually updating every part that depends on a change is slow and error-prone. Changing how a task works means rewriting big chunks of code, risking bugs. The code becomes hard to read, maintain, and extend, making future updates a nightmare.

The Solution

Behavioral patterns like Observer, Strategy, and Command organize how objects communicate and change behavior. They let you separate concerns, react automatically to changes, and swap behaviors easily without rewriting code, making your software flexible and easier to manage.

Before vs After

✗ Before

if (event) { updatePartA(); updatePartB(); updatePartC(); } // repeated everywhere
if (taskType == 1) { doTaskA(); } else if (taskType == 2) { doTaskB(); }

✓ After

subject.attach(observerA);
subject.attach(observerB);
subject.notify();
strategy = new ConcreteStrategyA();
context.setStrategy(strategy);
context.executeStrategy();

What It Enables

It enables building software that adapts smoothly to change, reacts automatically to events, and keeps code clean and easy to update.

Real Life Example

Think of a news app where users get notified instantly when new articles arrive (Observer), can choose how to sort articles (Strategy), and can queue actions like saving or sharing articles (Command) without changing the app's core.

Key Takeaways

Behavioral patterns help manage communication and behavior changes in software.

They reduce repeated code and make programs easier to maintain.

Using these patterns leads to flexible, scalable, and clean software design.

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