Builder and Factory Patterns

When object creation becomes complex, Builder and Factory patterns provide clean solutions. This article covers fluent builders, step builders, static factory methods, and Spring integration.

📋 At a Glance

Aspect	Details
Patterns	Builder, Step Builder, Factory Method, Abstract Factory
Use Cases	Complex construction, many parameters, conditional creation
Tools	Lombok @Builder, Records

🔬 Deep Dive

Pattern 1: Fluent Builder

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Pattern 2: Step Builder (Compile-Time Safety)

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Pattern 3: Static Factory Methods

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Pattern 4: Factory with Spring

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Lombok @Builder

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🐛 Debug This: The Mutable Builder Trap

A developer reports: "We're getting random test failures. The same builder creates different objects in different tests!"

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Why do orders have different items? Find the state mutation bug!

✅ Solution:

The builder is mutable and reused - each addItem() call modifies the shared items list:

baseBuilder.addItem(item2) - adds item2 to the list (now has item1, item2)
baseBuilder.addItem(item3) - adds item3 to the SAME list (now has item1, item2, item3)
Both order1 and order2 reference the same mutated list!

Fix 1: Defensive copy in build()

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Fix 2: Immutable builder (each method returns new builder)

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The lesson: Builders should either be single-use or return new instances on modification. Mutable shared state causes subtle bugs.

💻 Exercises

Exercise 1: Implement Fluent Builder

⭐ Difficulty: Easy | ⏱️ Time: 15 minutes

Task: Create a fluent builder for this class.

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✅ Solution:

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Exercise 2: Step Builder Pattern

⭐⭐ Difficulty: Medium | ⏱️ Time: 20 minutes

Task: Create a step builder that enforces: customerId → items → build.

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✅ Solution:

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Exercise 3: Static Factory Methods

⭐⭐ Difficulty: Medium | ⏱️ Time: 15 minutes

Task: Replace constructors with static factory methods.

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✅ Solution:

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Exercise 4: Abstract Factory

⭐⭐⭐ Difficulty: Medium-Hard | ⏱️ Time: 20 minutes

Task: Create an Abstract Factory for UI components that supports different themes.

✅ Solution:

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Exercise 5: Lombok @Builder with Validation

⭐⭐⭐⭐ Difficulty: Hard | ⏱️ Time: 25 minutes

Task: Create a Lombok builder with custom validation and defaults.

✅ Solution:

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🎤 Senior-Level Interview Questions

Q1: Builder vs Constructor - when to use each?

Constructor: 1-3 required parameters, all simple types
Builder: 4+ parameters, optional parameters, complex objects
Rule of thumb: If you need to look at JavaDoc to understand parameter order, use Builder

Q2: How do you make builders thread-safe?

Return new builder instance from each method (immutable builder)
Or make builder single-use and not shared
Or synchronize access (rarely needed)

Q3: What's the difference between Static Factory and Factory Method pattern?

Static Factory: Static method that returns instance (Money.of(100, USD))
Factory Method: Abstract method overridden by subclasses to create objects
Static Factory is about naming and caching, Factory Method is about polymorphic creation

📝 Summary

Pattern	Use When
Fluent Builder	Many optional params, complex objects
Step Builder	Required fields, compile-time safety
Static Factory	Named construction, caching
Abstract Factory	Family of related objects

📅 Review Schedule for This Article

Day	Task	Time
Day 1	Review Builder vs Constructor decision table	5 min
Day 3	Redo Exercise 1 (Fluent Builder)	15 min
Day 7	Answer interview questions without looking	10 min
Day 14	Redo Debug This (Mutable Builder Trap)	10 min
Day 30	Find one constructor in your project that should be a builder	15 min

Next: [Part 10: Composition Over Inheritance]

📋 At a Glance

🔬 Deep Dive

Pattern 1: Fluent Builder

Pattern 2: Step Builder (Compile-Time Safety)

Pattern 3: Static Factory Methods

Pattern 4: Factory with Spring

Lombok @Builder

🐛 Debug This: The Mutable Builder Trap

💻 Exercises

Exercise 1: Implement Fluent Builder

Exercise 2: Step Builder Pattern

Exercise 3: Static Factory Methods

Exercise 4: Abstract Factory

Exercise 5: Lombok @Builder with Validation

🎤 Senior-Level Interview Questions

Q1: Builder vs Constructor - when to use each?

Q2: How do you make builders thread-safe?

Q3: What's the difference between Static Factory and Factory Method pattern?

📝 Summary

📅 Review Schedule for This Article

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