Creational Patterns • Builder PatternEasy⏱️ ~2 min
Builder Pattern: Definition and Purpose
Definition
Builder Pattern is a creational design pattern that separates the construction of a complex object from its representation, allowing the same construction process to create different representations.
Problem It Solves:
When an object requires many constructor parameters (especially optional ones), traditional approaches become unmanageable. Consider creating a
Three Problematic Approaches:
First, Telescoping Constructor Pattern creates multiple constructors with increasing parameters. This becomes unmaintainable with 5+ parameters and makes it unclear which values correspond to which attributes.
Second, JavaBeans Pattern uses setters but leaves objects in inconsistent states during construction. The object might be used before all required fields are set, violating immutability.
Third, Single Constructor with Many Parameters forces clients to pass null or default values for optional parameters, reducing readability:
The Builder Solution:
Builder provides a fluent interface to construct objects step by step. It enforces required parameters through the builder constructor, allows optional parameters through method chaining, and produces an immutable object only when
The construction becomes readable:
When an object requires many constructor parameters (especially optional ones), traditional approaches become unmanageable. Consider creating a
HotelRoom object with attributes like bed type, view, floor, amenities, and special requests.Three Problematic Approaches:
First, Telescoping Constructor Pattern creates multiple constructors with increasing parameters. This becomes unmaintainable with 5+ parameters and makes it unclear which values correspond to which attributes.
Second, JavaBeans Pattern uses setters but leaves objects in inconsistent states during construction. The object might be used before all required fields are set, violating immutability.
Third, Single Constructor with Many Parameters forces clients to pass null or default values for optional parameters, reducing readability:
new HotelRoom("401", 4, BedType.KING, null, null, true, false, null).The Builder Solution:
Builder provides a fluent interface to construct objects step by step. It enforces required parameters through the builder constructor, allows optional parameters through method chaining, and produces an immutable object only when
build() is called.The construction becomes readable:
room = HotelRoomBuilder("401").setBedType(KING).setView(OCEAN).setMiniBar(true).build()Interview Tip: Emphasize that Builder is about CONSTRUCTION complexity, not just parameter count. If an object has 10 parameters but they are all required and simple, a regular constructor might suffice. Builder shines when you have optional parameters, complex validation logic, or need to construct objects in multiple steps.
💡 Key Takeaways
✓Separates object construction from representation
✓Solves problems with telescoping constructors and JavaBeans patterns
✓Produces immutable objects with readable, fluent interfaces
✓Required parameters go in builder constructor, optional ones use setter methods
✓Construction is validated only when build() is called
📌 Examples
1HotelRoom with bed type, view, floor, amenities
2Email with recipient, subject, body, CC, BCC, attachments
3Query builder for database with SELECT, WHERE, JOIN, ORDER BY clauses