The Builder wraps a product to extract out the steps of constructing. Constructing steps are optional, not all of the objects will have the same implementation.

Components

• Abstract Builder
  • It holds the abstract product
  • It provides methods to setup product’s properties
  • It provides build method to finalize the construction
• Concrete Builder
• Abstract Product
• Concrete Product
• Director (Optional): Encapsulate certain steps for certain product.
  • It can be less likely used if we also use Prototype pattern: Implementation

V.S. Factory Method

In implementation,

• The Builder has been holding an unfinished product instance already (not a strict rule) and builds it step-by-step; the build() method just returns the instance. It emphasizes how a product is made (step-by-step)
• The Factory Method creates the instance at the call of create(). It emphasizes which product is made (via polymorphism).

Since the Builder doesn’t initialize the product strictly, two patterns can combine together (it is not about Factory Method wraps the Builder, it is about Builder and Factory Method are at the same level). (example in the following note)

V.S. Template Method

V.S. Builder

SOLID Principle

✅ Single Responsibility Principle (SRP): The Builder only contains methods related to the target product.

⚠️ Open/Closed Principle (OCP): Builder is not open to be extended.

✅ Liskov Substitution Principle (LSP) / Interface Segregation Principle (ISP): The Builder only contains methods related to the target product.

✅ Dependency Inversion Principle (DIP): Builder depends on an Abstract Product.

Implementation