Recap: Builder Pattern

Assemble an object one part at a time and validate once, where named arguments are not enough.

Builder moves the construction of a complex object into a builder that sets one part per call and validates the whole in build().

Intent. Separate the construction of a complex object from its representation, so the same construction process can create different representations.
Participants. The builder declares a step per part and build(); the concrete builder stores the parts and creates the product; the optional director runs fixed sequences of steps.
Recognise it by shape. Calls that each set one named part, often chained, then one final call that returns the finished object.
In the wild. HttpRequest.newBuilder() in Java 11 and later sets the URI, headers, timeout and method, and build() returns an immutable request.
In Python. Keyword arguments with defaults and a frozen dataclass replace a builder whenever every part is known at the call.
When it pays. Many optional parts, an object assembled across statements, or parts that accumulate. A short constructor whose arguments are already clear does not need one.

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