Which pattern helps create immutable objects by assembling state at construction time?

Which pattern helps create immutable objects by assembling state at construction time?

• A. Constructors
• B. Factories
• C. Prototypes
• D. Builders

Answer: (D)

Constructing immutable objects from a lot of configuration options is smoother with a builder. The builder is mutable and collects values for each field, often providing a fluent way to set optional parameters. When everything you need is gathered, a build method creates the final object in a single step. The resulting instance has all fields initialized in its constructor and exposes no mutators, so its state cannot change after creation. This approach keeps the construction logic separate from the immutable object's definition, avoiding long, unwieldy constructors while still delivering a fully initialized, immutable result. Other patterns can help with creation in different ways—direct constructors can become cumbersome with many options, factories choose how to create objects or which concrete type to instantiate, and prototypes clone existing objects—but they don’t specifically address assembling a complex set of state over time to produce one immutable object.

Constructing immutable objects from a lot of configuration options is smoother with a builder. The builder is mutable and collects values for each field, often providing a fluent way to set optional parameters. When everything you need is gathered, a build method creates the final object in a single step. The resulting instance has all fields initialized in its constructor and exposes no mutators, so its state cannot change after creation.

This approach keeps the construction logic separate from the immutable object's definition, avoiding long, unwieldy constructors while still delivering a fully initialized, immutable result. Other patterns can help with creation in different ways—direct constructors can become cumbersome with many options, factories choose how to create objects or which concrete type to instantiate, and prototypes clone existing objects—but they don’t specifically address assembling a complex set of state over time to produce one immutable object.