Builder and Factory both create objects, but they target different problems: Factory hides which concrete class gets instantiated behind a single creation call, while Builder manages the step-by-step construction of a single complex object with many optional parts.
Key Points: • Factory returns a fully constructed object in one method call, typically choosing among several related classes. • Builder constructs an object incrementally, letting the caller set only the fields or parts that are relevant. • Factory is best for simple objects where creation logic mainly involves picking the right subtype. • Builder is best for complex objects with many optional parameters, avoiding telescoping constructors. • Builder typically returns the same type of object built with different configurations, while Factory can return different subtypes entirely.
Interview Tip: A concise interview answer is:
"Factory is about choosing which class to instantiate and handing back a ready object in one call, which works well for simple, relatively uniform objects. Builder is about assembling a single complex object step by step, which is the better fit when an object has many optional parameters and a constructor would otherwise need a dozen overloads."