Component-based programming is currently carried out using mainstream object-oriented languages. These languages have to be used in a highly disciplined way to guarantee flexible component composition and extensibility. This paper investigates abstractions for component-oriented programming on the programming language level. We propose a simple prototype-based model for first-class components on top of a class-based object-oriented language. The model is formalized as an extension of Featherweight Java. Our calculus includes a minimal set of primitives to dynamically build, extend, and compose software components, while supporting features like explicit context dependencies, late composition, unanticipated component extensibility, and strong encapsulation. We present a type system for our calculus that ensures type-safe component definition, composition, and evolution.