Software reusability has two main purposes: to increase the reliability of software and to reduce the cost of software development. Most current approaches to objectoriented reusability are based on empirical methods focussing on reuse of implementation code. However the most effective forms of reuse are generally found at more abstract levels of software design. A model for the definition of the structure of a reusable component allowing descriptions of object class hierarchies at different levels of abstraction is presented. This model integrates algebraic specification and concrete classes in an object-oriented language. An objectoriented programming method with reuse, based on the model, is described. Our approach reconciles formal specifications with their concrete implementations.