Some context-oriented programming languages use block-structured constructs to redirect method invocation. However, these constructs do not support dynamic layer addition and activation, which increases the binary size of the program. To address the problem, this paper proposes a new approach that uses fitness testing to support method redirection, and presents a runtime fitness testing calculus, an extension of featherweight Java calculus with context-based method resolution and object transformation, to describe the approach. Based on the calculus, the paper analyzes the influence approach as on the type safety of the program, imposes constraints on it, and formally proves that it preserves the type safety of the program when these constraints are satisfied, which validates the approach. The paper also shows the implementation of the approach and assesses the implementation, which indicates that the approach is feasible. The calculus and the implementation illustrate how to extend Java-like languages to support runtime behavior adjustment that is context-based and type-safe.