Classes are the fundamental concepts in the object-oriented paradigm. They are the basic units of object-oriented programs, and serve as the units of encapsulation, which promotes the modifiability and the reusability of them. However, improper modeling in the design phase or uncontrolled changes during the maintenance phase can degrade the quality of classes, which leads to programs cumbersome to maintain or extend. This paper proposes a method for assessing the quality of classes and restructuring them. We define an abstraction as the most cohesive form of a class, and the cohesion indicates the extent to which a class approaches the abstraction. Assessing and restructuring are both based on the notion of cohesion of a class. Less cohesive classes can be transformed into a form of an abstraction by restructuring them.