Abstract classes in Java define incomplete classes that cannot be instantiated directly. Declared with 'abstract class Name', they serve as templates for subclasses. Abstract classes can contain both abstract methods (no implementation) and concrete methods (with implementation). They enable partial implementation—define common behavior while deferring specific details to subclasses. You cannot create instances with 'new AbstractClass()'. Understanding abstract classes enables designing extensible frameworks and class hierarchies.

Mastering abstract classes enables you to create flexible, reusable frameworks. Being able to define partial implementations that subclasses complete is essential for framework design. In professional development, abstract classes appear in template method patterns and framework designs. For example, an abstract 'HttpServlet' provides common servlet behavior while requiring subclasses to implement request handling. Abstract classes enable code reuse while ensuring customization points. Use abstract classes when you want to share code among related classes.

By learning abstract classes effectively, you'll design better inheritance hierarchies. Understanding when to use abstract classes versus interfaces is crucial for good design. Abstract classes are for "is-a" relationships with shared implementation. Prerequisites include understanding inheritance, methods, and polymorphism.