Abstract classes and interfaces are fundamental concepts in Java programming, each serving distinct purposes in defining class hierarchies and contracts for implementing classes. Abstract Classes Abstract classes in Java are declared with the abstract keyword. They cannot be instantiated on their own and are designed to be inherited by subclasses. They may contain abstract methods—methods without a body—that subclasses must implement. When to Use Abstract Classes Use abstract classes when you want to provide a default implementation that subclasses can override. Abstract classes are suitable when you have a strong relationship between the base class and its subclasses. They allow you to define non-public members, such as protected fields and methods. Interfaces Interfaces in Java define a contract that classes can choose to implement. They contain method signatures but no implementation details. A class can implement multipl