Final answer:
In Java, abstract data types are primarily expressed through interfaces and classes, where interfaces provide a contract for classes to implement certain methods, and classes serve as blueprints for objects encapsulating data and behavior.
Step-by-step explanation:
Abstract data types (ADTs) are fundamental concepts in computer science, particularly with regard to data structure design and encapsulation. In the Java programming language, ADTs can be expressed using interfaces and classes. An interface in Java is a reference type that can contain abstract methods, default methods, static methods, and constants. It represents a contract where classes that implement the interface agree to perform certain actions. On the other hand, a class in Java is a blueprint from which individual objects are created. A class can define state in the form of fields, and behavior in the form of methods.
To answer the question more directly, abstract data types are mainly expressed through B. Interfaces and C. Classes in Java, as these structures are used to encapsulate and model the data and its interactions.