What type of methods can abstract classes contain?

Abstract classes are designed to provide a common base for other classes and can contain a variety of method types. The correct answer indicates that abstract classes can have both abstract methods and concrete methods.

Abstract methods are those that are declared without an implementation, meaning they must be overridden by subclasses. This allows subclasses to provide specific functionality while adhering to the interface set forth by the abstract class. On the other hand, concrete methods are fully defined within the abstract class. They can contain implementation code that can be reused by subclasses, promoting code reusability and reducing redundancy.

The inclusion of both types of methods grants developers the flexibility to set up a structured framework while still allowing for concrete implementations where necessary. This feature is particularly useful in frameworks and libraries, where the base class provides vital methods that can be leveraged without requiring every implementing class to rewrite common functionality.