In modern Java (8+), when would you choose an abstract class over an interface? Compare the two.

An interface defines a contract. A class can implement many interfaces, and since Java 8 interfaces can have default and static methods (Java 9 added private methods), but they can't hold instance state or constructors, and their fields are implicitly public static final constants.

An abstract class can have instance fields, constructors, protected members and a partial implementation, but a class can extend only one. Choose an abstract class when related classes share state or a template of behavior (for example an AbstractRepository with common fields and a template method). Choose an interface to define a capability across unrelated types (Comparable, Runnable) or when you need multiple inheritance of type. A common design is both: an interface for the contract plus an abstract skeletal implementation, like List and AbstractList.

Key points:
- A class can implement many interfaces but extend only one class
- Interfaces can have default/static methods (Java 8+) but no instance state
- Abstract classes can have fields, constructors and protected members
- Use an abstract class for shared state or behavior among related classes
- Use an interface for a capability or contract across unrelated types