Difference between interface and abstract class in Java?

Tests understanding of OOP abstraction mechanisms in Java and how their distinction has evolved.

Key differences:

• Interface: multiple inheritance supported, all fields are public static final, methods default to public abstract (Java 8+ allows default/static methods; Java 9+ allows private methods)
• Abstract class: single inheritance, can have instance fields, constructors, and methods of any visibility; use when sharing state or partial implementation

Rule of thumb: prefer interfaces to define contracts/capabilities; use abstract classes when common state or template-method patterns are needed.

Version-specific follow-ups: Java 8 default methods blurring the line, Java 9 private interface methods, sealed classes (Java 17) restricting hierarchies.