Java FAQ: Top Questions

Abstract classes and interfaces both provide abstraction, but they differ in structure and use.

• Abstract Class:
  • Declared with abstract keyword; can have abstract and non-abstract methods.
  • Can have instance fields and constructors.
  • Supports single inheritance (a class can extend only one abstract class).
  • Used for sharing code among related classes.
• Interface:
  • Declared with interface keyword; traditionally only abstract methods (now default , static ).
  • Only constants (static final fields), no instance fields or constructors.
  • Supports multiple inheritance (a class can implement multiple interfaces).
  • Used to define a contract for unrelated classes.