Differences between Java Interfaces and Abstract Classes: When to Use Interfaces and When to Use Abstract Classes

Java abstract classes and interfaces are important concepts for designing flexible code, with core differences as follows: **Definition**: Abstract classes are declared with `abstract class`, containing both abstract and concrete methods, as well as member variables. Interfaces use `interface` declarations, where members are constants (`public static final`), and methods were all abstract before JDK 8 (now support default/static methods). **Inheritance/Implementation**: Abstract classes allow single inheritance. Interfaces support multiple implementations (a class can implement multiple interfaces) and can extend multiple interfaces themselves. **Constructors**: Abstract classes have constructors; interfaces do not. **Design Purpose**: Abstract classes emphasize "what is" (`is-a` relationship), providing shared code and partial implementations. Interfaces emphasize "what can be done" (`can-do`), used for multiple implementations or behavioral specifications without inheritance relationships. **Applicable Scenarios**: Abstract classes are suitable for shared code, strong inheritance, and partial implementations. Interfaces apply to multiple implementations, behavioral specifications, or common behaviors with no inheritance ties. **Summary**: Abstract classes are "templates" (shared + partial implementation), while interfaces act as "contracts" (multiple implementations + specifications). When uncertain, choose abstract classes for inheritance-based sharing and interfaces for multiple implementations or specifications.