- The document discusses interfaces and abstract classes in Kotlin, including their differences and usage scenarios. It covers interface declarations and implementations, default interface methods, abstract classes, and the discrepancies between Kotlin and Java interfaces. - Interfaces can provide default method implementations in Kotlin but abstract functions in abstract classes cannot have bodies. A class can extend only one abstract class but implement multiple interfaces. - Interfaces represent contracts while abstract classes follow an "is-a" relationship from a design perspective.

1. Interface and Abstract Classes
Kotlin Study Group
John

2. Hi, I am John
- Android Developer
- Work on Rakuten Viki
- Use Kotlin for 2 years
- Linkedin:
https://www.linkedin.com/in/chang-john-0a95237a/

3. Outline
- Preface
- Interfaces
- Default Implementation
- Abstract Classes
- Difference

4. Preface
- Be aware the discrepancy of interface between Java(8 later and 8 prior) and Kotlin
- Be aware how to implement interface and abstract class between Java and Kotlin
- Be aware different between interface and abstract class in Kotlin
- Know about scenario of interface and abstract class usage

5. Interface - declaration

6. Interface - implementation

7. Interface - default implementation

8. Properties in interfaces
Computed Property
Property

9. Resolving overriding conflicts
default implementation
C only need to override bar because A already provide
default implementation
When interface has same signature function, whatever interface has default
implementation, D still override explicitly.
If D want to call default implementation, use super<T>.function(), like
super<A>.foo()

10. Discrepancy between Kotlin and Java
Kotlin Java 8 Java 8 prior
Field Final static via companion
object
Final static Final static
Property(field+accessor) Yes(but no backing field) No No
Function abstract/default/static abstract/default/static abstract

11. Abstract Classes

12. Difference
- Interface is stateless(they can’t have a state), abstract class is stateful(they can
have a state)
- An interface cannot specify a constructor.
- Interface can provide default implementation, but abstract function in abstract
class can’t have body
- A class can extend only one abstract class, but can implement many interface.
- From design view, interface is contract and abstract class is “is-a”