The document discusses different ways to use objects in Kotlin including object declarations, object expressions, companion objects, nested classes, data classes, enumerated classes, sealed classes, operator overloading, and destructuring operators. It provides examples of how to define and use these object types in Kotlin as well as comparisons to their usage in Java. Key topics covered include singleton design patterns with object declarations, anonymous implementations with object expressions, instance and factory design with companion objects, and benefits of data classes like toString(), equals(), copy(), and destructuring.

1. Objects
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
- Object
- Object declarations
- Object expressions
- Companion Objects
- Nested Classes
- Data Classes
- ADT(Algebraic Data Type)
- Enumerated Classes
- Sealed Classes
- Operator overloading
- destructure operator

4. Preface
- Understand usage scenario
- Know discrepancy of concept between Kotlin and Java

5. What’s meaning of object keyword?
If you need to hold on to a single instance with state that is consistent throughout the
time that your program is running, consider defining a singleton.
According different usages, there are three ways to use the object keyword:
- object declarations(singleton design pattern)
- object expressions(anonymous implementation)
- companion objects(instance/factory design pattern)

6. Object declarations

7. Object expressions
Object expression is replacement of anonymous class in Java
Object expression is inner class !!!

8. Companion objects - define

9. Companion objects - usage in Kotlin

10. Companion Object - usage in Java

11. Nested Class

12. Nested Class - tricky question

13. Nested Class is not equal to Inner Class

14. Data Class
data classes are classes designed specifically for holding data, and they
come with some powerful data manipulation benefits
- toString()
- equals()
- copy()
- destructuring

15. Data Class

16. Class without marked data

17. Data Class Rule

18. Enumerated Classes

19. What enum class is similar between Kotlin and Java?

20. What enum class are different between Kotlin and Java?

21. Enumerated Class - even override

22. Sealed class

23. Kotlin would checks ADT type Exhaustively

24. Operator Overloading

25. Destructure Operator