Kotlin types for Java Developers

Presented at BlrKotlin (meetup.com/BlrKotlin). This talk is meant to give a kick start to Java Developers about Kotlin's Type system.

Transcript

1. Kotlin Types for Java Developers Chandra Sekar @iChanSek
2. Type System in Java
3. Type System in Java • Primitive Types
4. Type System in Java • Primitive Types • Class Types
5. Primitive Types
6. Primitive Types - Java
7. Primitive Types - Java • long
8. Primitive Types - Java • long • int
9. Primitive Types - Java • long • int • short
10. Primitive Types - Java • long • int • short • byte
11. Primitive Types - Java • long • int • short • byte • char
12. Primitive Types - Java • long • int • short • byte • char • double
13. Primitive Types - Java • long • int • short • byte • char • double • float
14. Primitive Types - Java • long • int • short • byte • char • double • float • boolean
15. Primitive Types - Kotlin
16. Primitive Types - Kotlin • Long
17. Primitive Types - Kotlin • Long • Int
18. Primitive Types - Kotlin • Long • Int • Short
19. Primitive Types - Kotlin • Long • Int • Short • Byte
20. Primitive Types - Kotlin • Long • Int • Short • Byte • Char
21. Primitive Types - Kotlin • Long • Int • Short • Byte • Char • Double
22. Primitive Types - Kotlin • Long • Int • Short • Byte • Char • Double • Float
23. Primitive Types - Kotlin • Long • Int • Short • Byte • Char • Double • Float • Boolean
24. Primitive Types - Comparison
25. Primitive Types - Comparison Java Kotlin
26. Primitive Types - Comparison Java Kotlin long = Long
27. Primitive Types - Comparison Java Kotlin long = Long int = Int
28. Primitive Types - Comparison Java Kotlin long = Long int = Int short = Short
29. Primitive Types - Comparison Java Kotlin long = Long int = Int short = Short byte = Byte
30. Primitive Types - Comparison Java Kotlin long = Long int = Int short = Short byte = Byte char = Char
31. Primitive Types - Comparison Java Kotlin long = Long int = Int short = Short byte = Byte char = Char double = Double
32. Primitive Types - Comparison Java Kotlin long = Long int = Int short = Short byte = Byte char = Char double = Double float = Float
33. Primitive Types - Comparison Java Kotlin long = Long int = Int short = Short byte = Byte char = Char double = Double float = Float boolean = Boolean
34. Wrapper Types - Java
35. Wrapper Types - Java • Long
36. Wrapper Types - Java • Long • Integer
37. Wrapper Types - Java • Long • Integer • Short
38. Wrapper Types - Java • Long • Integer • Short • Byte
39. Wrapper Types - Java • Long • Integer • Short • Byte • Character
40. Wrapper Types - Java • Long • Integer • Short • Byte • Character • Double
41. Wrapper Types - Java • Long • Integer • Short • Byte • Character • Double • Float
42. Wrapper Types - Java • Long • Integer • Short • Byte • Character • Double • Float • Boolean
43. Primitive Types - Comparison
44. Primitive Types - Comparison Java Kotlin Java(Wrappers)
45. Primitive Types - Comparison Java Kotlin Java(Wrappers) long = Long ~ Long
46. Primitive Types - Comparison Java Kotlin Java(Wrappers) long = Long ~ Long int = Int ~ Integer
47. Primitive Types - Comparison Java Kotlin Java(Wrappers) long = Long ~ Long int = Int ~ Integer short = Short ~ Short
48. Primitive Types - Comparison Java Kotlin Java(Wrappers) long = Long ~ Long int = Int ~ Integer short = Short ~ Short byte = Byte ~ Byte
49. Primitive Types - Comparison Java Kotlin Java(Wrappers) long = Long ~ Long int = Int ~ Integer short = Short ~ Short byte = Byte ~ Byte char = Char ~ Character
50. Primitive Types - Comparison Java Kotlin Java(Wrappers) long = Long ~ Long int = Int ~ Integer short = Short ~ Short byte = Byte ~ Byte char = Char ~ Character double = Double ~ Double
51. Primitive Types - Comparison Java Kotlin Java(Wrappers) long = Long ~ Long int = Int ~ Integer short = Short ~ Short byte = Byte ~ Byte char = Char ~ Character double = Double ~ Double float = Float ~ Float
52. Primitive Types - Comparison Java Kotlin Java(Wrappers) long = Long ~ Long int = Int ~ Integer short = Short ~ Short byte = Byte ~ Byte char = Char ~ Character double = Double ~ Double float = Float ~ Float boolean = Boolean ~ Boolean
53. Example - Primitive Type
54. Example - Primitive Type int x = 10; // Java - Primitive int
55. Example - Primitive Type int x = 10; // Java - Primitive int // is Same as val x: Int = 10 // Kotlin - Primitive int
56. Example - Primitive Type int x = 10; // Java - Primitive int // is Same as val x: Int = 10 // Kotlin - Primitive int // Even more simpler way val x = 10 // Kotlin - Type inferred as Int
57. Example - Wrapper Type
58. Example - Wrapper Type Integer x = new Integer(10); // Java - Wrapper Type Integer y = 10; // Java - Wrapper Type (Auto Boxing)
59. Example - Wrapper Type Integer x = new Integer(10); // Java - Wrapper Type Integer y = 10; // Java - Wrapper Type (Auto Boxing) // Lets assume they are same as val x: Int = 10 val y = 10
60. Example - Wrapper Type Integer x = new Integer(10); // Java - Wrapper Type Integer y = 10; // Java - Wrapper Type (Auto Boxing) // Lets assume they are same as val x: Int = 10 val y = 10 // then, how to represent this in Kotlin? Integer z = null;
61. Nullable Types
62. Nullable Types
63. Nullable Types • Kotlin has separate type to handle null.
64. Nullable Types • Kotlin has separate type to handle null. • By default all types are non nullable.
65. Nullable Types • Kotlin has separate type to handle null. • By default all types are non nullable. • If you want to assign null, then you have to declare them as nullable.
66. Nullable Types • Kotlin has separate type to handle null. • By default all types are non nullable. • If you want to assign null, then you have to declare them as nullable. • Syntax - postfix corresponding type with ‘?’
67. Nullable Types • Kotlin has separate type to handle null. • By default all types are non nullable. • If you want to assign null, then you have to declare them as nullable. • Syntax - postfix corresponding type with ‘?’ • Example - Int?, String?, User?
68. Back to Previous Example
69. Back to Previous Example // then, how to represent this in Kotlin? Integer z = null;
70. Back to Previous Example // then, how to represent this in Kotlin? Integer z = null; // And, the answer is val z: Int? = null
71. Primitive Types - Comparison
72. Primitive Types - Comparison Java Kotlin Java(Wrappers)
73. Primitive Types - Comparison Java Kotlin Java(Wrappers) long = Long ~ Long? = Long
74. Primitive Types - Comparison Java Kotlin Java(Wrappers) long = Long ~ Long? = Long int = Int ~ Int? = Integer
75. Primitive Types - Comparison Java Kotlin Java(Wrappers) long = Long ~ Long? = Long int = Int ~ Int? = Integer short = Short ~ Short? = Short
76. Primitive Types - Comparison Java Kotlin Java(Wrappers) long = Long ~ Long? = Long int = Int ~ Int? = Integer short = Short ~ Short? = Short byte = Byte ~ Byte? = Byte
77. Primitive Types - Comparison Java Kotlin Java(Wrappers) long = Long ~ Long? = Long int = Int ~ Int? = Integer short = Short ~ Short? = Short byte = Byte ~ Byte? = Byte char = Char ~ Char? = Character
78. Primitive Types - Comparison Java Kotlin Java(Wrappers) long = Long ~ Long? = Long int = Int ~ Int? = Integer short = Short ~ Short? = Short byte = Byte ~ Byte? = Byte char = Char ~ Char? = Character double = Double ~ Double? = Double
79. Primitive Types - Comparison Java Kotlin Java(Wrappers) long = Long ~ Long? = Long int = Int ~ Int? = Integer short = Short ~ Short? = Short byte = Byte ~ Byte? = Byte char = Char ~ Char? = Character double = Double ~ Double? = Double float = Float ~ Float? = Float
80. Primitive Types - Comparison Java Kotlin Java(Wrappers) long = Long ~ Long? = Long int = Int ~ Int? = Integer short = Short ~ Short? = Short byte = Byte ~ Byte? = Byte char = Char ~ Char? = Character double = Double ~ Double? = Double float = Float ~ Float? = Float boolean = Boolean ~ Boolean? = Boolean
81. Take Away - Primitive Types
82. Take Away - Primitive Types • Kotlin doesn’t have Primitive types explicitly.
83. Take Away - Primitive Types • Kotlin doesn’t have Primitive types explicitly. • Kotlin compiler chooses Primitive type when the type is mentioned as non nullable.
84. Take Away - Primitive Types • Kotlin doesn’t have Primitive types explicitly. • Kotlin compiler chooses Primitive type when the type is mentioned as non nullable. • If the type is mentioned as nullable, Kotlin compiler treats it as a wrapper type.
85. Class Types
86. Class Types - Java
87. Class Types - Java • class String
88. Class Types - Java • class String • interface CharSequence
89. Class Types - Java • class String • interface CharSequence • enum Color
90. Class Types - Java • class String • interface CharSequence • enum Color • class User
91. Class Types - Java • class String • interface CharSequence • enum Color • class User
92. Class Types - Java • class String • interface CharSequence • enum Color • class User
93. Class Types - Java • class String • interface CharSequence • enum Color • class User Object is common to all.
94. Supermost Type
95. Supermost Type Object (in Java)
96. Supermost Type Object (in Java) is Same as
97. Supermost Type Object (in Java) is Same as Any (in Kotlin)
98. Type Hierarchy - Java
99. Type Hierarchy - Java Object
100. Type Hierarchy - Java Object String
101. Type Hierarchy - Java Object Integer String
102. Type Hierarchy - Java Object Integer String User
103. Type Hierarchy - Kotlin
104. Type Hierarchy - Kotlin Any
105. Type Hierarchy - Kotlin Any String
106. Type Hierarchy - Kotlin Any Int String
107. Type Hierarchy - Kotlin Any Int String User
108. Examples
109. Examples Object obj = new Object(); Object user = new User(); Object str = "Test"; Object integer = 10;
110. Examples val any = Any() val user: Any = User() val str: Any = "Test" val int: Any = 10 Object obj = new Object(); Object user = new User(); Object str = "Test"; Object integer = 10;
111. Examples val any = Any() val user: Any = User() val str: Any = "Test" val int: Any = 10 Object obj = new Object(); Object user = new User(); Object str = "Test"; Object integer = 10; User user = new User(); String str = "Test"; Integer i = 10;
112. Examples val any = Any() val user: Any = User() val str: Any = "Test" val int: Any = 10 Object obj = new Object(); Object user = new User(); Object str = "Test"; Object integer = 10; User user = new User(); String str = "Test"; Integer i = 10; val user: User = User() val str: String = "Test" val i: Int = 10
113. Examples val any = Any() val user: Any = User() val str: Any = "Test" val int: Any = 10 Object obj = new Object(); Object user = new User(); Object str = "Test"; Object integer = 10; User user = new User(); String str = "Test"; Integer i = 10; val user = User() val str = "Test" val i = 10 val user: User = User() val str: String = "Test" val i: Int = 10
114. Examples val any = Any() val user: Any = User() val str: Any = "Test" val int: Any = 10 Object obj = new Object(); Object user = new User(); Object str = "Test"; Object integer = 10; User user = new User(); String str = "Test"; Integer i = 10; val user = User() val str = "Test" val i = 10 val user: User = User() val str: String = "Test" val i: Int = 10 Type Inferrence
115. Examples (with null)
116. Examples (with null) Object obj = null; User user = null; String str = null; Integer i = null;
117. Examples (with null) Object obj = null; User user = null; String str = null; Integer i = null; val any: Any = null val user: User = null val str: String = null val i: Int = null
118. Examples (with null) Object obj = null; User user = null; String str = null; Integer i = null;
119. Examples (with null) Object obj = null; User user = null; String str = null; Integer i = null; val any: Any? = null val user: User? = null val str: String? = null val i: Int? = null
120. Type Hierarchy (Final) - Java
121. Type Hierarchy (Final) - Java Object
122. Type Hierarchy (Final) - Java Object String
123. Type Hierarchy (Final) - Java Object Integer String
124. Type Hierarchy (Final) - Java Object Integer String User
125. Type Hierarchy (Final) - Java Object Integer String User null
126. Type Hierarchy - Kotlin
127. Type Hierarchy - Kotlin Any
128. Type Hierarchy - Kotlin Any String
129. Type Hierarchy - Kotlin Any Int String
130. Type Hierarchy - Kotlin Any Int String User
131. Type Hierarchy - Kotlin Any Int String User null
132. Type Hierarchy - Kotlin Any Int String User null then what???
133. Nothing Type
134. Nothing Type
135. Nothing Type • A special type that exists but doesn’t represent any value.
136. Nothing Type • A special type that exists but doesn’t represent any value. • Should be used to mark any code that can never be reached.
137. Nothing Type • A special type that exists but doesn’t represent any value. • Should be used to mark any code that can never be reached. • We can also use it while type inference.
138. Examples - Nothing Type
139. Examples - Nothing Type val s = person.name ?: throw IllegalArgumentException("Name required") println(s) // 's' is known to be initialized at this point
140. Examples - Nothing Type val s = person.name ?: throw IllegalArgumentException("Name required") println(s) // 's' is known to be initialized at this point val x = null // 'x' has type `Nothing?` val l = listOf(null) // 'l' has type `List<Nothing?> var user: User? = x
141. Type Hierarchy - Kotlin
142. Type Hierarchy - Kotlin Any
143. Type Hierarchy - Kotlin Any String
144. Type Hierarchy - Kotlin Any Int String
145. Type Hierarchy - Kotlin Any Int String User
146. Type Hierarchy - Kotlin Any Int String User Nothing
147. Type Hierarchy - with null - Kotlin
148. Type Hierarchy - with null - Kotlin Any? Any
149. Type Hierarchy - with null - Kotlin Any? Any String? String
150. Type Hierarchy - with null - Kotlin Any? Any String? String Any? String?
151. Type Hierarchy - with null - Kotlin Any? Any String? String Any? String? Any? String
152. Type Hierarchy (Final) - Kotlin
153. Type Hierarchy (Final) - Kotlin Any
154. Type Hierarchy (Final) - Kotlin Any String
155. Type Hierarchy (Final) - Kotlin Any String Int
156. Type Hierarchy (Final) - Kotlin Any String Int User
157. Type Hierarchy (Final) - Kotlin Any String Int User Nothing
158. Type Hierarchy (Final) - Kotlin Any String Int User Nothing Any?
159. Type Hierarchy (Final) - Kotlin Any String Int User Nothing Any? String?
160. Type Hierarchy (Final) - Kotlin Any String Int User Nothing Any? String? Int?
161. Type Hierarchy (Final) - Kotlin Any String Int User Nothing Any? String? Int? User?
162. Type Hierarchy (Final) - Kotlin Any String Int User Nothing Any? String? Int? User? Nothing?
163. Type Hierarchy (Final) - Kotlin Any String Int User Nothing Any? String? Int? User? Nothing?
164. void == Unit
165. Examples - void and Unit
166. Examples - void and Unit private void test() {} private void test() { return; }
167. Examples - void and Unit private void test() {} private void test() { return; } private fun test(): Unit {} private fun test(): Unit { return Unit } private fun test() {} private fun test(): Unit {}
168. Array Type
169. Array Type
170. Array Type • Kotlin has no [] syntax for creating Arrays.
171. Array Type • Kotlin has no [] syntax for creating Arrays. • Instead it has a special classs called Array<T>
172. Examples - Array Type
173. Examples - Array Type String[] names = new String[10]; String[] colors = new String[] {"Red", "Green", "Blue"};
174. Examples - Array Type String[] names = new String[10]; String[] colors = new String[] {"Red", "Green", "Blue"}; val names: Array<String> = emptyArray() val colors: Array<String> = arrayOf("Red", "Green", "Blue") val nulls: Array<String?> = arrayOfNulls(10)
175. Examples - Array of Primitive Type
176. Examples - Array of Primitive Type int[] numbers = new int[10]; int[] nums = new int[] {1, 2, 3, 4, 5};
177. Examples - Array of Primitive Type int[] numbers = new int[10]; int[] nums = new int[] {1, 2, 3, 4, 5}; val numbers: Array<Int> = emptyArray() val nums: Array<Int> = arrayOf(2, 3, 4) val nulls: Array<Int?> = arrayOfNulls(10)
178. Examples - Array of Primitive Type int[] numbers = new int[10]; int[] nums = new int[] {1, 2, 3, 4, 5}; val numbers: Array<Int> = emptyArray() val nums: Array<Int> = arrayOf(2, 3, 4) val nulls: Array<Int?> = arrayOfNulls(10) } Integer[]
179. Examples - Array of Primitive Type int[] numbers = new int[10]; int[] nums = new int[] {1, 2, 3, 4, 5}; val numbers: Array<Int> = emptyArray() val nums: Array<Int> = arrayOf(2, 3, 4) val nulls: Array<Int?> = arrayOfNulls(10) val nums: IntArray = intArrayOf(1, 2, 3) val longs: LongArray = longArrayOf(1L, 2L, 3L) } Integer[]
180. Examples - Array of Primitive Type int[] numbers = new int[10]; int[] nums = new int[] {1, 2, 3, 4, 5}; val numbers: Array<Int> = emptyArray() val nums: Array<Int> = arrayOf(2, 3, 4) val nulls: Array<Int?> = arrayOfNulls(10) val nums: IntArray = intArrayOf(1, 2, 3) val longs: LongArray = longArrayOf(1L, 2L, 3L) } Integer[] } int[]
181. Nullable Types in Java
182. Nullable Types - Interoperability
183. Nullable Types - Interoperability Since Java doesn’t have nullable types, we need to take extra care of our code while taking adventage of interoperability.
184. Nullable Types - Interoperability
185. Nullable Types - Interoperability // Test.java public class Test { public String test() { return null; } }
186. Nullable Types - Interoperability // Test.java public class Test { public String test() { return null; } } // TestKotlin.kt class Test { private test() { val test = Test() test.test().toString() } }
187. Nullable Types - Interoperability // Test.java public class Test { public String test() { return null; } } // TestKotlin.kt class Test { private test() { val test = Test() test.test().toString() } } This will lead to NPE
188. How to avoid such NPE?
189. How to avoid such NPE? // Test.java public class Test { @Nullable public String test() { return null; } }
190. How to avoid such NPE? // TestKotlin.kt class Test { private test() { val test = Test() test.test().toString() } } // Test.java public class Test { @Nullable public String test() { return null; } }
191. How to avoid such NPE? // TestKotlin.kt class Test { private test() { val test = Test() test.test().toString() } } Now no NPE as it fails at Compile Time // Test.java public class Test { @Nullable public String test() { return null; } }
192. Take Away
193. Take Away • Kotlin has no primitive and wrapper type speartely.
194. Take Away • Kotlin has no primitive and wrapper type speartely. • Nullable and Non-Nullable values are two separate types.
195. Take Away • Kotlin has no primitive and wrapper type speartely. • Nullable and Non-Nullable values are two separate types. • By default all types are Non-Nullable.
196. Take Away • Kotlin has no primitive and wrapper type speartely. • Nullable and Non-Nullable values are two separate types. • By default all types are Non-Nullable. • Type and Type? are not same.
197. Take Away - Continued…
198. Take Away - Continued… • Object in Java is same as Any in Kotlin.
199. Take Away - Continued… • Object in Java is same as Any in Kotlin. • void in Java is same as Unit in Kotlin.
200. Take Away - Continued… • Object in Java is same as Any in Kotlin. • void in Java is same as Unit in Kotlin. • Nothing is not same as Unit or void.
201. Take Away - Continued… • Object in Java is same as Any in Kotlin. • void in Java is same as Unit in Kotlin. • Nothing is not same as Unit or void. • Nothing is converted into void in byte code level as JVM doesn’t has it.
202. Take Away - Continued… • Object in Java is same as Any in Kotlin. • void in Java is same as Unit in Kotlin. • Nothing is not same as Unit or void. • Nothing is converted into void in byte code level as JVM doesn’t has it. • Always annotate your Java types to avoid NPE in Kotlin world.
203. Thank You Chandra Sekhar @iChanSek