Kotlin - Data Types

Data Types in Kotlin

Kotlin is a statically typed programming language that runs on the Java Virtual Machine (JVM). It is fully interoperable with Java and has a concise syntax that reduces boilerplate code. One of the fundamental concepts in any programming language is its data types. In Kotlin, data types are categorized primarily into built-in types, user-defined types, and special types. Understanding Kotlin's data types is crucial for effective programming and building robust applications.

1. Introduction to Data Types in Kotlin

Every variable in Kotlin has a data type, either explicitly defined or inferred by the compiler. Kotlin is statically typed, which means types are checked at compile-time rather than at runtime. This reduces errors and increases performance.

1.1 Type Inference

Kotlin supports type inference, allowing the compiler to automatically deduce the type of a variable based on the assigned value. For example:

val name = "John" // The compiler infers it as String
val age = 30      // The compiler infers it as Int

2. Kotlin Basic Data Types

Kotlin has several built-in data types that cover most use cases. These can be categorized into:

• Numeric Types
• Character
• Boolean
• String

2.1 Numeric Types

Numeric types in Kotlin include the following:

• Byte: 8-bit signed integer (-128 to 127)
• Short: 16-bit signed integer (-32,768 to 32,767)
• Int: 32-bit signed integer (-2^31 to 2^31-1)
• Long: 64-bit signed integer (-2^63 to 2^63-1)
• Float: 32-bit floating point number
• Double: 64-bit floating point number

Example:

val myByte: Byte = 120
val myShort: Short = 30000
val myInt: Int = 1_000_000
val myLong: Long = 9_000_000_000L
val myFloat: Float = 3.14F
val myDouble: Double = 3.14159265358979

2.2 Type Conversion

Kotlin does not perform implicit type conversions between smaller and larger numeric types. You must explicitly convert between types using conversion functions like toInt(), toLong(), toFloat(), etc.

val a: Int = 100
val b: Long = a.toLong()

2.3 Characters

Char represents a single 16-bit Unicode character. It is defined using single quotes.

val letter: Char = 'A'

2.4 Booleans

Boolean type in Kotlin has only two possible values: true and false.

val isActive: Boolean = true

2.5 Strings

String is a sequence of characters. Kotlin provides string interpolation and multi-line strings.

val name: String = "Kotlin"
val greeting = "Hello, $name"

val multiline = """
    This is a
    multi-line string.
""".trimIndent()

3. Arrays in Kotlin

Arrays are used to store multiple values of the same type. Kotlin provides a built-in Array class and several utility functions.

val numbers = arrayOf(1, 2, 3, 4, 5)
val strings = arrayOf("One", "Two", "Three")

3.1 Primitive Arrays

To optimize memory, Kotlin has primitive array types like:

• IntArray
• ByteArray
• CharArray
• BooleanArray
• FloatArray
• DoubleArray

val intArray = intArrayOf(1, 2, 3)
val booleanArray = booleanArrayOf(true, false)

4. Nullable Types

Kotlin provides null safety at the language level. A variable is not nullable by default. If you want a variable to hold null, you must explicitly declare it as nullable using ?.

var name: String? = null

4.1 Safe Calls

Use ?. to safely access properties and methods of nullable objects.

val length = name?.length

4.2 Elvis Operator

The Elvis operator ?: provides a default value if an expression evaluates to null.

val length = name?.length ?: 0

4.3 Not-Null Assertion

The !! operator throws a NullPointerException if the value is null. Use it carefully.

val length = name!!.length

5. Type Aliases

Kotlin allows you to define type aliases for existing types to improve readability and maintainability.

typealias Username = String

val user: Username = "admin"

6. Any, Unit, and Nothing

6.1 Any

Any is the root of the Kotlin class hierarchy. Every class in Kotlin inherits from Any.

It contains three methods: equals(), hashCode(), and toString().

6.2 Unit

Unit is similar to void in Java. It is the return type of functions that return nothing meaningful.

fun greet(): Unit {
    println("Hello")
}

6.3 Nothing

Nothing represents a value that never exists. It is used to mark code paths that never return (like exceptions).

fun fail(message: String): Nothing {
    throw IllegalArgumentException(message)
}

7. Type Checking and Casting

7.1 is and !is Operators

Use is to check the type of a variable and !is to check if a variable is not of a certain type.

if (name is String) {
    println(name.length)
}

7.2 Smart Casts

If a variable is checked using is, it is automatically cast in that scope.

7.3 Explicit Casts

Use as for explicit casting. as? is the safe cast operator which returns null if the cast fails.

val obj: Any = "Hello"
val str: String = obj as String
val safeStr: String? = obj as? String

8. Kotlin Type System Highlights

8.1 Strong Type Safety

Kotlin enforces strong typing to prevent unexpected behavior. Implicit conversions (like from Int to Long) are not allowed.

8.2 Kotlin vs Java Types

Unlike Java, Kotlin distinguishes between nullable and non-nullable types at the type level.

9. Custom Data Types

In addition to built-in types, you can define your own custom data types using classes, data classes, enums, and sealed classes.

9.1 Data Classes

Data classes automatically provide equals(), hashCode(), toString(), and copy functions.

data class Person(val name: String, val age: Int)

9.2 Enum Classes

Enum classes are used for fixed sets of constants.

enum class Direction {
    NORTH, SOUTH, EAST, WEST
}

9.3 Sealed Classes

Sealed classes are used for restricted class hierarchies, useful in when expressions.

sealed class Result
class Success(val data: String): Result()
class Error(val error: Exception): Result()

Understanding Kotlin's data types is essential for writing clean, efficient, and safe code. Kotlin offers a modern type system with null safety, concise syntax, and powerful features like smart casting and custom data types. Whether you're building Android apps, backend services, or multiplatform projects, mastering Kotlin's type system is key to success.