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Type Inference

Part of the Fundamentals section of Coddy's Kotlin journey. Lesson 6 of 93.

Kotlin can automatically figure out the type of a value. This is called type inference:

fun main() {
    val name = "Alice"   // Kotlin infers this is a String
    val age = 25         // Kotlin infers this is an Int
    val price = 9.99     // Kotlin infers this is a Double
}

But you can also explicitly declare the type using a type annotation. You write a colon after the name, followed by the type:

fun main() {
    val name: String = "Alice"
    val age: Int = 25
    val price: Double = 9.99
    val isActive: Boolean = true
}

Type annotations are useful when:

  • You want to be extra clear about what type a value is
  • You declare a variable without an initial value
fun main() {
    var score: Int   // Declared but not yet assigned
    score = 100      // Assigned later
}

Without the type annotation, Kotlin wouldn't know what type score should be.

challenge icon

Challenge

Beginner

Create four val declarations without explicit type annotations: city holding "Oslo", platform holding 4, fare holding 12.5, and isExpress holding false. Let Kotlin infer their types. Print their values on four separate lines in that order.

Try it yourself

fun main() {
    // Declare the four values without type annotations.
    // Print them in the requested order.
}
quiz iconTest yourself

This lesson includes a short quiz. Start the lesson to answer it and track your progress.

All lessons in Fundamentals

1Introduction

The LanguageHello World!CommentsRecap - Launch Message

4Operators Part 1

Arithmetic OperatorsModulo OperatorAugmented AssignmentComparison OperatorsRecap - Simple Math

7Basic IO

Println FunctionString TemplatesReadLine InputType ConversionRecap - Years Until RetirementRecap - True or False

10Functions

Declare A FunctionParameters And ArgumentsReturn ValuesNamed ArgumentsDefault ValuesSingle Expression FunctionsRecap - Sigma FunctionRecap - Validation Function

13Iterating Over Sequences

Iterating Over ElementsWithIndex MethodIterating Over Strings P1Iterating Over Strings P2Recap - Indexed Letters

16Final Challenges

Flat Asterisk PyramidPattern FinderWhat To Buy

2Variables

Val vs VarType InferenceNumbersStringBooleanNaming ConventionsRecap - Initialize Variables

5Operators Part 2

Logical Operators Part 1Logical Operators Part 2Logical Operators Part 3Ternary With If ExpressionRecap - Simple Logic

8Bill Split Calculator

Welcome MessageGetting InputProject lessonCalculating Tip And TotalProject lessonSplitting The BillProject lessonFormatted OutputProject lesson

11FizzBuzz with a Twist

Game OverviewThe FizzBuzz FunctionProject lessonLooping The NumbersProject lessonAdding The TwistProject lesson

14Lists Advanced

List Slicing With SubListList Slicing With Take DropSequence OperatorsThe Contains MethodRecap - Search a Page

3Nullability

What Is Null SafetyNullable TypesSafe Call OperatorElvis OperatorFunction Challenge BasicsNot Null AssertionRecap - Safe Access

6Decision Making

If StatementIf - ElseIf As An ExpressionWhen ExpressionWhen With RangesRecap - Simple Calculator

9Loops

For LoopWhile LoopDo-While LoopBreakContinueRanges In LoopsNested LoopRecap - FactorialRecap - Dynamic Input

12Lists Basics

List vs MutableListAccessing ElementsModifying ListsList MethodsPair And TripleRecap - Product ListRecap - Reversed List

15Daily Expense Tracker

Project OverviewExit The ProgramProject lessonAdd ExpenseProject lessonView All ExpensesProject lessonTotal And AverageProject lessonClear AllProject lessonHandling ErrorsProject lesson

Practice on your own: Kotlin playground