Lernen Type Inference in Kotlin | Kotlin Variables and Data Types

Kotlin uses a feature called type inference to automatically determine the type of a variable when you declare it and assign a value. This means you do not always need to specify the type explicitly—the compiler can often figure it out from the value you provide. Type inference helps make code more concise and readable, but there are situations where you still need to declare the type explicitly.

Here is how type inference works in practice:

Main.kt


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package com.example

fun main() {
    // Type inferred as Int
    val number = 10

    // Type inferred as String
    val greeting = "Hello, Kotlin!"

    // Type inferred as Double
    val pi = 3.14159

    // Explicit type declaration
    val age: Int = 25

    // Explicit type declaration for clarity
    val name: String = "Alice"

    // Type inference with var (mutable variable)
    var city = "New York"
    city = "San Francisco" // Still a String

    // Uncommenting the following line would cause a type error:
    // city = 123 // Error: The integer 123 is not a String

    println("number: $number")
    println("greeting: $greeting")
    println("pi: $pi")
    println("age: $age")
    println("name: $name")
    println("city: $city")
}

In the code above, you see both inferred and explicitly declared types. When you write val number = 10, Kotlin knows number must be an Int because 10 is an integer. The same logic applies to greeting, pi, and city. You only need to specify a type—such as val age: Int = 25—if you want to make the type clear, if the value could be of multiple possible types, or if you are declaring a variable without assigning a value immediately. Explicit type declarations are also useful for readability or when working with APIs that expect a specific type.

Type inference keeps your code simple, but always remember: if the compiler cannot determine the type, or if you want to prevent accidental mistakes, specify the type explicitly.

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