Answer

Both const val and val are used to declare immutable (read-only) properties in Kotlin, but they differ in when and how their values are determined.

const val - Compile-Time Constants

const val declares compile-time constants whose values must be known during compilation.

Requirements:

• Must be declared at top-level, inside an object, or in a companion object
• Can only be primitive types (Int, Double, Boolean) or String
• Cannot be assigned from function calls or complex expressions
• Value is inlined into bytecode at compile-time for better performance

const val API_URL = "https://api.example.com" // Valid const val MAX_RETRY = 3 // Valid const val TIME = System.currentTimeMillis() // Invalid - function call

val - Runtime Constants

val declares runtime constants that are initialized when the code executes.

Characteristics:

• Can be declared anywhere (top-level, inside classes, functions)
• Supports any data type and complex expressions
• Can use function calls and custom getters
• Value is stored in memory and accessed at runtime

val currentTime = System.currentTimeMillis() // Valid - runtime initialization val userName = getCurrentUser() // Valid - function call val config: String get() = if (isDev) "dev" else "prod" // Valid - custom getter

When to Use Which?

Use const val for:

• Fixed configuration values, API URLs, constants
• Values that never change and are known beforehand
• Better performance due to compile-time inlining[5]

Use val for:

• Values determined at runtime (timestamps, user input)
• Function results or complex calculations
• Any scenario requiring runtime flexibility

Key Difference

The main distinction is timing: const val is evaluated at compile-time and inlined for performance, while val is evaluated at runtime and stored in memory.