# Object Declarations and Companion Objects — Kotlin

Source: https://www.skillbyai.com/en/kotlin/o-objects

> Singletons, factories and anonymous objects.

## Objects instead of static

An `object` declaration defines a class and its single instance at once: a thread-safe, lazily initialised **singleton**. A `companion object` inside a class holds members called on the class name, such as factory functions and constants; Kotlin has no `static` keyword. `const val` declares compile-time constants of primitive or String type. **Object expressions** (`object : Listener { ... }`) create anonymous objects, similar to Java anonymous classes. `data object` gives a singleton a readable `toString` and is useful in sealed hierarchies.

## A factory in a companion object

Private constructor plus validation.

```kotlin
class Email private constructor(val value: String) {
    companion object {
        const val MAX_LENGTH = 254

        fun parse(raw: String): Email? {
            val trimmed = raw.trim().lowercase()
            return if (trimmed.length <= MAX_LENGTH && "@" in trimmed) Email(trimmed) else null
        }
    }
}

object AppConfig {
    val baseUrl: String = System.getenv("BASE_URL") ?: "http://localhost:8080"
}

fun main() {
    val email = Email.parse(" Ada@Example.com ")      // called on the class name
    val url = AppConfig.baseUrl                        // singleton access
}
```

## The front desk of a building

A companion object is the front desk attached to a building: you go to it by the building name to get things made, without needing a specific apartment (instance) first.

**Quiz:** What does an `object` declaration create?

- [ ] A new instance on every access
- [ ] A data class
- [ ] An abstract class
- [x] A singleton: a class with exactly one lazily created instance

*Answer:* A singleton: a class with exactly one lazily created instance. object declarations are Kotlin singletons.