Objects and companion

Kotlin singletons use the object keyword—one instance per class loader. Companion objects replace Java static members on the outer class, living inside the class body.

object and companion object

object AppConfig {
    const val VERSION = "1.0"
}

class Factory {
    companion object {
        fun create() = Factory()
    }
}

From Java, companion members appear as Factory.Companion.create() unless annotated with @JvmStatic.

Compared to Java static

Java uses static on the class; Kotlin prefers companion objects and top-level functions in files for namespaced helpers.

Important interview questions and answers

1. Q: object vs companion object?
   A: object is a true singleton type; companion object is one instance tied to the outer class, often for factories and constants.
2. Q: How do Java callers see companion members?
   A: As static-like methods on a generated Companion class, or true statics with @JvmStatic.

Self-check

1. What keyword declares a singleton?
2. Where do factory methods often live?

Tip: Use companion object for factories; add @JvmStatic for Java Class.method() calls.