Create Singletons with Object

Key points

• Use object instead of class to create a singleton (single instance of that class)
• Useful for “utility” classes

Rules:

• No constructor parameters
• Can contain:
  • Properties
  • Functions
  • Initializer blocks
• Objects can inherit from interfaces and classes
• There is a difference between object expressions and declarations
  • Expressions are used on the right hand side of expressions
  • Declarations are when you use object to create a singleton or companion object

Singleton example

object MathUtils {
    init {
        println("MathUtils init was called")
    }
    val ONE = 1
    fun plus1(i: Int) = i + 1
}

fun main(args: Array<String>) {
    val x = MathUtils.plus1(MathUtils.ONE)
    println(x)
}

Prints:

MathUtils init was called
2

Inheritance

This is from the Kotlin Language Reference (TODO: write my own example):

// objects can have supertypes:
object DefaultListener : MouseAdapter() {
    override fun mouseClicked(e: MouseEvent) { ... }
    override fun mouseEntered(e: MouseEvent) { ... }
}