Careeroza

Object Expressions

Anonymous Objects

An object expression creates a one-off object of an anonymous class - Kotlin's version of Java anonymous inner classes. Use it when an interface has more than one function (so a lambda is not enough).

Example

interface DownloadListener {
    fun onProgress(percent: Int)
    fun onComplete(file: String)
}

fun download(file: String, listener: DownloadListener) {
    for (p in listOf(25, 50, 100)) listener.onProgress(p)
    listener.onComplete(file)
}

fun main() {
    var updates = 0
    download("report.pdf", object : DownloadListener {
        override fun onProgress(percent: Int) {
            updates++                            // can use outer variables
            println("Progress: $percent%")
        }
        override fun onComplete(file: String) = println("Saved $file after $updates updates")
    })
}

Output

Progress: 25%
Progress: 50%
Progress: 100%
Saved report.pdf after 3 updates

Ad-hoc Objects

Example

Inside a function you can also create a quick object with properties.

fun main() {
    val point = object {
        val x = 10
        val y = 20
        fun sum() = x + y
    }
    println("x=${point.x}, y=${point.y}, sum=${point.sum()}")
}

Output

x=10, y=20, sum=30