What are 'object expressions' and 'object declarations' in Kotlin?

Object expressions (anonymous objects) are used to create objects of anonymous classes. They are useful for implementing one-time interfaces or inheriting from classes in a single instance. Syntax: object : <type(s)>.

Object declarations create a singleton. This is a class with a single instance that is globally accessible by its name. Syntax: object <name>.

Comparison table:

Feature	Object expressions	Object declarations
Purpose	Creating an anonymous object	Creating a singleton
Usage	Local, inside expressions or functions	Public, by object name
Instance creation	Each time the expression is used	One instance upon first access
Naming	Anonymous	Named
Application	One-time event listeners, adapters	Utilities, configurations, state storage

Example of object expression:

// Creating a click handler "on the fly"
button.setOnClickListener(object : View.OnClickListener {
    override fun onClick(v: View?) {
        // Action on click
    }
})

Example of object declaration:

// Declaring a singleton object for logging
object Logger {
    fun log(message: String) {
        println(message)
    }
}

// Using the singleton
fun processData(data: String) {
    Logger.log("Processing data...")
    // Data processing
}