Singleton objects

Introduction#

An object is a special kind of class, which can be declared using object keyword. Objects are similar to Singletons (a design pattern) in java. It also functions as the static part of java. Beginners who are switching from java to kotlin can vastly use this feature, in place of static, or singletons.

Use as repalcement of static methods/fields of java

object CommonUtils {

    var anyname: String ="Hello"

    fun dispMsg(message: String) {
        println(message)
    }
}

From any other class, just invoke the variable and functions in this way:

CommonUtils.anyname
CommonUtils.dispMsg("like static call")

Use as a singleton

Kotlin objects are actually just singletons. Its primary advantage is that you don’t have to use SomeSingleton.INSTANCE to get the instance of the singleton.

In java your singleton looks like this:

public enum SharedRegistry {
    INSTANCE;
    public void register(String key, Object thing) {}
}

public static void main(String[] args) {
    SharedRegistry.INSTANCE.register("a", "apple");
    SharedRegistry.INSTANCE.register("b", "boy");
    SharedRegistry.INSTANCE.register("c", "cat");
    SharedRegistry.INSTANCE.register("d", "dog");
}

In kotlin, the equivalent code is

object SharedRegistry {
    fun register(key: String, thing: Object) {}
}

fun main(Array<String> args) {
    SharedRegistry.register("a", "apple")
    SharedRegistry.register("b", "boy")
    SharedRegistry.register("c", "cat")
    SharedRegistry.register("d", "dog")
}

It’s obvoiusly less verbose to use.