This post demonstrates how to create a Singleton object in Kotlin.

Requirements

IntelliJ IDEA Ultimate 2016.3
- The Community Edition may be enough but we have not tried it.
Kotlin Version 1.1
Windows 10 Enterprise

Singleton in Java

In Java, there is no straight-forward construct to create a Singleton object. Normally, we would have a private constructor, and a thread-safe static method to retrieve a private instance of the class from which to invoke appropriate thread-safe methods. Consider the following example.

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import java.util.HashMap;
import java.util.Map;

public class Singleton {

	private static volatile Singleton singleton;
	
	private Map<String, String> map = new HashMap<>();
	
	private Singleton() {}
	
	public static Singleton getInstance() {
		if(singleton == null) {
			synchronized(Singleton.class) {
				singleton = new Singleton();
			}
		}
		return singleton;
	}

	public synchronized String getValue(String key) {
		return map.get(key);
	}
	
	public synchronized void setValue(String key, String value) {
		map.put(key, value);
	}
	
	
	public static void main(String[] args) {
		Singleton s = Singleton.getInstance();
		
		 s.setValue("app.settings.name", "Singleton Demo");
		 s.setValue("app.settings.version", "1.0");
		
		 System.out.println(s.getValue("app.settings.name"));
		 System.out.println(s.getValue("app.settings.version"));

	}
}

import java.util.HashMap;

import java.util.Map;

public class Singleton {

private static volatile Singleton singleton;

private Map<String, String> map = new HashMap<>();

private Singleton() {}

public static Singleton getInstance() {

if(singleton == null) {

synchronized(Singleton.class) {

singleton = new Singleton();

}

return singleton;

}

public synchronized String getValue(String key) {

return map.get(key);

}

public synchronized void setValue(String key, String value) {

map.put(key, value);

}

public static void main(String[] args) {

Singleton s = Singleton.getInstance();

s.setValue("app.settings.name", "Singleton Demo");

s.setValue("app.settings.version", "1.0");

System.out.println(s.getValue("app.settings.name"));

System.out.println(s.getValue("app.settings.version"));

}

Singleton in Kotlin

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In Kotlin, there is a keyword object used to create a Singleton object of a class. Consider the example below. The keyword class is replaced with the keyword object.

object Singleton {

    private var map: HashMap<String, String> = HashMap()

    @Synchronized
    fun getValue(key: String): String? {
        return this.map.getValue(key)
    }

    @Synchronized
    fun setValue(key: String, value: String) {
        map.put(key, value)
    }
}

fun main(args: Array<String>) {
    Singleton.setValue("app.settings.name", "Singleton Demo")
    Singleton.setValue("app.settings.version", "1.0")

    println(Singleton.getValue("app.settings.name"))
    println(Singleton.getValue("app.settings.version"))

}