Singleton class in Kotlin

Question

I want to know way to create singleton class, so that my Util class instantiate only once per app. However when I converted my Java class to kotlin, below code was generated

Answer 1

In Kotlin you should get rid of the whole notion of the utility singleton class. The idiomatic way is to simply move all declarations to the top level.

Java:

public final class Util {
    public static final Util UTIL = new Util();

    private int prefixLength = 4;

    private Util() {}

    public void setPrefixLength(int newLen) {
        prefixLength = newLen;
    }

    public String extractVin(String input) {
        return input.substring(prefixLength);
    }
}

Usage:

String vin = UTIL.extractVin("aoeuVN14134230430")

In Kotlin just create a separate file called util.kt with the following:

var prefixLength = 4

fun String.extractVin() = this.substring(prefixLength)

Usage:

val vin = "aoeuVN14134230430".extractVin()

But... you're polluting the top-level namespace!

If your Java intuition triggers a red flag here, simply remember that the package is the namespacing construct and as opposed to Java, Kotlin doesn't conflate the concerns of namespacing and encapsulation. There's no "package-private" access level so you're free from the burden of deciding that something must stay within the same package so it can be made package-private.

So, where in Java you create a degenerate class as a workaround, in Kotlin you just create a file in its own package.