Private constructor in Kotlin

Question

In Java it's possible to hide a class' main constructor by making it private and then accessing it via a public static method inside that class:

public final class Foo {
    /* Public static method */
    public static final Foo constructorA() {
        // do stuff

        return new Foo(someData);
    }

    private final Data someData;

    /* Main constructor */
    private Foo(final Data someData) {
        Objects.requireNonNull(someData);

        this.someData = someData;
    }

    // ...
}

How can the same be reached with Kotlin without separating the class into a public interface and a private implementation? Making a constructor private leads to it not being accessible from outside the class, not even from the same file.

Answer 1

You can even do something more similar to "emulating" usage of public constructor while having private constructor.

class Foo private constructor(val someData: Data) {
    companion object {
        operator fun invoke(): Foo {
            // do stuff

            return Foo(someData)
        }
    }
}

//usage
Foo() //even though it looks like constructor, it is a function call

Answer 2

This is possible using a companion object:

class Foo private constructor(val someData: Data) {
    companion object {
        fun constructorA(): Foo {
            // do stuff

            return Foo(someData)
        }
    }

    // ...
}

Methods inside the companion object can be reached just like if they were members of the surrounding class (e.g. Foo.constructorA())

Answer 3

See kotlin documentation here:

https://kotlinlang.org/docs/reference/classes.html#constructors

https://kotlinlang.org/docs/reference/visibility-modifiers.html#constructors

class DontCreateMe private constructor () { /*...*/ }

Answer 4

This is the Answer

class Hide private constructor(val someData: Data) {

}

By declaring the constructor private, we can hiding constructor.