This post demonstrates how to overload class constructors 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

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Primary vs Secondary Constructors

In simplest terms, the type of constructor in Kotlin depends on where we declare/reference the constructor.

Below are examples of primary and secondary constructors. They are equivalent.

Primary Constructor Example

// All class implicitly extend java.lang.Object
class Parent(): Object() {
   // Other codes here
}

// All class implicitly extend java.lang.Object

class Parent(): Object() {

// Other codes here

}

Secondary Constructor Example

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// All classes implicitly extend java.lang.Object
class Parent {
    constructor(): super() {
        // Other codes
    }

    // Other codes
}

// All classes implicitly extend java.lang.Object

class Parent {

constructor(): super() {

// Other codes

}

// Other codes

}

Overloading Constructors

For instance, we have a class with overloaded constructors. Note, all classes implicitly extend from java.lang.Object.

class Base {

    /**
     * The compiler inserts a call to Object construct. For example,
     *
     * In Kotlin:
     * constructor():super() {  }
     *
     * In Java:
     *
     * public Base() { super(); }
     *
     */
    constructor() { }

    /**
     * The compiler inserts a call to Object construct. For example,
     *
     * In Kotlin:
     *   constructor(p: String):super() {  }
     *
     * to refer to the no-arg constructor of the current class:
     *
     *   constructor(p: String):this() {  }
     *
     * In Java:
     *
     *   public Base(String p) { super(); }
     *
     * to refer to the no-arg constructor of the current class:
     *
     *   public Base(String p) { this(); }
     *
     */
    constructor(p: String): this() { }
}

class Base {

/**

* The compiler inserts a call to Object construct. For example,

* In Kotlin:

* constructor():super() { }

* In Java:

* public Base() { super(); }

constructor() { }

/**

* The compiler inserts a call to Object construct. For example,

* In Kotlin:

* constructor(p: String):super() { }

* to refer to the no-arg constructor of the current class:

* constructor(p: String):this() { }

* In Java:

* public Base(String p) { super(); }

* to refer to the no-arg constructor of the current class:

* public Base(String p) { this(); }

constructor(p: String): this() { }

}

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Using overloading constructors

fun main(args: Array<String>) {
    
    var parent: Parent = Parent()

    var base1: Base = Base()
    var base2: Base = Base("base01")

}

fun main(args: Array<String>) {

var parent: Parent = Parent()

var base1: Base = Base()

var base2: Base = Base("base01")

}

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Requirements

Primary vs Secondary Constructors

Primary Constructor Example

Secondary Constructor Example

Overloading Constructors

Using overloading constructors

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