Primary Constructor vs Secondary Constructor

1 minute read

A constructor is a special function that creates a new object. Using var or val in the parameter list makes them accessible from outside the object.

class Hero(val name: String)

val hero = Hero("superman")
print(hero.name)

In the case above, Kotlin wrote the constructor code fro us. For more customization, we can use the init section. init section is executed during the object creation:

class Hero(val name: String) {
  private val about: String
  init {
    about = "$name is a brave hero"
  }
  fun toString() = about
}

val superman = Hero("superman")
print(superman.toString())

Secondary constructor

When you are creating an object, sometimes, you might require several ways to construct. You may use named and default arguments - they are easier approach, but sometimes, you must create multiple overloaded constructors.

In Kotlin, overloaded constructors are called secondary constructors. To create a secondary constructor, use the constructor keyword followed by a parameter list that’s distinct from all other primary and secondary lists.

class SampleSecondary(i: Int) {
  init {
    print("Primary $i")
  }
  constructor(s: String) : this(s.first()) {
    print("Secondary $s")
  }
}

Summary

A constructor is the combination of its constructor parameter list – initialized before entering the class body and the init section executed during object creation. Kotlin allows multiple init sections, which are executed in definition order. But having multiple sections, may produce maintenance issues for programmers who are accustomed to a single init section.

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