What are best practices for choosing between property initializer vs property getter in Kotlin?

Question

In Kotlin, we have a choice between:

class|object X {
  [override] val y = Z
}

and

class|object X {
  [override] val y: Y
    get() = Z
}

Which one should be chosen and when?

Answer 1

Decide based on when you want evaluation to happen. A property initializer results in memoization semantics: the initializing expression is evaluated only at instantiation time:

class X {
    val y = Z // Z evaluated only at instantiation time
}

A property getter is evaluated every time you access the property:

class X { 
    val y get() = Z // evaluated on every access of y
}

There are several factors that may make you prefer one or the other:

• If the evaluation of Z changes over time, you probably want to evaluate it every time.
• Even if Z always yields the same result, you may want to postpone the evaluation untill first access. In that case you may use a lazy property delegate.
• Possibly the size of the result is large and you prefer not to retain it in memory while not being actively used. In that case prefer the custom getter.