- What:

1. Used particularly in functional programming languages.
2. Chances to increase performance by not doing the computation until needed.
3. Has book-keeping overhead.

- When:

1. You want something to be initialized only once (exmaple below)
2. Partial accessing a large set of data
   • ex: iterator.next() instead of getAll().get(n)

- Examples:

• You want something to be initialized once: - [test it online]

  class Doncic {
    def doInit() : Int = {
        println("Hello Basketball")
        77
    }
    lazy val x = doInit()
    def dribbling() {
        println(x)
    }
  }
  val luka = new Doncic
  luka.dribbling()
  luka.dribbling()
  luka.dribbling()
  

Outputs:

Hello Basketball
77
77
77

You can see the “Hello Basketball” will be called only once, so when it’s something heavy computation, you know it’s going to save sometime when it’s applicable.

• Delay of evaluation - [test it online]

  val x = { println ("doncic") ; 77 }
  println ("luka")
  println (x)
  

  prints: doncic, then luka then 77.

  lazy val y = { println ("doncic") ; 77 }
  println ("luka")
  println (y)
  

  prints: luka, then doncic then 77.

- References:

http://matt.might.net/articles/implementing-laziness/ https://en.wikipedia.org/wiki/Lazy_evaluation https://stackoverflow.com/questions/19955995/when-to-use-lazy-values-in-scala https://stackoverflow.com/questions/19469464/do-something-only-once-without-state http://twitter.github.io/effectivescala/#Functional%20programming-Laziness