What does a lazy val do

Question

I noticed that Scala provide lazy vals  But I don t get what they do   scala gt  val x   15 x  Int   15  scala gt  lazy val y   13 y  Int    lt lazy gt   scala gt  x res0  Int   15  scala gt  y res1  Int   13   The REPL shows that y is a lazy val  but how is it different from a normal val

User · Answer

This feature helps not only delaying expensive calculations, but is also useful to construct mutual dependent or cyclic structures. E.g. this leads to an stack overflow:

trait Foo { val foo: Foo }
case class Fee extends Foo { val foo = Faa() }
case class Faa extends Foo { val foo = Fee() }

println(Fee().foo)
//StackOverflowException

But with lazy vals it works fine

trait Foo { val foo: Foo }
case class Fee extends Foo { lazy val foo = Faa() }
case class Faa extends Foo { lazy val foo = Fee() }

println(Fee().foo)
//Faa()

User · Answer

I understand that the answer is given but I wrote a simple example to make it easy to understand for beginners like me  var x     println  quot x quot    15   lazy val y     println  quot y quot    x   1   println  quot ----- quot   x   17 println  quot y is   quot    y   Output of above code is  x ----- y y is  18  As it can be seen  x is printed when it s initialized  but y is not printed when it s initialized in same way  I have taken x as var intentionally here - to explain when y gets initialized   Next when y is called  it s initialized as well as value of last  x  is taken into consideration but not the old one  Hope this helps

User · Answer

The difference between them is  that a val is executed when it is defined whereas a lazy val is executed when it is accessed the first time   scala gt  val x     println  x    15   x x  Int   15  scala gt  lazy val y     println  y    13   y  Int    lt lazy gt   scala gt  x res2  Int   15  scala gt  y y res3  Int   13  scala gt  y res4  Int   13   In contrast to a method  defined with def  a lazy val is executed once and then never again  This can be useful when an operation takes long time to complete and when it is not sure if it is later used   scala gt  class X   val x     Thread sleep 2000   15     defined class X  scala gt  class Y   lazy val y     Thread sleep 2000   13     defined class Y  scala gt  new X res5  X   X 262505b7    we have to wait two seconds to the result  scala gt  new Y res6  Y   Y 1555bd22    this appears immediately   Here  when the values x and y are never used  only x unnecessarily wasting resources  If we suppose that y has no side effects and that we do not know how often it is accessed  never  once  thousands of times  it is useless to declare it as def since we don t want to execute it several times   If you want to know how lazy vals are implemented  see this question

User · Answer

A lazy val is most easily understood as a  memoized  no-arg  def    Like a def  a lazy val is not evaluated until it is invoked   But the result is saved so that subsequent invocations return the saved value   The memoized result takes up space in your data structure  like a val   As others have mentioned  the use cases for a lazy val are to defer expensive computations until they are needed and store their results  and to solve certain circular dependencies between values   Lazy vals are in fact implemented more or less as memoized defs   You can read about the details of their implementation here   http   docs scala-lang org sips pending improved-lazy-val-initialization html

User · Answer

Also lazy is useful without cyclic dependencies  as in the following code   abstract class X     val x  String   println   x is   x length     object Y extends X   val x    Hello    Y   Accessing Y will now throw null pointer exception  because x is not yet initialized  The following  however  works fine   abstract class X     val x  String   println   x is   x length     object Y extends X   lazy val x    Hello    Y   EDIT  the following will also work    object Y extends   val x    Hello    with X    This is called an  early initializer   See this SO question for more details

User · Answer

A demonstration of lazy - as defined above - execution when defined vs execution when accessed   using 2 12 7 scala shell      compiler says this is ok when it is lazy scala gt  lazy val t  Int   t  t  Int    lt lazy gt    however when executed  t recursively calls itself  and causes a StackOverflowError scala gt  t              java lang StackOverflowError         when the t is initialized to itself un-lazily  the compiler warns you of the recursive call scala gt  val t  Int   t  lt console gt  12  warning  value t does nothing other than call itself recursively    val t  Int   t

User · Answer

scala gt  lazy val lazyEight              println  I am lazy              8          lazyEight  Int    lt lazy gt   scala gt  lazyEight I am lazy   res1  Int   8    All vals are initialized during object construction Use lazy keyword to defer initialization until first usage Attention  Lazy vals are not final and therefore might show performance drawbacks

[scala] What does a lazy val do?

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