Defining Variables Using the `def` Keyword in Scala

Senthil Nayagan
Senthil Nayagan           

Defining Variables Using the `def` Keyword in Scala

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The title reads as follows: “Defining Variables Using the def Keyword.” Before we continue, let’s see if this title is appropriate. It’s not!

Let’s first be clear that a def is not a variable declaration. It’s a declaration of a function or method instead.

Consider the following code:

scala> def i = 3
def i: Int

scala> i.getClass
val res0: Class[Int] = int

scala> println(i)

scala> val j = 3
val j: Int = 3

scala> j.getClass
val res1: Class[Int] = int

scala> println(j)

scala> i + j
val res2: Int = 6

As we see above, variable definition works using def. Unlike a val or var declaration, a def is not a variable declaration. It’s a function declaration. What we are doing is creating a function or method that returns a constant number 3. Having said that, both i and j are functions that do not take any arguments.

Unless the lazy keyword is prefixed, variables declared with the val or var are evaluated immediately, whereas def is evaluated on call, which means it is lazy evaluated. Having said, def evaluation happens when called explicitly.

Note that Scala does not permit the creation of lazy variables, i.e., var. Only lazy values (val) are allowed.

Difference between lazy val and def

Difference between lazy val and def:

  • When accessed for the first time, lazy evaluates and caches the result.
  • The def declaration is evaluated every time we call method name.

Let’s see the difference between lazy val and def in action:

scala> lazy val a = { println("a value evaluated"); 1}
lazy val a: Int

scala> a
a value evaluated
val res0: Int = 1

scala> a
val res1: Int = 1

scala> def b = { println("b function evaluated"); 2}
def b: Int

scala> a
b function evaluated
val res3: Int = 2

scala> a
b function evaluated
val res4: Int = 2

As we see above, a is evaluated only once, and any subsequent invocations of a return the cached result. This indicates that lazy val only needs to be evaluated once and then the result is stored forever.

On the other hand, a def is evaluated every time it is invoked. In this case, we see println output every time we invoke the function.

I hope this explains how def differs from val in terms of evaluation frequency.

Happy learning!


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