Scala Book

The Type is Optional


As we showed in the previous lesson, when you create a new variable in Scala you can explicitly declare its type, like this:

val count: Int = 1
val name: String = "Alvin"

However, you can generally leave the type off and Scala can infer it for you:

val count = 1
val name = "Alvin"

In most cases your code is easier to read when you leave the type off, so this inferred form is preferred.

The explicit form feels verbose

For instance, in this example it’s obvious that the data type is Person, so there’s no need to declare the type on the left side of the expression:

val p = new Person("Candy")

By contrast, when you put the type next to the variable name, the code feels unnecessarily verbose:

val p: Person = new Person("Leo")

In summary:

val p = new Person("Candy")           // preferred
val p: Person = new Person("Candy")   // unnecessarily verbose

Use the explicit form when you need to be clear

One place where you’ll want to show the data type is when you want to be clear about what you’re creating. That is, if you don’t explicitly declare the data type, the compiler may make a wrong assumption about what you want to create. Some examples of this are when you want to create numbers with specific data types. We show this in the next lesson.

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