Scala allows you to decide whether a variable is immutable (read-only) or not (readwrite) when you declare it.

immutable “variable”

an immutable “variable” is declared with the keyword val (think value object):

val array: Array[String] = new Array(5)

A val must be initialized when it is declared.

mutable variable

Similarly, a mutable variable is declared with the keyword var and it must also be initialized immediately, even though it can be changed later.

var stockPrice: Double = 100.0

Note
There are a few exceptions to the rule that you must initialize vals and vars when they are declared. For example, keyword(val or var) can be used with a constructor parameter.

code example

class Person(val name: String, var age: Int)

Why use immutable “variable”

Use immutable values whenever possible to eliminate a class of bugs caused by mutability.

For example, a mutable object is dangerous as a key in hash-based maps.
More common is unexpected behavior when an object you are using is being changed by someone else.

Ref

《Programming Scala》

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Variable Declarations

immutable “variable”

mutable variable

Why use immutable “variable”

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