Scala var vs. val Keywords

Scala is a programming language that mixes object-oriented and functional programming styles. It emphasizes using immutable values and functional programming principles. A big part of this is the use of the keywords var and val for declaring variables. You should understand the differences between var and val before writing Scala code. We will explain these two keywords in detail.

The var Keyword : Mutable Variables

The var keyword in Scala is used to declare mutable variables. A mutable variable is one whose value can be changed after it has been initialized. This is similar to variable declarations in many other programming languages like Java, Python, or C++.

Example

mutableVariable = 20  // This is valid
println(mutableVariable)  // Output: 20

When to Use var?

Although you can reassign variables using var. However, it is generally better to use immutable values (val) for several reasons −

• Predictability − Mutable variables can change unexpectedly. So, the code can be harder to understand and debug.
• Concurrency − In concurrent and parallel programming, immutable values are safer because mutable variables can cause race conditions and require complex synchronization.
• Functional Programming − Scala supports a functional programming style. The functions are pure and do not depend on and change external states.

The val Keyword: Immutable Variables

The val keyword declares immutable variables. Once you assign a value to a val, it cannot be changed. This is similar to constants in other languages. But with a subtle difference: a val can hold references to mutable objects, even though the reference itself cannot be reassigned.

Example

val immutableVariable = 10
// immutableVariable = 20  // This would cause a compile-time error
println(immutableVariable)  // Output: 10

When to use val?

There are various advantages to using val variables in Scala. These are −

• Security − The chance of unexpected side effects is low. So, the code can be more reliable.
• Clarity − There is more clarity due to the immutable property of val variables.
• Functional Programming − You need immutable data structure functional programming because of its essentiality. Therefore, secure concurrent operations can occur without the need for locks.

Example

val mutableList = scala.collection.mutable.ListBuffer(1, 2, 3)
mutableList += 4  // The reference to mutableList is immutable, but the contents can change
println(mutableList)  // Output: ListBuffer(1, 2, 3, 4)

Performance Considerations

There can be performance differences between var and val data structures in Scala. The immutable data structures (i.e., val) can have slow performance because you need to copy data structures when changes are made. But there are advantages of immutability too. For example, it is safer and easier-to-maintain code which is usually more important than these drawbacks. We usually use val instead of val in Scala programs because the Scala style prefers immutability, so the code will be more predictable and robust.

Differences between var and val keywords in Scala

var x = 10

val x = 10

var counter = 0; 
counter += 1

val counter = 0