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  • Post category:Scala
  • Post last modified:May 9, 2024
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1. Introduction

In this post, we will learn the difference between var and val in Scala. Scala developers encounter a significant difference between val and var when defining variables. It is one of the most significant features that Scala introduced. These keywords are used in declaring variables, but have distinctly different approaches. Understanding the contrast between the two keywords enables developers to come up with concise and expressive code that is easy to maintain.


2. Distinct Features Between Var and Val in Scala

2.1. Immutability vs. Mutability

One of the most important distinctions between val and var keywords is in the concept of mutability and immutability. Scala uses val to declare a variable that is immutable, while it uses var to define a mutable variable. When you declare a variable using the val keyword, what you are essentially doing is defining a constant. Once Scala assigns a value to val variable, it will not be possible to change it. It will remain fixed throughout its entire scope.

//example of defining a variable using val keyword
val pi = 3.14159
// Attempting to reassign pi will result in a compilation error.

The use of val (Value) is powerful when creating stable and predictable code. It gives the guarantee that the value of a variable will not change, which means there will be fewer bugs. It also leads to a easier way of debugging.

On the other hand, var (Variable) will create a variable which is mutable. It means that one can reassign values to it many times within its scope.

//code snippet to show use of var keyword
var counter = 0
counter = 1 // This is allowed

the use of var makes code flexible. However, they can introduce complexity in code, thus making hard to interrogate and introduce any changes to them. This is especially true in multi-threaded, and concurrent environments. Developers prefer to use var sparingly, and val often, when immutability is appropriate.

2.2. Functional vs. Imperative

The design of Scala is such that is object-oriented and functional. The use of val leans more towards functional programming principles. In functional programming, data is more often immutable, while functions do not have a lot of side effects. On the other hand, var leans towards imperative programming more. In imperative programming, variables are prone to change, and there are side effects for functions.

Functional programming has become popular because of the testability, maintainability, and predictability of code. When developers write code using val keyword, they are adopting immutability of code and thus writing functional code more often.

2.3. Thread safety of Var and Val in Scala

The immutability feature of val keyword, is useful in realizing the safety of threads. In an environment where there are multiple threads, the safety of threads is guaranteed since it is impossible to change them after their creation. They rarely change after the developer creates them. It means that many threads are able to read val variables safely without any issues with synchronization. This is not the case with var variables. Since their values keep on changing, they will require explicit synchronization so as to realise thread safety. If there are no proper synchronization mechanisms with var variables, concurrent writes to them can lead to unwanted and erroneous behaviors.

2.4. Code clarity and maintainability

Developers are encouraged to use immutable variables, by using the keyword val. This approach ensures that the code is cleaner, and easy to maintain. When the developer knows that the value of a variable will not change after assigning, it will be easier to interrogate and reason about it. Their behaviours are also easy to follow up on.

//code to explain immutability
object ImmutabilityExample {
def main(args: Arrays[String]): Unit = {
// Using val for immutability
val taxRate = 0.20
val purchasePrice = 100.0
val taxAmount = purchasePrice * taxRate
val totalAmount = purchasePrice + taxAmount

In the code above, it is clear that the four variables will not change anyhow, thus it is easy to explain the working of the code.

Conversely, mutable variables are difficult to comprehend and maintain because they can change unexpectedly.

object MutabilityExample {
def main(args: Arrays[String]): Unit = {
// Using var for mutability
var balance = 1000.0
// Some code that modifies balance...

From the program code above, it is clear that it becomes hard to reason out about the changes that might take  place. It is hard to know where the code might be modified, and so reasoning out the program becomes a challenge.

2.5. Performance considerations

While immutability is a desirable feature for functional programming, it is critical to consider the performance parameters, especially in situations where it is critical to consider performance.

Because of the immutability characteristic of val every reassignment will lead to a new instance creation, thus likely leading to new memory use. This new memory requirement will lead affect the performance. However, Scala’s optimization mechanisms, and modern JVMs are able to mitigate this overhead.

In situations where performance is very critical, developers might have to choose to use var for variables that are prone to change often within the scope.

3. Choice between var and val

Use val by default in functional and concurrent programming because you will be assured of thread safety. It is useful and recommended for variables that should not change after they have been initialized.

On the other hand, var should be used for variables that are mutable. If you have enough reason to change the value of variable over its entire scope, then use var. when this is the case, make sure you document and encapsulate this usage so as to avoid unnecessary side effects in your code

4. Conclusion

Understanding the difference between var and val in Scala is important in order to write clean, maintainable, and concise code. It is also a good strategy in order to write code that secure multi-threads. The difference between the two is in their mutability, where val variables are immutable, while var variables are mutable. The choice between the two should align with your program requirements. Generally, val is preferred so as to enhance code clarity and reduce the risk of side effects. The use of var should be for cases where mutability is a requirement, and well-documented.