Internal Mechanism of `val` in Kotlin

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ProAndroidDev

2 min readJun 29, 2024

Kotlin’s val keyword is fundamental for declaring immutable variables. But have you ever wondered how Kotlin ensures that a val can only be assigned once and remains immutable throughout its lifecycle? Let's take a journey from understanding its declaration to the bytecode level, diving deep into how Kotlin achieves this.

Declaring a `val` in Kotlin

In Kotlin, a val is used to declare a read-only variable, meaning its value can be assigned once and cannot be changed afterwards. Here’s how you declare and use a val:

val x = 5

In this simple declaration, x is initialized with the value 5 and cannot be reassigned later. But how does Kotlin enforce this immutability?

Understanding Kotlin’s Approach

1. Declaration and Initialization

When you declare a val like val x = 5, Kotlin ensures that:

The variable x is assigned an initial value (5 in this case).
This assignment happens during the initialization phase and cannot be changed thereafter.

2. Internal Mechanism

Internally, Kotlin generates bytecode that reflects the immutability of val variables. Let’s break down what happens under the hood:

Kotlin to Java Bytecode Translation

Kotlin code is eventually compiled into Java bytecode, which runs on the Java Virtual Machine (JVM). Here’s a simplified explanation of how val works at the bytecode level:

Initialization: When you initialize val x = 5, Kotlin translates this into bytecode instructions that assign 5 to x.
final Modifier: Behind the scenes, Kotlin marks val variables with the equivalent of Java’s final modifier. This ensures that once x is assigned a value, it cannot be reassigned.

Exploring Bytecode

Let’s look at an example of how Kotlin bytecode might look for a simple val declaration:

public final class ValExample {
   private final int x;

   public ValExample() {
      this.x = 5;
   }
}

In this bytecode snippet:

private final int x; declares x as a final field.
this.x = 5; initializes x with the value 5 in the constructor.

Why It Matters

Understanding how val works under the hood helps in writing safer and more predictable Kotlin code:

Thread Safety: Immutable variables (val) are inherently thread-safe because their values do not change.
Code Clarity: By using val, you communicate intent clearly: this variable’s value won’t change.
Performance: Kotlin’s approach to immutability and val allows for optimizations, especially in concurrent programming scenarios.

Conclusion

Next time you use val in Kotlin, remember that it’s not just about immutability at the surface level—it’s about how Kotlin compiles your code into bytecode that enforces this immutability throughout the lifecycle of your program. Whether you’re assigning a simple value or dealing with more complex scenarios, Kotlin’s val ensures that what you assign once stays that way.

Internal Mechanism of val in Kotlin

Declaring a val in Kotlin