final Keyword

The final keyword enforces immutability and restricts extension. It can be applied to variables, methods, and classes to establish code invariants.

final Variables: Value vs. Reference Immutability

When declaring a variable as final, understand the type behavior differences:

• Primitive variables: The value is immutable and cannot be changed (e.g., final int x = 5; cannot be reassigned).
• Reference variables: The reference variable cannot be reassigned to point to another object. However, the internal state of the referenced object can still be mutated (e.g. elements can be added to a final ArrayList).
• Blank final variables: A final variable declared without an initial value. It must be initialized in every constructor (for instance fields) or in the static block (for static fields) before compilation completes.

Constant folding optimization

If a final variable is initialized with a compile-time constant expression (e.g., public static final int LIMIT = 100;), the compiler performs Constant Folding:

The compiler replaces all references to LIMIT directly with the literal value 100 in the compiled bytecode, eliminating runtime lookup overhead.

Common Pitfalls

• Assuming Reference Immutability: Assuming a final collection (like final List<String> list) is read-only, and getting caught when elements are mutated.
• Omitting initialization: Declaring a blank final instance variable and failing to initialize it in one of the class constructors.
• Over-restricting inheritance: Declaring classes final when inheritance extension is needed for testing or customization.

Best Practices

• Mark variables as final by default, especially in multi-threaded contexts, to minimize mutable state.
• When declaring constants, compile them as public static final to leverage compiler constant folding.
• If you want a collection to be truly read-only, wrap it using List.of() or Collections.unmodifiableList(), rather than just marking the reference as final.

Interview-Relevant Information