What does `final` mean for a variable, a method, and a class?

Deep dive

`final` means “cannot be changed in that dimension”, but the meaning depends on where it’s applied.

• `final` local variable / parameter: you can’t reassign the reference.
• `final` field: must be assigned once (constructor, initializer). The reference can’t change, but the object can still be mutable.
• `final` method: prevents overriding (useful for API stability/security).
• `final` class: prevents inheritance (e.g., `String`).

Concurrency note

Final fields have special visibility guarantees after construction (when the object is safely published). This is why immutable objects with final fields are easier to reason about.

Example

final List<String> xs = new ArrayList<>();
xs.add("a");      // ok (mutating object)
// xs = new ArrayList<>(); // not ok (reassign)

Common pitfalls

• Confusing `final` reference with immutability.
• Relying on `final` for thread-safety (it helps, but doesn’t make mutable state safe).