Why Java’s Anonymous Inner Classes Require Final Variables (And How Kotlin Differs)

Phenomenon Description

Before Java 8, using an external variable inside an anonymous inner class caused a compilation error: “Cannot refer to a non‑final variable arg inside an inner class defined in a different method”.

After Java 8, the same code no longer shows that error, but attempting to modify the variable still results in a warning such as “Variable 'num' is accessed from within inner class, need to be final or effectively final”.

Kotlin, by contrast, does not impose this restriction.

Reason Analysis

Compiling the code with javac generates several .class files. The TestInnerClass$1.class file represents the compiled anonymous inner class. Decompiling it reveals that the compiler treats the anonymous class like a regular class: it copies primitive variables into the inner class and passes references for object variables. Because the primitive copy is separate, modifying it inside the inner class would cause inconsistency with the outer variable, so the compiler forbids the modification.

Scenario Comparison

In Kotlin, the compiler wraps primitive variables, turning value passing into reference passing, which allows the inner class to modify the value without affecting the outer scope.

When a variable is not passed at all, Kotlin’s compilation shows no extra handling, confirming that the extra wrapping only occurs for captured primitives.