Understanding Why Java Lambda Captured Variables Must Be Effectively Final

My understanding is as follows; please feel free to correct any mistakes.

This behavior is determined by Java's implementation of lambda expressions. In Java, lambda expressions are a further simplification of anonymous class syntax, but fundamentally they still invoke methods on objects. Method calls in Java use pass‑by‑value, so operations on variables inside a lambda work on a copy of the original variable and do not affect the original value.

Consequently, assuming there is no requirement for external variables used in a lambda to be declared final, a developer might mistakenly think that the external variable's value can be changed within the lambda, which is actually impossible; therefore the compiler enforces the final (or effectively final) requirement to guarantee that users cannot modify the original variable inside a lambda.

Moreover, support for lambda expressions embraces functional programming, and functional programming itself should not introduce state into functions; from this perspective, requiring external variables to be final also aligns with that characteristic.

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