Variable used in lambda expression should be final or effectively final

Question

Variable used in lambda expression should be final or effectively final

When I try to use calTz it is showing this error.

Answer 1

Although other answers prove the requirement, they don't explain why the requirement exists.

The JLS mentions why in §15.27.2:

The restriction to effectively final variables prohibits access to dynamically-changing local variables, whose capture would likely introduce concurrency problems.

To lower risk of bugs, they decided to ensure captured variables are never mutated.