Why Variables in Lambda Should Be Final?

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.

Why should local variables be final or effectively final?

The local variables that a lambda expression may use are referred to as “effectively final”. An effectively final variable is one whose value doesn't change after it's first assigned. There is no need to explicitly declare such a variable as final, although doing so would not be an error.

Why local variables referenced from a lambda expression must be final or effectively final?

3. Local Variables in Capturing Lambdas. start is a local variable, and we are trying to modify it inside of a lambda expression. ... Forcing the variable to be final avoids giving the impression that incrementing start inside the lambda could actually modify the start method parameter.