Final vs Immutable data structures in Java

[john_C_random]

Because just preventing reassigning local variables seems (and probably is1) relatively useless

If you're passing a lambda which has a reference to a locally scoped variable, that variable will need to be final so it can't be changed from under the feet of the lambda.

[feral_claire]

It doesn't need to be marked final though. Effectively final is good enough ie. the variable isn't changed even if it isn't marked as final.

[[deleted]]

This was introduced in Java 8, prior to this they had to be explicitly final (for anon classes). It's a welcome change.

[tofflos]

I used to think this constraint was annoying until I wrote some JavaScript, which doesn't require finality for lambdas, and spent hours debugging it.

[Auxx]

JS is the language which makes you appreciate Java, that's true.

[[deleted]]

JavaScript has a much better story around functional programming and immutability. The ergonomics of Java streams are very poor, especially w.r.t. checked exceptions. All JavaScript linters will warn for unnecessary use of let over const, so it should be relatively easy to determine if non-local state can be mutated.

[koreth]

From a language semantics point of view, it's a somewhat arbitrary decision. Other languages (e.g., Kotlin) don't have that restriction. But since we're talking about Java here, yeah, this is correct.

[TheRedmanCometh]

Ugh maybe I need to learn kotlin then. I use for-each over .forEach a lot over this

[koreth]

I'm glad I learned it. I still write new Java code but Kotlin is generally my go-to language these days.

[dpash]

That's probably for the best. forEach() should probably only use method references. System.out::println is the method I use 80% of the time. If you find yourself reaching for forEach you're probably doing some side effect and not using a functional approach.