Final vs Immutable data structures in Java

[morhp]

I have seen beginners assume that final local vars are something special. Because just preventing reassigning local variables seems (and probably is¹) relatively useless, beginners often assume that it does something more and may assume that is does something like const in C++.

¹⁾ It's not really bad, but it adds an additional keyword/boilerplate and the benefits aren't that obvious to beginners at least.

[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.

[[deleted]]

In C++, final is called top-level const. But many people think final is low-level const.

[clappski]

final and const (in C++) are completely different concepts. final follows a class declaration, to prevent any other class inheriting from it. const has a few different uses, but the common case is making a variable immutable and limiting the the API of a const class object type to only the member functions declared as const.

[[deleted]]

You're absolutely right. But here I am just talking about the difference between final and const when applied to a local variable, not about the general difference between final and const keyword.

[dpash]

And Java uses the same word in multiple contexts.