what is the sense of final ArrayList?

Question

Which advantages/disadvantages we can get by making ArrayList (or other Collection) final? I still can add to ArrayList new elements, remove elements and update it. But what is effect making it's final?

Solution 1

But what is effect making it's final?

This means that you cannot rebind the variable to point to a different collection instance:

final List<Integer> list = new ArrayList<Integer>();
list = new ArrayList<Integer>(); // Since `list' is final, this won't compile

As a matter of style, I declare most references that I don't intend to change as final.

I still can add to ArrayList new elements, remove elements and update it.

If you wish, you can prevent insertion, removal etc by using Collections.unmodifiableList():

final List<Integer> list = Collections.unmodifiableList(new ArrayList<Integer>(...));

Solution 2

It just means that you can't re-assign its reference. Attempting to do something like the below will lead to compiler error.

final List<String> list = new ArrayList<String>();

list = new LinkedList<String>();
     ^
     Compiler error here

If you really want an immutable list, you should use the Collections.unmodifiableList() method.

Solution 3

You won't be able to modify its reference using new ArrayList for example.

Solution 4

final has a lot of consequences under multi threading.

1. JMM clearly defines a final field's initialization completion is guaranteed.

What's not clearly defined is:

1. Compilers are free to reorder them across memory barriers.
2. Compilers can always read a cached copy.

Solution 5

Making the variable final makes sure you cannot re-assign that objest reference after it is assigned. As you mention you can still use that lists methods to make changes.

If you combine the final keyword with the use of Collections.unmodifiableList, you ge the behaviour you are probably trying to achieve, for instance:

final List fixedList = Collections.unmodifiableList(someList);

This has as result that the list pointed to by fixedList cannot be changed. Beware however that it can still be change through the someList reference (so make sure it is out of scope after this asignment.)

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Comments

[Vishy]

+1 Using final fields can improve clarity as classes can be quite long. I don't use final in methods so much as I try to break up long methods.

[Coder]

how does final improve clarity?