How does the “final” keyword in Java work? (I can still modify an object.)

Question

In Java we use final keyword with variables to specify its values are not to be changed. But I see that you can change the value in the constructor / methods of

Answer 1

First of all, the place in your code where you are initializing (i.e. assigning for the first time) foo is here:

foo = new ArrayList();

foo is an object (with type List) so it is a reference type, not a value type (like int). As such, it holds a reference to a memory location (e.g. 0xA7D2A834) where your List elements are stored. Lines like this

foo.add("foo"); // Modification-1

do not change the value of foo (which, again, is just a reference to a memory location). Instead, they just add elements into that referenced memory location. To violate the final keyword, you would have to try to re-assign foo as follows again:

foo = new ArrayList();

That would give you a compilation error.

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Now, with that out of the way, think about what happens when you add the static keyword.

When you do NOT have the static keyword, each object that instantiates the class has its own copy of foo. Therefore, the constructor assigns a value to a blank, fresh copy of the foo variable, which is perfectly fine.

However, when you DO have the static keyword, only one foo exists in memory that is associated with the class. If you were to create two or more objects, the constructor would be attempting to re-assign that one foo each time, violating the final keyword.