GotchaDifficulty: Advanced1 min

The final Keyword on Classes, Methods and Fields

One keyword, three build failures, and a final list that can still be added to.

final means "cannot be replaced", and what that stops depends on where it is written. A final class cannot be extended, a final method cannot be overridden, and a final field cannot be assigned again once set, although the object it points at can still change.

You extend a class from a library and the build fails. You override a method and the build fails. You assign to a field in a second method and the build fails. One keyword, three different messages:

What final prevents in each of the three places it can appear
Written on	Prevents	Still allowed
A class	Anything extending it	Creating instances, calling everything on it
A method	A subclass overriding it	Calling it, and overloading the name
A field	Assigning to it after it is set	Changing the object it points at

java

final class Money { }

class Account {
    final List<String> history = new ArrayList<>();

    void log(String line) {
        history.add(line);          // fine
        history = new ArrayList<>(); // will not compile
    }
}

The last row is the one that surprises people. final on a reference field freezes the address, not the object at that address. Line 7 changes the list and is legal; line 8 tries to point the field somewhere else and is not.

Reach for final on fields by default: it says the value is decided once, and the compiler will tell you if any path fails to set it. Reach for it on classes and methods deliberately, because it is a promise to everyone that this behaviour will never be replaced.

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