Question

I have an immutable object that doesn’t have its fields or class marked as final. I could do this, but is there really any benefit to doing this? I could see it saving the compiler a little bit of time figuring things out, but I can’t see it being “worth” it (other then the fact that it will make future developers rethink doing something to the object to make it mutable).

Answer 1

Apart from the points you make (future developer modifying the fields being a very sensible one, another one is that someone could subclass your class and make it mutable), explicitly marking the fields as final provides you with visibility guarantees in a multi threaded environment.

Take this class – it is effectively immutable:

public class SomeClass {
    private int i;
    public SomeClass(int i) { this.i = i; }
    public int getI() { return this.i; }
}

In a multi-threaded environment, it is possible that a thread T1 creates a SomeClass sc = new SomeClass(1); and that another thread T2 reads sc.getI() and sees 0.

If i is made final, this can not happen any more (assuming you don’t let this escape during construction, as explained in the quote below).

Reference: JLS #17.5 – emphasis mine

final fields also allow programmers to implement thread-safe immutable objects without synchronization. […]
The usage model for final fields is a simple one: Set the final fields for an object in that object’s constructor; and do not write a reference to the object being constructed in a place where another thread can see it before the object’s constructor is finished. If this is followed, then when the object is seen by another thread, that thread will always see the correctly constructed version of that object’s final fields.