Creating an immutable object without final fields?

Question

Can we create an immutable object without having all fields final?

If possible a couple of examples would be helpful.

Answer 1

Declare all fields private and only define getters:

public final class Private{
    private int a;
    private int b;

    public int getA(){return this.a;}
    public int getB(){return this.b;}
}

citing @Jon Skeet's comment, final class modifier is useful for:

While an instance of just Private is immutable, an instance of a subclass may well be mutable. So code receiving a reference of type Private can't rely on it being immutable without checking that it's an instance of just Private.

So if you want to be sure the instance you are referring to is immutable you should use also final class modifier.

Answer 2

Yes, it is - just make sure that your state is private, and nothing in your class mutates it:

public final class Foo
{
    private int x;

    public Foo(int x)
    {
        this.x = x;
    }

    public int getX()
    {
        return x;
    }
}

There's no way of mutating the state within this class, and because it's final you know that no subclasses will add mutable state.

However:

• The assignment of non-final fields doesn't have quite the same memory visibility rules as final fields, so it might be possible to observe the object "changing" from a different thread. See section 17.5 of the JLS for more details on the guarantees for final fields.
• If you're not planning on changing the field value, I would personally make it final to document that decision and to avoid accidentally adding a mutating method later
• I can't remember offhand whether the JVM prevents mutating final fields via reflection; obviously any caller with sufficient privileges could make the x field accessible in the above code, and mutate it with reflection. (According to comments it can be done with final fields, but the results can be unpredictable.)