Why is Java's BigDecimal class not declared as final?

Question

While checking the source code of Java's BigDecimal class, I was surprised that it was not declared as a final class:

Class BigDecimal

public class BigDecimal
extends Number
implements Comparable<BigDecimal>

Immutable, arbitrary-precision signed decimal numbers.

(from the Oracle Docs)

Is there a specific reason for this or did the developers just forget to add that keyword? Is it a good practice to not declare immutable classes as final?

The same goes for BigInteger, but not for String which is declared as final.

Answer 1

Quote from https://blogs.oracle.com/darcy/entry/compatibly_evolving_bigdecimal:

However, there is a possible complication here since BigDecimal is not final and since it has public constructors, it can be subclassed. (As discussed in Effective Java, Item 13, Favor Immutability, this was a design oversight when the class was written.)

(emphasis mine).

Since Java has always favored backward compatibility, making it final now is out of question: it would break existing subclasses.

That said, just as when using Date, I would simply assume that no-one ever subclasses BigDecimal, and that BigDecimal should thus be used as if it were truly immutable.