Are all final class immutable?

Question

Are all final classes in Java immutable. String and Integer both are final classes and both are immutable i beleive.

Answer 1

Further to the other responses, if you look at the code for java.lang.String you'll see it contains a field: hash, which is mutable and is in fact computed and stored when hashCode() is called for the first time.

However, the class is still immutable: The hash field cannot be accessed directly or modified outside of the class.

Also, you may notice a common approach within the JDK is the implementation of immutable wrappers that can be used to expose an object's internal state without allowing it to be modified; e.g.

private final List values;

public List getValues() {
  return Collections.unmodifiableList(values);
}