If a class that was not abstract is changed to be declared abstract, then pre-
existing binaries that attempt to create new instances of that class will throw either
an InstantiationError at link time, or an InstantiationException at run
time (if the method newInstance (§20.3.6) of class Class is used); such a change
is therefore not recommended for widely distributed classes.
Changing a class that was declared abstract to no longer be declared abstract does not break compatibility with pre-existing binaries.