In almost all programs inheritance is for re-using code, so it is implementation inheritance. The big problem with that approach is that in most cases there is

In almost all programs inheritance is for re-using code, so it is implementation inheritance. The big problem with that approach is that in most cases there is no contract for sub-classes, the properties that a sub-class is required to have. This can lead to very messy class hierarchies. Also note that a lot of problems disappear when objects are immutable.