How to avoid multiple inheritance?

Question

For a project, I have following classes:

• SuperClass
• Subclass 1
• Subclass 2

The two subclasses extend the superclass. Now, I need a third class with the EXACT behaviour (read, same overriden method implementations) of both SubClass 1 and Subclass 2. Because Subclass 1 overrides only 1 method in SuperClass, and Subclass 2 doesn't override that method, I want to make the third class inherit Superclass and just implement it with the methods of Subclass 1 and Subclass 2. Now, is this good OO-design? I see no other solution because multiple inheritance in Java just isn't possible. Are there any alternatives?

Answer 1

Java8 introduced default and static methods for interfaces. To a certain degree, that allows for multiple inheritance. But most likely, the correct solution would be to rework your design.

You see, inheritance is not about code re-use. It is about creating useful abstractions; and make good use of polymorphism for example.

In your case: maybe those functionalities could/should be put into smaller interfaces; and then segregated into their own, independent classes. And then you use composition of objects instead of inheritance to build the thing you need.