Answer by Alex B for java traits or mixins pattern?

I would encapsulate all of the business logic into a new class BusinessLogic and have each class that needs BusinessLogic make calls to the class. If you need a single rooted heirarchy for your classes that make calls to BusinessLogic, you'll have to create an interface as well (BusinessLogicInterface?)

In pseudo-code:

interface BusinessLogicInterace
{
    void method1();
    void method2();
}

class BusinessLogic implements BusinessLogicInterface
{
    void method1() { ... }
    void method2() { ... }
}

class User 
    extends OtherClass 
    implements BusinessLogicInterface
{
    BusinessLogic logic = new BusinessLogic();

    @Override
    void method1() { logic.method1(); }

    @Override
    void method2() { logic.method2(); }
}

This isn't the prettiest implementation to work around a lack of multiple inheritance and it becomes quite cumbersome when the interface has a lot of methods. Most likely, you'll want to try and redesign your code to avoid needing mixins.