java traits or mixins pattern?

Question

Is there a way to emulate mixins or traits in java? basically, I need a way to do multiple inheritance so I can add common business logic to several classes

Answer 1

Not the way you want to do it. Effective Java recommends that you "Favor composition over inheritance". Meaning you move the common logic to other classes and delegate. This is how you get around the lack of multiple inheritance in java.

Answer 2

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.