Implement Mixin In Java? [closed]

Question

You could use CGLIB for that. The class Mixin is able to generate a dynamic class from several interfaces / object delegates:

static Mixin    create(java.lang.Class[] interfaces,
                        java.lang.Object[] delegates)
static Mixin    create(java.lang.Object[] delegates)
static Mixin    createBean(java.lang.Object[] beans) 

Default Methods

I know the question said Java 6, but in Java 8 we'll have a pretty decent alternative: default methods.

We'll be able to add 'default' implementations of interface methods, so we can add new methods without breaking every class that implements the interface.

As long as your mixin doesn't need state, you can write code in an interface. Then your class can implement as many of these interfaces as it wants and boom, you've got mixins.

Is this an abuse of the system? A little bit, but it doesn't get into any multiple inheritance issues because there's no state.

Of course, that's also the biggest disadvantage with this approach.

I'd say just use object composition. Every time you want to throw in new functionality, compose another object into the class as a member. If you want to make all of your mixed-in classes of the same type, you can use an array as a member object where each element is composed with all of the others, and you can dispatch to a particular element.

Since Java only supports single inheritance, that is not possible. Have a look at WP: Mixin.

EDIT: Because of the comments about interfaces: The cool thing about mixins is that you can combine them without writing the combination's code. With interfaces you have to implement the combination's functionality yourself (except one class you can extend)!

The simplest approach is to use static imports. It allows for code reuse that 'looks' like it's part of the class, but is really defined elsewhere.

Pros:

• really easy
• you can 'mixin' as many static imports as you like

Cons:

• the static methods won't have access to 'this', so you'd have to pass it in manually
• no state: your static methods can't have their own instance fields. They can only define their own static fields, which are then shared by any object calling the static method.
• can't define public methods on the client class (the one with code being mixed into it). In Ruby, importing a mixin will actually define those public methods as public methods on your class. In Java, inheritance would be a better solution in this case (assuming you don't need to extend multiple classes)

Example:

import static my.package.MyHelperUtility.methodDefinedInAnotherClass;

public class MyNormalCode {
    public void example() {
        methodDefinedInAnotherClass();
    }
}

In the sense that a Ruby mix-in is the equivalent of a Java abstract class, no, you cannot implement a mix-in in Java. You can come close by using interfaces and thus defining absolutely no code in your mix-in, but you cannot directly achieve the same behavior as in a Ruby mix-in.

UPDATE: Qi4j is now Apache Polygene, https://polygene.apache.org

Qi4j's definition of Mixins are probably quite unique, as it doesn't start with a base class. By going to that extreme, a whole new paradigm of how applications are built up emerges, and we call that Composite Oriented Programming. The Composite is the 'object' equivalent and not only are Mixins wired together, but also Constraints (validation), Concerns (around advice) and SideEffects (can't alter the method outcome).

So I think Qi4j has a very strong Mixin story to tell. Mixins can be 'typed' or 'generic', they can be public (accessible outside the composite) or purely private (within the Composite). Qi4j strongly defines what Properties are, and goes on to have built-in persistence, which doesn't leak the storage implementation into your domain (caveat; Qi4j leaks to your domain). And once persisted entities enters the picture, strong definition of a Associations are also required (and included in Qi4j).

See http://www.qi4j.org/state-modeling.html for a good overview.

In Qi4j, ONLY Mixins have state. Constraints/Concerns/SideEffects can't have state (if they do they need to referens a private mixin).

To define a composite in Qi4j, it is possible to either do it structurally on the types themselves, OR at bootstrap time when the runtime model is created.

Structurally;

@Mixins({PetrolEngfineMixin.class, FourWheelsMixin.class})
public interface Car extends HasEngine, HasWheels, EntityComposite
{}

At boot time;

public interface Car
{}

public class CarModuleAssembler implements Assembler { public void assemble( ModuleAssembly module ) { module.entities( Car.class ) .withMixins( PetronEngineMixin.class, FourWheelsMixin.class ); } }

Yet, this is just touching on the surface of the features in Qi4j.