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[[tut-composites-step2,Step 2 - Creating a Transient Composite]]
= Step 2 - Creating a Transient Composite =

Previous step was <<tut-composites-step1>>.

In this step we will create a TransientComposite interface that ties all pieces together. The TransientComposite 
interface is a regular Java interface which extends the interfaces you want to expose from your domain model, and which 
uses various annotations to declare what Fragments to include. Fragments include Mixins, Concerns, SideEffects and 
Constraints. In this tutorial we will only use Mixins. When a TransientComposite is instantiated at runtime the 
framework will inspect the interface to determine what the TransientComposite instance should look like in terms of 
used Fragments.

In Polygene™ all method parameters are considered mandatory unless marked as @Optional. Therefore you can remove the null
checks in the Mixin. If a null value is passed in an exception will be thrown by Polygene™.

Steps for this tutorial:

- Create an interface that extends the domain interface +HelloWorld+.
- Add a @Mixins annotation to it with the name of the Mixin as argument.

== Solution ==

These ones remain unchanged:

- +HelloWorld.java+
- +HelloWorldBehaviour.java+
- +HelloWorldState.java+

*HelloWorldComposite.java*
[snippet,java]
----
source=tutorials/composites/src/main/java/org/apache/polygene/tutorials/composites/tutorial3/HelloWorldComposite.java
tag=solution
----

*HelloWorldMixin.java*
[snippet,java]
----
source=tutorials/composites/src/main/java/org/apache/polygene/tutorials/composites/tutorial2/HelloWorldMixin.java
tag=solution
----


Next step is <<tut-composites-step3>>