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[[tut-composites-step5,Step 5 - Constraints]]
= Step 5 - Constraints =

Previous step was <<tut-composites-step4>>.

In this step we will look at how to use Constraints. When we pass parameters to methods in regular Java code the only
restriction we can make is to denote a type. Any other constraints on the input value, such as whether the parameter is
optional, integer ranges, string regular expressions, and so on, cannot be expressed, and so we have to put this into
Javadoc, and then manually add these checks in the implementation class.

In Polygene™ there is the option to use Constraints, which are further restrictions on the parameters. This is implemented
by having an annotation that describes what the Constraint does, and then an implementation class that checks whether a
specific value fulfills the Constraint or not.

NOTE: The previous steps had a dependency to the <<core-api>> only. The constraints you've used in this step,
introduce a new dependency to the <<library-constraints>>, where all the constraint related classes reside. So
update your classpath settings accordingly.

There are a number of pre-written constraints in Polygene™ which you can use. The null check of the original HelloWorld
version is already handled by default since Polygene™ considers method parameters to be mandatory if not explicitly marked
with the @Optional annotation. So, instead of doing that check we will add other checks that are useful to make, such
as ensuring that the passed in string is not empty.

The only thing you have to do is add the annotation @NotEmpty to the method parameters you want to constrain in this
way. The annotation has a default implementation declared in it by using the @Constraints annotation. You can either
just use this, which is the common case, or override it by declaring your own @Constraints annotation in the
TransientComposite type.

You can add as many Constraint annotations you want to a parameter. All of them will be checked whenever a method is
called.

Steps for this tutorial:

- Add @NotEmpty to the state parameters.

== Solution ==

If you have successfully completed the task, you should end up with the following artifacts;

These ones remain unchanged:

- +HelloWorld.java+
- +HelloWorldComposite.java+
- +HelloWorldStateMixin.java+


*HelloWorldBehaviour.java*
[snippet,java]
----
source=tutorials/composites/src/main/java/org/apache/polygene/tutorials/composites/tutorial6/HelloWorldBehaviour.java
tag=solution
----

*HelloWorldBehaviourMixin.java*
[snippet,java]
----
source=tutorials/composites/src/main/java/org/apache/polygene/tutorials/composites/tutorial6/HelloWorldBehaviourMixin.java
tag=solution
----

*HelloWorldBehaviourConcern.java*
[snippet,java]
----
source=tutorials/composites/src/main/java/org/apache/polygene/tutorials/composites/tutorial6/HelloWorldBehaviourConcern.java
tag=solution
----

*HelloWorldState.java*
[snippet,java]
----
source=tutorials/composites/src/main/java/org/apache/polygene/tutorials/composites/tutorial6/HelloWorldState.java
tag=solution
----

Next step is <<tut-composites-step6>>