Tutorial 5 - Constraints

In this tutorial 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 Qi4j 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.

There are a number of pre-written constraints in Qi4j which you can use. The null check of the original HelloWorld version is already handled by default since Qi4j 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:

1. Add @NotEmpty to the state parameters.

Note:
The previous steps had a dependency to the qi4j-core-api only. The constraints you've used in this step, introduce a new dependency to the qi4j-lib-constraints library, where all the constraint related classes reside. So update your classpath settings accordingly.

Solution

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

HelloWorld.java
HelloWorldBehaviour.java
HelloWorldBehaviorConcern.java
HelloWorldBehaviourMixin.java
HelloWorldComposite.java
HelloWorldState.java
HelloWorldStateMixin.java