This whole tutorial describes how to step-by-step modify a typical HelloWorld "application" into a full-fledged Qi4j Composite Oriented application. Here is the initial code of HelloWorld.

Tutorial 1 - Interface refactoring

In this tutorial we start with a basic Java class, which when invoked will concatenate the two properties "phrase" and "name". If invoked with the properties set to "Hello" and "World" respectively it will hence return "Hello World".

Qi4j relies heavily on the use of interfaces. This makes it possible for an object to externally implement a number of interfaces which internally is backed by a number of Mixins, some of which you may have written yourself, and some of which may have been reused. This also makes it easy to introduce Modifiers (aka "interceptors", aka "advice"), which are Fragments which execute before and after the method on the Mixin is invoked.

The first task is therefore to refactor the code so that the method is implemented from an interface instead. We should then also separate the state into one interface and the behaviour into another. This will make things easier for us later when state and behaviour becomes implemented by separate Mixins.

Steps for this tutorial:

1. Refactor the class into interface and implementation
2. Refactor the interface so that it extends one interface called HelloWorldBehaviour with behaviour and one called HelloWorldState with state (get/set methods)

Solution

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

HelloWorld.java
HelloWorldBehaviour.java
HelloWorldMixin.java
HelloWorldState.java