At this stage, you're probably wondering just how is it technically possible, using code, to substitute or inject (to use the correct term) different compiled code during a test execution. To illustrate the various options for Dependency Injection let's start with a simple code example. We will explore how unit testing can be applied to Apex Enterprise patterns later in this chapter.
The following diagram shows the Unified Modeling Language (UML) for a Car class model, which has been designed with SOC in mind. Responsibilities such as engine, dashboard, and the digital readout display have been separated. This is a pure Apex code example to illustrate how dependencies between classes can be managed with Dependency Injection (DI):
The following code is for the Car class. It has a dependency on methods from the Dashboard and Engine classes. The caller must set up and provide instances of these classes for the methods to function correctly:
public...