We will start with writing a simple example; a business service talking to another data service. Most Java classes depend on other classes. These are called dependencies of that class.
Take a look at an example class BusinessServiceImpl, as follows:
public class BusinessServiceImpl { public long calculateSum(User user) { DataServiceImpl dataService = new DataServiceImpl(); long sum = 0; for (Data data : dataService.retrieveData(user)) { sum += data.getValue(); } return sum; } }
Typically, all well-designed applications have multiple layers. Every layer has a well-defined responsibility. The business layer contains the business logic. The data layer talks to the external interfaces and/or databases to get ...
