ioc-example.java

// In the example above, the userLister is not initialized
class DependencyProblem {
    private UserLister userLister;

    // gets users from DB and renders to the view (just a trivial example)
    public void render() {
        List<User> users = userLister.getUsers(); 
        view.render(users);
    }
}

// If we initialize it like below, we increase coupling between classes (tight coupling)
// UserLister userLister = new UserListerDB();
// Also, if we want to change it, we need to use UserListerCommaSeparatedFile();
// This change is no problem for a small project but a nightmare for a huge project

// Inversion of Control Solution
class SpringIOC {
  // Just adding the annotation is enough. Spring manages the dependency (DI).
  @Inject // this and @Autowired are almost identical
  private UserLister userLister;
  
  public void render() {
        List<User> users = userLister.getUsers();
        view.render(users);
  }
}