Wiring & Refactoring

Wiring dependencies by hand

Frameworks (Spring) do dependency injection for you; intermediates learn to wire by hand first — it demystifies the container:

public static void main(String[] args) {
    ExpenseRepository repo = new InMemoryExpenseRepository();
    ReportService reports = new ReportService(repo);
    CliController cli = new CliController(reports);
    cli.run();   // the composition root — the ONLY place that knows everything
}

The composition root is the single place where concrete classes meet. Everything below it sees interfaces only.

Refactoring a god class, in order:

1. find the seams (inputs/outputs of each responsibility)
2. extract one responsibility into a collaborator
3. inject it via constructor
4. test the extracted class alone
5. repeat until the original class only orchestrates

Each step keeps the program working — refactoring is a series of small verified moves, not a rewrite.