2026-03-21-21-13-private-fields-coursestate-summary.md

Summary: Make CourseState fields private

What changed

File	Change
CourseState.java	Fields name, configuration, clock, courseView, durationInMinutes changed from protected to private. Added transitionToOnGoing() and transitionToFinished(Instant) protected methods. Added Instant and ChronoUnit imports.
YetToStartCourseState.java	start() now delegates to transitionToOnGoing() instead of directly constructing OnGoingCourseState.
OnGoingCourseState.java	end() now delegates to transitionToFinished(startTime) instead of directly computing duration and constructing FinishedCourseState. Removed ChronoUnit import.

Why

Encapsulating CourseState's fields as private enforces the Law of Demeter within the hierarchy: subclasses interact with the parent through its interface (methods), not its data. This makes the base class's internals easier to change without ripple effects across subclasses.

Side effects / follow-up

• None. The refactoring is purely structural; no behaviour was altered.
• FinishedCourseState required no changes — it never accessed the fields directly.

Code review checklist

• All five fields in CourseState are now private.
• transitionToOnGoing() and transitionToFinished(Instant) are protected (accessible to subclasses, not public API).
• YetToStartCourseState.start() delegates to transitionToOnGoing().
• OnGoingCourseState.end() delegates to transitionToFinished(startTime).
• No subclass directly references a field from CourseState.
• All existing tests still pass (mvn test).