Unit 5: Designing Your First Class

Every program up to here has been one long main method holding loose variables that nothing protected. This unit gives students a way to name a thing, decide what it knows and what it does, and build as many of them as they need. It covers the class as a blueprint against the object as the thing, choosing fields and methods, new and the moment an object exists, what a reference points at, why a field is private, getters and setters and the ones not to write, constructors that guarantee a valid start, a second constructor, this with a shadowed parameter, and methods that change state on purpose.

The performance task is The Object Worth Designing. Students model something real from their own life in a class with at least four fields of at least two types, at least two constructors and at least two methods that are not getters or setters, plus a driver that builds three valid objects and attempts one build with bad data. The design sheet is completed before any code is written, and the students write their own definition of what makes an object invalid first. The heaviest criterion is that no code outside the class can put an object into an invalid state, and the run transcript is the evidence.

The unit is honest about what it cannot yet do. A constructor here cannot reject bad data by throwing, so it substitutes a safe value and reports what it did, and Lesson 12 says so and names Unit 10. A teacher gets a plan and a substitute plan for each of the eighteen lessons, answer keys, SUPPORT and STRETCH tiers of the three hardest pages, and every answer key that shows an object changing state shows the value of every field after each step.

The unit runs 18 instructional days.