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📜 WAYPOINT LESSON

Objects are state machines

⭐⭐⭐ advanced⏳ 10 min read📍 Lesson 189 of 256

Fields are state, methods are transitions; the FRQ2 shape is one guard, one boundary, one accessor.

An object is a state machine: fields are the state, methods are transitions, constructors set the start state. Exam FRQs hide a state machine behind a story (parking garage, scoreboard, vending machine), and the rubric cares about exactly three things:

  1. The transition rule is the whole problem. arrive returns false when full — one comparison decides three test cases.
  2. Order of checks matters: is a duplicate plate rejected before or after the capacity check? The spec's sentence order is the check order.
  3. Derived state (count(), isFull()) must be computed from the real state, never stored twice — a second copy of the truth will drift.

The classic FRQ2 shape: constructor stores parameters (with this.), one boolean-returning transition with a guard, one mutator that reaches a boundary (depart on empty → false), and one accessor. Every graded behavior is one of those four.