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📜 WAYPOINT LESSON

The error clinic

⭐⭐ intermediate⏳ 12 min read📍 Lesson 131 of 256

Classify, locate, name the missing check, re-solve from scratch.

The error clinic — a method for reviewing your OWN wrong answers (and the debug challenges in this course):

  1. Classify before re-solving. Which family? (Write it down.) Off-by-one? Reference confusion? Missed precondition?
  2. Locate the exact statement where your reasoning diverged from the truth — one line, not "I guessed wrong".
  3. Name the missing check. What probe would have caught it? (Empty? Tie? Absence?) Add it to your personal checklist — by exam day it has ~15 items and every MCQ runs through it.
  4. Re-solve from scratch, not from "where I went wrong" — right answers built on patched reasoning don't transfer to the next question.

Example: you chose "compilation error" for ArrayList<Integer> list; list.add(5); int x = list.get(0) + 1;. Classification: unbox family. Divergence: thought list.get(0) was still an Integer in arithmetic. Missing check: "arithmetic forces the unbox." The next time any choice offers "compilation error" for autoboxed arithmetic, your checklist fires.

The clinic loop is why this course's wrong-solution challenges matter: every W solution in the harness is a classified, named error — the same taxonomy, practiced from the inside.