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📜 WAYPOINT LESSON

Edge discipline

⭐⭐ intermediate⏳ 12 min read📍 Lesson 121 of 256

Base cases that answer with zero information; the two failure signatures.

Foundations taught recursion's shape; the exam grades its edge discipline. Every recursive method is two contracts:

  1. Base case — the smallest input, answered directly, with NO recursive call. For arrays/strings it is usually the empty or single-element input.
  2. Recursive case — reduce toward the base case, combine, return. The reduction must be provably smaller: n - 1, i + 1, substring(1).

The two failure signatures (both exam favorites):

  • Wrong base value: fact(0) = 1 by definition; returning 0 there multiplies everything to 0. One wrong seed poisons the whole tree — the code looks right and every non-trivial test fails.
  • Missing base case (or an unreachable one): infinite recursion → StackOverflowError. On a trace question, count frames: if n never reaches the base value, the method never returns.

The decision rule: when the base case runs, the answer must be constructible with zero additional information. If your base case still needs to call something, it is not a base case.