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📜 WAYPOINT LESSON

Index arithmetic toolkit

⭐⭐⭐ advanced⏳ 10 min read📍 Lesson 177 of 256

The middle-two formula for both parities, adjacent comparison from index 1, and substring endpoints said out loud.

You already know charAt, substring, indexOf, equals, compareTo. At exam level the differentiators are index arithmetic and building results:

  • The middle of a string of length n starts at (n - 1) / 2 for the two-center convention — one formula, both parities. Verify: n=4 → index 1 ("code"), n=5 → index 2 ("abcde" pair starts at 2).
  • Comparing adjacent characters (charAt(i) vs charAt(i - 1)) needs a loop from 1 — not 0 — and an empty-string guard.
  • Run-length problems track a run counter and flush it at the last index. The flush after the loop is where most wrong answers live.
  • Building results: StringBuilder is legal but plain concatenation is fine for exam sizes; what matters is appending in the right order under a conditional.

Substring endpoints remain the #1 source of lost points: substring(a, b) includes a, excludes b, and throws if b > length or a > b. Before you write one, say the included and excluded indexes out loud.