The trap taxonomy
Eight families of wrong answers — recognize the bait on sight.
The trap taxonomy — every wrong answer on the exam belongs to a family. Learn the families, recognize the bait:
The < vs <= family. Loop bounds and tie-breaking. A choice that
differs only by one included/excluded element is testing this.
The -1 vs 0 family. indexOf absence, sentinel defaults, "no
match" conventions. What IS the contract's absence value?
The == vs .equals family. References vs content. Strings and
Integers; the answer that "works" in some tests but is wrong by
contract.
The copy vs alias family. x = y for objects copies the arrow.
A choice assuming independent objects is bait.
The unbox family. Integer arithmetic auto-unboxes (true), Integer
== Integer compares references (false for 200, true for small cached
values). A choice that says "compilation error" for list.get(0) + 1
is bait.
The mutation-order family. remove/insert during forward iteration; second pass reading first pass's mutations. The choice "prints the original values" is bait.
The base-case family. Off-by-one recursion bases (0 vs 1), missing bases (StackOverflow), wrong seeds (fact(0) = 0).
The division family. Integer division truncation, 9/2 = 4, and
the (double) cast that fixes it. Modulo sign on negatives (rarely
tested in AP scope but the division family is everywhere).
Practicing with families converts "tricky" into "recognized".