First-error localization

The exam does not ask "is this code right?" — it asks "which change makes it right?" and grades the diagnosis. The localization loop:

1. Restate the contract in one sentence.
2. Trace the smallest failing case — the one the prompt shows failing (its examples ARE the failing tests).
3. Find the FIRST statement where the state table diverges from what the contract needs. Everything before that statement is correct — do not touch it.
4. The fix is usually one token: a comparison operator, an initialization value, a loop bound, a missing this., an argument in the wrong slot. If your fix rewrites half the method, you mislocated the error.

Why "first" matters: a later correct-looking line that misbehaves is usually a victim of the earlier bug, not the bug. Fixing victims adds bugs; fixing the source collapses the cascade.

Worked micro-example — contract: "sum of even elements":

int total = 0;
for (int i = 0; i <= arr.length; i++) {   // victim: throws at the end
    if (arr[i] % 2 == 0) { total += arr[i]; }
}

The exception at the LAST iteration tempts you to "fix the array". The first divergence is the <= bound itself. One token: <.