The five boundary traps

Traps are just patterns wearing a disguise. Each of these fails a plausible-looking test:

Trap 1 — the runaway initializer.

int max = 0;                    // wrong when all values are negative
for (int v : arr) { if (v > max) { max = v; } }

Fix: seed with arr[0] (requires arr.length >= 1, which the spec will state as a precondition).

Trap 2 — the fencepost loop. Print 1 2 3 with dashes between: - must be printed before every element except the first.

for (int i = 0; i < arr.length; i++) {
    if (i > 0) { System.out.print("-"); }
    System.out.print(arr[i]);
}

The general rule: N items need N−1 separators. Count the posts, count the fences.

Trap 3 — the off-by-one range. "Elements at odd positions" (positions 1, 3, 5...) means indexes 0, 2, 4 in 0-based Java. "Every second element starting at the first" is indexes 0, 2, 4 too. Draw index numbers over the elements before writing the loop bounds.

Trap 4 — the boundary comparison. <= vs < in i < arr.length decides whether the last element participates. When a loop should touch every element, < with arr.length is right; adding <= walks off the end with ArrayIndexOutOfBoundsException.

Trap 5 — early exit forgotten. Scanning for one match without break/return overwrites the result (Module 1's findFirst). Ask of every search loop: "can the answer change after it is already decided?" If yes, you needed an exit.