The four traversal modes

Arrays are the exam's heaviest topic (Data Collections is 30–40% of the MCQ section). Every array question is a traversal in one of four modes — name the mode before writing code:

Forward index scan — the default; use when you need indexes:

for (int i = 0; i < arr.length; i++) { ... arr[i] ... }

For-each — when you only read values, never indexes:

for (int v : arr) { ... }

Reverse scan — when removals or right-to-left order matter:

for (int i = arr.length - 1; i >= 0; i--) { ... }

Pairwise (adjacent) scan — neighbor logic; loop to length - 1 (and read arr[i + 1]), or start at 1 and read arr[i - 1]:

for (int i = 0; i < arr.length - 1; i++) { compare arr[i] and arr[i + 1]; }

Choosing the wrong mode is the #1 structural mistake: modifying while forward-iterating, or needing the index inside a for-each (impossible — the loop variable is a copy of the element, so v = 5 changes nothing).

Boundary drill: for {10, 20, 30} — arr.length is 3, valid indexes are 0..2, arr[arr.length - 1] is the last element, and arr[arr.length] throws ArrayIndexOutOfBoundsException. Every MCQ that "throws an exception" is testing exactly this.