Data Collections — the one-page version

Tip: Same rule as CSP's Big Idea 5: the correct answer minimizes collection, requires consent, considers who's harmed, and acknowledges bias. Absolute or one-sided options are distractors.

Tip: Questions here are conceptual: which structure fits which data, and what step (cleaning, filtering, aggregating) a description refers to. The code for all of it comes in the following topics.

Tip: Every array question: what's the first index (0), the last index (length − 1), and what's the default value? An index equal to length is the exam's favorite exception. And length without parentheses — with parentheses it won't compile.

Tip: If the question modifies the array through the loop variable of an enhanced for loop, the array is unchanged — that's the trick. If it calls a mutator method on an object element, the object is changed. Index needed? Standard loop.

Tip: Any algorithm touching arr[i + 1] must stop at length - 1; touching arr[i - 1] must start at 1. For "any" vs "all," the return inside the loop is the opposite of the return after it.

Tip: Three things the exam checks: the throws FileNotFoundException clause, the hasNext…/next… pair matching (hasNextLine with nextLine, hasNextInt with nextInt), and closing the Scanner. If a method opens a file and has no throws clause, it won't compile.

Tip: You can't write ArrayList<int> — that's a guaranteed compile-error question. Everywhere else, treat Integer like int and let autoboxing work, except: never compare Integers with ==, and remember a null Integer crashes when unboxed.

Tip: Rewrite the list in brackets after every call. set doesn't change size; add(i, x) and remove(i) do, and they shift everything after i. remove and set both return the element that was there — questions sometimes use that return value.

Tip: If a question removes elements in a forward loop, trace it literally — the answer usually has "leftover" elements that were skipped. If asked for the correct version, pick the backward loop or the one with i-- after remove.

Tip: FRQ 3 rubrics typically award: correct loop over the list, correct accessor call on each element, correct condition, correct add/remove/return. Write the enhanced for loop first, then the if, then what happens inside. Don't forget to return the new list.

Tip: Read m[a][b] as "row a, column b" every time — never reverse it. length counts rows; [0].length counts columns. A question that flips these is the standard distractor.

Tip: Look at which index the outer loop controls. Outer over r → row-major. Outer over c → column-major. When asked for the printed output, write out the grid and read it in that order.

Tip: FRQ 4 part (a) is usually a 1D-style helper on one row or column; part (b) uses it across the grid. Write the nested loops with r and c named clearly and keep row bound = length, column bound = [0].length. If a return inside nested loops is needed, remember it exits everything.

Tip: For "how many times is the target compared," trace mid: write low, high, mid, and the element at mid on each pass. For "which search should be used," unsorted → linear; sorted and large → binary. And binary search's mid uses integer division — truncation matters.

Tip: Given a mid-sort snapshot: if the first few elements are the globally smallest values in order, it's selection sort. If the first few are in order but a smaller value still sits later in the array, it's insertion sort.

Tip: Write the calls in a column, arguments decreasing, until the base case returns a concrete value. Then fill in upward. For methods that print, note whether the print happens before or after the recursive call — that reverses the order of output.

Tip: For merge sort traces, the snapshot after the recursive sorts but before the final merge shows two independently sorted halves — that's the give-away. For recursive binary search, count the calls the same way as 4.14 counts comparisons.