Collections: ArrayList, HashMap, TreeSet

Collections: ArrayList, HashMap, TreeSet and sorting with Arrays.sort.

Collections are Java's ready-made data structures. The three most useful: • ArrayList — a variable-length list (the equivalent of C++'s vector); • HashMap — a "key → value" dictionary for counting and grouping; • TreeSet — a duplicate-free set that keeps its elements sorted. Plus the static Arrays.sort method for sorting plain arrays.

JavaTry it yourself — the code is editable

Input (sample — feel free to edit)

Sorting an array. Enter: 5, then 3 1 4 1 5

The loop for (int x : a) is the short "for each element" form. Below, a HashMap counts how many times each number occurs: the getOrDefault method returns the accumulated value, or 0 when the key is not there yet.

JavaTry it yourself — the code is editable

Input (sample — feel free to edit)

Counting occurrences. Enter: 6, then 1 2 2 3 3 3

Task: read n numbers and print how many distinct values there are (hint: put everything into a TreeSet and print its size()). Mark the lesson as completed — the final lesson on fast input/output is ahead.