Sets, Iterating & the Collections Class

HashSet, LinkedHashSet, TreeSet

The Set family mirrors the Map family exactly, because each is internally backed by a corresponding map: HashSet→HashMap, LinkedHashSet→LinkedHashMap, TreeSet→TreeMap. Same ordering trade-offs apply.

Removing while iterating

Modifying a collection with list.remove(...) while a for-each loop walks it throws ConcurrentModificationException. Use the Iterator's own remove(), or removeIf, instead.

List<Integer> nums = new ArrayList<>(List.of(1, 2, 3, 4, 5));

// Safe:
nums.removeIf(n -> n % 2 == 0);

// Also safe:
Iterator<Integer> it = nums.iterator();
while (it.hasNext()) {
    if (it.next() > 3) it.remove();
}

java.util.Collections helpers

The Collections utility class (not to be confused with the Collection interface) offers static helpers: sort a list in place, wrap one as unmodifiable, or make it thread-safe.

List<Integer> nums = new ArrayList<>(List.of(3, 1, 2));
Collections.sort(nums);
Collections.reverse(nums);

List<Integer> readOnly = Collections.unmodifiableList(nums);
List<Integer> safe = Collections.synchronizedList(nums);