CSE 122 Q1

Stack

Last-In First-Out (LIFO) collection. Use push() to add

Queue

First-In First-Out (FIFO) collection. Use add() or offer() to enqueue

PriorityQueue

A type of Queue that orders elements based on natural ordering or a Comparator. Not FIFO.

Deque (Double-Ended Queue)

Can act as both a queue (FIFO) and stack (LIFO). Supports addFirst()

HashSet

Stores unique elements

TreeSet

Sorted set that keeps elements in natural order (or a custom Comparator). Backed by a Red-Black tree

LinkedHashSet

Maintains insertion order while still preventing duplicates. Slightly slower than HashSet.

Set iteration method

Use enhanced for loop (for (Type x : set)) or an Iterator (Iterator it = set.iterator()).

HashMap

Key-value pairs

TreeMap

Sorted map ordered by keys (natural or Comparator). No null keys allowed. O(log n) operations.

LinkedHashMap

Maintains insertion order of keys. Slightly slower than HashMap.

Map iteration (keys)

for (K key : map.keySet()) { … }

Map iteration (values)

for (V value : map.values()) { … }

Map iteration (entries)

for (Map.Entry<K

Map get vs put

get(key) retrieves value or null if missing

put(key

value) inserts or replaces value.

Map containsKey vs containsValue

containsKey() checks if a key exists

containsValue() checks if any key maps to that value (slower).

Reference Semantics

When you assign or pass objects

Value Semantics

When primitive values or immutable objects (like Strings) are copied by value

Iterator syntax

Iterator it = collection.iterator()

while (it.hasNext()) { Type element = it.next()

}

Iterator remove()

Safely removes elements while iterating to avoid ConcurrentModificationException.

Enhanced for loop (for-each)

for (Type x : collection) { // read-only access to elements }

HashSet vs TreeSet

HashSet — unordered

TreeSet — sorted

slower (O(log n)).

HashMap vs TreeMap

HashMap — unordered

TreeMap — sorted

no null key.

LinkedHashSet vs TreeSet

LinkedHashSet keeps insertion order

TreeSet keeps sorted order.

Map<String

List>

Set

Set containing lists — duplicates determined by list equality (equals()). Order of lists in set not guaranteed.

Map<String

Set>