java.lang.Iterable
    |
java.util.Collection
    |
    +-- List          --> ArrayList, LinkedList, Vector, Stack
    |
    +-- Queue         --> PriorityQueue, ArrayDeque, LinkedList
    |       |
    |       +-- Deque --> ArrayDeque, LinkedList
    |
    +-- Set           --> HashSet, LinkedHashSet
            |
            +-- SortedSet --> NavigableSet --> TreeSet

java.util.Map (separate hierarchy - does NOT extend Collection)
    |
    +-- SortedMap --> NavigableMap --> TreeMap
    |
    Implementations: HashMap, LinkedHashMap, TreeMap, Hashtable, EnumMap, WeakHashMap

public interface Iterable<T> {
    Iterator<T> iterator();                              // Returns an iterator
    default void forEach(Consumer<? super T> action);   // Java 8 - iterates and applies action
    default Spliterator<T> spliterator();               // Java 8 - for parallel iteration
}

List<String> list = List.of("A", "B", "C");
list.forEach(s -> System.out.println(s));  // uses forEach

public interface Iterator<E> {
    boolean hasNext();           // Returns true if more elements exist
    E next();                    // Returns next element, advances cursor
    default void remove();       // Removes last element returned by next() - optional operation
    default void forEachRemaining(Consumer<? super E> action); // Java 8 - applies action to remaining elements
}

List<Integer> list = new ArrayList<>(Arrays.asList(1, 2, 3, 4));
Iterator<Integer> it = list.iterator();
while (it.hasNext()) {
    int val = it.next();
    if (val % 2 == 0) it.remove(); // safe removal
}
// list is now [1, 3]

public interface ListIterator<E> extends Iterator<E> {
    // Forward traversal (inherited)
    boolean hasNext();
    E next();
    int nextIndex();         // Index of element that would be returned by next()

    // Backward traversal
    boolean hasPrevious();
    E previous();            // Returns previous element, moves cursor backward
    int previousIndex();     // Index of element that would be returned by previous()

    // Modification
    void remove();           // Removes last element returned by next() or previous()
    void set(E e);           // Replaces last element returned by next() or previous()
    void add(E e);           // Inserts element before the element that would be returned by next()
}

List<String> list = new ArrayList<>(Arrays.asList("A", "B", "C"));
ListIterator<String> lit = list.listIterator(list.size()); // start from end
while (lit.hasPrevious()) {
    System.out.print(lit.previous() + " "); // prints C B A
}

public interface Collection<E> extends Iterable<E> {

    // Size and emptiness
    int size();                                      // Number of elements
    boolean isEmpty();                               // True if size() == 0

    // Membership
    boolean contains(Object o);                     // True if element exists (uses equals())
    boolean containsAll(Collection<?> c);           // True if all elements of c are present

    // Adding
    boolean add(E e);                               // Adds element; returns true if collection changed
    boolean addAll(Collection<? extends E> c);      // Adds all elements of c

    // Removing
    boolean remove(Object o);                       // Removes single instance of o; returns true if changed
    boolean removeAll(Collection<?> c);             // Removes all elements also in c (set difference)
    boolean retainAll(Collection<?> c);             // Retains only elements also in c (intersection)
    default boolean removeIf(Predicate<? super E> filter); // Java 8 - removes elements matching predicate
    void clear();                                   // Removes all elements

    // Conversion
    Object[] toArray();                             // Returns array of all elements
    <T> T[] toArray(T[] a);                         // Returns typed array; uses a if large enough
    default <T> T[] toArray(IntFunction<T[]> generator); // Java 11 - e.g., toArray(String[]::new)

    // Stream support (Java 8)
    default Stream<E> stream();                     // Returns a sequential Stream
    default Stream<E> parallelStream();             // Returns a parallel Stream

    // Iterator
    Iterator<E> iterator();
    default Spliterator<E> spliterator();
}

public interface List<E> extends Collection<E> {

    // Positional access
    E get(int index);                               // Returns element at index
    E set(int index, E element);                    // Replaces element at index; returns old element
    void add(int index, E element);                 // Inserts at index; shifts subsequent elements right
    E remove(int index);                            // Removes element at index; returns it
    boolean addAll(int index, Collection<? extends E> c); // Inserts all at index

    // Search
    int indexOf(Object o);                          // First index of o; -1 if not found
    int lastIndexOf(Object o);                      // Last index of o; -1 if not found

    // Sub-list
    List<E> subList(int fromIndex, int toIndex);    // View of portion [fromIndex, toIndex); NOT a copy

    // Sorting (Java 8)
    default void sort(Comparator<? super E> c);     // Sorts list using comparator (null = natural order)

    // Replacement (Java 8)
    default void replaceAll(UnaryOperator<E> operator); // Replaces each element with result of operator

    // Iterators
    ListIterator<E> listIterator();                 // ListIterator starting at index 0
    ListIterator<E> listIterator(int index);        // ListIterator starting at given index

    // Factory methods (Java 9) - returns unmodifiable list
    static <E> List<E> of(E... elements);
    static <E> List<E> copyOf(Collection<? extends E> coll); // Java 10
}

ArrayList<String> list = new ArrayList<>();

// Constructors
new ArrayList<>()                        // empty, initial capacity 10
new ArrayList<>(int initialCapacity)     // empty with given initial capacity
new ArrayList<>(Collection<? extends E> c) // populated from collection

// Capacity management
void ensureCapacity(int minCapacity);    // Ensures array is at least minCapacity
void trimToSize();                       // Trims backing array to current size (saves memory)

ArrayList<String> list = new ArrayList<>();
list.add("Java");
list.add("Python");
list.add(0, "C++");            // insert at index 0
System.out.println(list.get(1)); // Java
list.remove(Integer.valueOf("Java")); // removes by object, not index
list.sort(Comparator.naturalOrder());

// Adding
void addFirst(E e);         // Inserts at beginning
void addLast(E e);          // Inserts at end (same as add(e))
boolean offerFirst(E e);    // Inserts at beginning (returns false if fails - never here)
boolean offerLast(E e);     // Inserts at end

// Removing
E removeFirst();            // Removes and returns first; throws NoSuchElementException if empty
E removeLast();             // Removes and returns last; throws NoSuchElementException if empty
E pollFirst();              // Removes and returns first; returns null if empty
E pollLast();               // Removes and returns last; returns null if empty

// Peeking
E getFirst();               // Returns first without removing; throws if empty
E getLast();                // Returns last without removing; throws if empty
E peekFirst();              // Returns first without removing; returns null if empty
E peekLast();               // Returns last without removing; returns null if empty

// Stack operations
void push(E e);             // Same as addFirst(e)
E pop();                    // Same as removeFirst()
E peek();                   // Same as peekFirst()

// Descending iterator
Iterator<E> descendingIterator(); // Iterates from tail to head

// Constructors
new Vector<>()
new Vector<>(int initialCapacity)
new Vector<>(int initialCapacity, int capacityIncrement) // custom growth amount
new Vector<>(Collection<? extends E> c)

// Legacy-specific methods (older names, same behavior as List methods)
void addElement(E obj);         // same as add(E)
void insertElementAt(E obj, int index); // same as add(int, E)
E elementAt(int index);         // same as get(int)
void setElementAt(E obj, int index);    // same as set(int, E)
void removeElement(Object obj); // same as remove(Object)
void removeElementAt(int index);// same as remove(int)
void removeAllElements();       // same as clear()
int indexOf(Object o, int index);       // search starting from index
int lastIndexOf(Object o, int index);   // search backwards from index
Enumeration<E> elements();      // legacy iterator (pre-Iterator)
void copyInto(Object[] anArray);// copies elements into array
int capacity();                 // returns current array capacity
void ensureCapacity(int minCapacity);
void trimToSize();
E firstElement();               // same as get(0)
E lastElement();                // same as get(size()-1)

E push(E item);             // Pushes item onto top of stack; returns item
E pop();                    // Removes and returns top item; throws EmptyStackException if empty
E peek();                   // Returns top item without removing; throws EmptyStackException if empty
boolean empty();            // Returns true if stack has no elements
int search(Object o);       // Returns 1-based position from top; -1 if not found

// Legacy way - Stack (synchronized, slow)
Stack<Integer> stack = new Stack<>();
stack.push(1); stack.pop();

// Modern way - ArrayDeque as stack (not synchronized, faster)
Deque<Integer> stack = new ArrayDeque<>();
stack.push(1); stack.pop();

public interface Queue<E> extends Collection<E> {
    boolean add(E e);       // Inserts element; throws IllegalStateException if capacity exceeded
    boolean offer(E e);     // Inserts element; returns false if not possible

    E remove();             // Retrieves and removes head; throws NoSuchElementException if empty
    E poll();               // Retrieves and removes head; returns null if empty

    E element();            // Retrieves but does not remove head; throws NoSuchElementException if empty
    E peek();               // Retrieves but does not remove head; returns null if empty
}

// Constructors
new PriorityQueue<>()                               // natural order, initial capacity 11
new PriorityQueue<>(int initialCapacity)
new PriorityQueue<>(Comparator<? super E> comparator)
new PriorityQueue<>(int initialCapacity, Comparator<? super E> comparator)
new PriorityQueue<>(Collection<? extends E> c)
new PriorityQueue<>(PriorityQueue<? extends E> c)
new PriorityQueue<>(SortedSet<? extends E> c)

// Methods (inherits all Queue methods, plus)
Comparator<? super E> comparator();  // Returns comparator, or null if natural ordering

PriorityQueue<Integer> maxHeap = new PriorityQueue<>(Comparator.reverseOrder());
maxHeap.offer(3); maxHeap.offer(1); maxHeap.offer(5);
System.out.println(maxHeap.poll()); // 5 (largest first)

Queue<String> queue = new LinkedList<>();
queue.offer("first");
queue.offer("second");
queue.poll();  // returns "first"

public interface Deque<E> extends Queue<E> {

    // Insert at front
    void addFirst(E e);         // throws IllegalStateException if full
    boolean offerFirst(E e);    // returns false if full

    // Insert at back
    void addLast(E e);          // throws IllegalStateException if full (same as add(e))
    boolean offerLast(E e);     // returns false if full (same as offer(e))

    // Remove from front
    E removeFirst();            // throws NoSuchElementException if empty (same as remove())
    E pollFirst();              // returns null if empty (same as poll())

    // Remove from back
    E removeLast();             // throws NoSuchElementException if empty
    E pollLast();               // returns null if empty

    // Examine front
    E getFirst();               // throws NoSuchElementException if empty (same as element())
    E peekFirst();              // returns null if empty (same as peek())

    // Examine back
    E getLast();                // throws NoSuchElementException if empty
    E peekLast();               // returns null if empty

    // Stack operations (treat deque as LIFO stack)
    void push(E e);             // same as addFirst(e)
    E pop();                    // same as removeFirst()
    E peek();                   // same as peekFirst()

    // Remove specific element
    boolean removeFirstOccurrence(Object o);  // removes first occurrence
    boolean removeLastOccurrence(Object o);   // removes last occurrence

    // Descending iterator
    Iterator<E> descendingIterator(); // iterates from tail to head
}

Set<String> s = Set.of("A", "B", "C");           // unmodifiable, no nulls, no duplicates
Set<String> s = Set.copyOf(existingCollection);   // Java 10

// Constructors
new HashSet<>()
new HashSet<>(int initialCapacity)
new HashSet<>(int initialCapacity, float loadFactor)
new HashSet<>(Collection<? extends E> c)

Set<String> set = new HashSet<>();
set.add("Apple"); set.add("Banana"); set.add("Apple"); // duplicate ignored
System.out.println(set.size()); // 2

// Constructors
new LinkedHashSet<>()
new LinkedHashSet<>(int initialCapacity)
new LinkedHashSet<>(int initialCapacity, float loadFactor)
new LinkedHashSet<>(Collection<? extends E> c)

// Constructors
new TreeSet<>()                                     // natural ordering
new TreeSet<>(Comparator<? super E> comparator)     // custom comparator
new TreeSet<>(Collection<? extends E> c)            // natural ordering from collection
new TreeSet<>(SortedSet<E> s)                       // same ordering as s

// All NavigableSet methods available - see Section 8

enum Day { MON, TUE, WED, THU, FRI, SAT, SUN }

// Factory methods (no public constructors)
EnumSet<Day> workDays = EnumSet.of(Day.MON, Day.TUE, Day.WED, Day.THU, Day.FRI);
EnumSet<Day> weekend  = EnumSet.of(Day.SAT, Day.SUN);
EnumSet<Day> allDays  = EnumSet.allOf(Day.class);
EnumSet<Day> noDays   = EnumSet.noneOf(Day.class);
EnumSet<Day> range    = EnumSet.range(Day.MON, Day.FRI);     // MON to FRI inclusive
EnumSet<Day> copy     = EnumSet.copyOf(workDays);
EnumSet<Day> comp     = EnumSet.complementOf(workDays);      // SAT and SUN

public interface SortedSet<E> extends Set<E> {
    Comparator<? super E> comparator(); // Returns comparator; null if natural ordering

    // Range views (returned views are backed by original set)
    SortedSet<E> subSet(E fromElement, E toElement);   // [fromElement, toElement) - from inclusive, to exclusive
    SortedSet<E> headSet(E toElement);                 // Elements strictly less than toElement
    SortedSet<E> tailSet(E fromElement);               // Elements >= fromElement

    // Endpoints
    E first();   // Returns first (lowest) element; throws NoSuchElementException if empty
    E last();    // Returns last (highest) element; throws NoSuchElementException if empty
}

public interface NavigableSet<E> extends SortedSet<E> {

    // Closest element queries
    E lower(E e);       // Greatest element strictly less than e; null if none
    E floor(E e);       // Greatest element <= e; null if none
    E ceiling(E e);     // Least element >= e; null if none
    E higher(E e);      // Least element strictly greater than e; null if none

    // Removal of extremes
    E pollFirst();      // Removes and returns first (lowest); null if empty
    E pollLast();       // Removes and returns last (highest); null if empty

    // Reverse order
    NavigableSet<E> descendingSet();            // Reverse-order view of this set
    Iterator<E> descendingIterator();           // Iterator in descending order

    // Controlled range views (more flexible than SortedSet)
    NavigableSet<E> subSet(E from, boolean fromInclusive, E to, boolean toInclusive);
    NavigableSet<E> headSet(E toElement, boolean inclusive);
    NavigableSet<E> tailSet(E fromElement, boolean inclusive);
}

TreeSet<Integer> ts = new TreeSet<>(Arrays.asList(1, 3, 5, 7, 9));
System.out.println(ts.floor(6));    // 5 (greatest <= 6)
System.out.println(ts.ceiling(6));  // 7 (least >= 6)
System.out.println(ts.lower(5));    // 3 (strictly less than 5)
System.out.println(ts.higher(5));   // 7 (strictly greater than 5)
System.out.println(ts.subSet(3, true, 7, false)); // [3, 5]

public interface Map<K, V> {

    // Size
    int size();
    boolean isEmpty();

    // Lookup
    boolean containsKey(Object key);            // True if key exists
    boolean containsValue(Object value);        // True if value exists (O(n) scan)
    V get(Object key);                          // Returns value for key; null if not found
    default V getOrDefault(Object key, V defaultValue); // Java 8 - returns defaultValue if key absent

    // Modification
    V put(K key, V value);                      // Associates key with value; returns old value (or null)
    void putAll(Map<? extends K, ? extends V> m); // Copies all mappings from m
    V remove(Object key);                       // Removes key; returns associated value (or null)
    boolean remove(Object key, Object value);   // Java 8 - removes only if key maps to value
    void clear();

    // Conditional put (Java 8)
    default V putIfAbsent(K key, V value);      // Puts only if key is absent or mapped to null; returns old value
    default boolean replace(K key, V oldValue, V newValue); // Replaces only if currently mapped to oldValue
    default V replace(K key, V value);          // Replaces only if key is currently mapped

    // Compute methods (Java 8)
    default V compute(K key, BiFunction<? super K, ? super V, ? extends V> remappingFunction);
        // Computes new value for key using function; removes entry if function returns null

    default V computeIfAbsent(K key, Function<? super K, ? extends V> mappingFunction);
        // If key absent or null, computes value using function and stores it; returns new value

    default V computeIfPresent(K key, BiFunction<? super K, ? super V, ? extends V> remappingFunction);
        // If key present and non-null, recomputes value; removes if function returns null

    default V merge(K key, V value, BiFunction<? super V, ? super V, ? extends V> remappingFunction);
        // If key absent: puts value. If key present: applies function to old and new value.
        // Removes if function returns null.

    // Iteration (Java 8)
    default void forEach(BiConsumer<? super K, ? super V> action);
    default void replaceAll(BiFunction<? super K, ? super V, ? extends V> function);

    // Views
    Set<K> keySet();                            // Set view of all keys
    Collection<V> values();                     // Collection view of all values
    Set<Map.Entry<K, V>> entrySet();            // Set view of key-value pairs

    // Nested interface
    interface Entry<K, V> {
        K getKey();
        V getValue();
        V setValue(V value);
        static <K extends Comparable<? super K>, V> Comparator<Entry<K,V>> comparingByKey();
        static <K, V extends Comparable<? super V>> Comparator<Entry<K,V>> comparingByValue();
        static <K, V> Comparator<Entry<K,V>> comparingByKey(Comparator<? super K> cmp);
        static <K, V> Comparator<Entry<K,V>> comparingByValue(Comparator<? super V> cmp);
    }

    // Factory methods (Java 9) - returns unmodifiable map
    static <K,V> Map<K,V> of(K k1, V v1, ...)           // up to 10 key-value pairs
    static <K,V> Map<K,V> ofEntries(Entry<K,V>... entries)
    static <K,V> Map<K,V> copyOf(Map<? extends K, ? extends V> map) // Java 10
    static <K,V> Map.Entry<K,V> entry(K k, V v)          // Java 9 - creates an Entry
}

Map<String, Integer> wordCount = new HashMap<>();

// computeIfAbsent: initialize if not present
wordCount.computeIfAbsent("hello", k -> 0);

// compute: increment counter (handles both absent and present)
wordCount.compute("hello", (k, v) -> (v == null) ? 1 : v + 1);

// merge: cleaner increment (value=1 is the default for new keys)
wordCount.merge("hello", 1, Integer::sum);

// Constructors
new HashMap<>()
new HashMap<>(int initialCapacity)
new HashMap<>(int initialCapacity, float loadFactor)
new HashMap<>(Map<? extends K, ? extends V> m)

// Constructors
new LinkedHashMap<>()
new LinkedHashMap<>(int initialCapacity)
new LinkedHashMap<>(int initialCapacity, float loadFactor)
new LinkedHashMap<>(Map<? extends K, ? extends V> m)
new LinkedHashMap<>(int initialCapacity, float loadFactor, boolean accessOrder)
    // accessOrder=true: iteration order is from least-recently-accessed to most-recently-accessed

// Additional method
protected boolean removeEldestEntry(Map.Entry<K,V> eldest);
    // Override to return true when eldest entry should be removed (used for caches)

int MAX = 3;
Map<Integer, String> lruCache = new LinkedHashMap<>(16, 0.75f, true) {
    protected boolean removeEldestEntry(Map.Entry<Integer, String> e) {
        return size() > MAX;
    }
};

// Constructors
new TreeMap<>()
new TreeMap<>(Comparator<? super K> comparator)
new TreeMap<>(Map<? extends K, ? extends V> m)
new TreeMap<>(SortedMap<K, ? extends V> m)

// All NavigableMap methods available - see Section 10

// Legacy-specific methods
Enumeration<K> keys();           // Legacy key iterator
Enumeration<V> elements();       // Legacy value iterator
boolean contains(Object value);  // Same as containsValue() - legacy method
int hashCode();

// Constructor - requires enum class to be specified
EnumMap<Day, String> schedule = new EnumMap<>(Day.class);
schedule.put(Day.MON, "Stand-up at 9am");

IdentityHashMap<Object, Boolean> visited = new IdentityHashMap<>();
String a = new String("hello");
String b = new String("hello");
visited.put(a, true);
System.out.println(visited.containsKey(b)); // false - different reference
System.out.println(visited.containsKey(a)); // true

WeakHashMap<Object, String> cache = new WeakHashMap<>();
Object key = new Object();
cache.put(key, "cached data");
key = null;           // key no longer strongly referenced
System.gc();          // entry may be removed from map after GC

public interface SortedMap<K, V> extends Map<K, V> {
    Comparator<? super K> comparator();                     // null if natural ordering

    // Sub-map views (backed by original map)
    SortedMap<K, V> subMap(K fromKey, K toKey);             // [fromKey, toKey)
    SortedMap<K, V> headMap(K toKey);                       // Keys strictly less than toKey
    SortedMap<K, V> tailMap(K fromKey);                     // Keys >= fromKey

    // First/last keys
    K firstKey();   // Throws NoSuchElementException if empty
    K lastKey();    // Throws NoSuchElementException if empty
}

public interface NavigableMap<K, V> extends SortedMap<K, V> {

    // Closest key/entry queries
    Map.Entry<K,V> lowerEntry(K key);    // Entry with greatest key strictly < key
    K lowerKey(K key);
    Map.Entry<K,V> floorEntry(K key);    // Entry with greatest key <= key
    K floorKey(K key);
    Map.Entry<K,V> ceilingEntry(K key);  // Entry with least key >= key
    K ceilingKey(K key);
    Map.Entry<K,V> higherEntry(K key);   // Entry with least key strictly > key
    K higherKey(K key);

    // Extremes
    Map.Entry<K,V> firstEntry();         // Lowest key entry; null if empty
    Map.Entry<K,V> lastEntry();          // Highest key entry; null if empty
    Map.Entry<K,V> pollFirstEntry();     // Removes and returns lowest key entry
    Map.Entry<K,V> pollLastEntry();      // Removes and returns highest key entry

    // Reverse order views
    NavigableMap<K,V> descendingMap();              // Reverse-order view
    NavigableSet<K> navigableKeySet();              // NavigableSet of keys
    NavigableSet<K> descendingKeySet();             // Keys in descending order

    // Controlled sub-map views
    NavigableMap<K,V> subMap(K from, boolean fromInclusive, K to, boolean toInclusive);
    NavigableMap<K,V> headMap(K toKey, boolean inclusive);
    NavigableMap<K,V> tailMap(K fromKey, boolean inclusive);
}

// Constructors
new ConcurrentHashMap<>()
new ConcurrentHashMap<>(int initialCapacity)
new ConcurrentHashMap<>(int initialCapacity, float loadFactor)
new ConcurrentHashMap<>(int initialCapacity, float loadFactor, int concurrencyLevel)
new ConcurrentHashMap<>(Map<? extends K, ? extends V> m)

// Atomic operations (all inherited from Map with atomic semantics)
putIfAbsent(key, value)
remove(key, value)
replace(key, oldValue, newValue)
replace(key, value)

// Bulk operations (Java 8) - run in parallel if parallelismThreshold met
void forEach(long parallelismThreshold, BiConsumer<? super K,? super V> action)
void forEachKey(long parallelismThreshold, Consumer<? super K> action)
void forEachValue(long parallelismThreshold, Consumer<? super V> action)
void forEachEntry(long parallelismThreshold, Consumer<? super Map.Entry<K,V>> action)

<U> U search(long parallelismThreshold, BiFunction<? super K,? super V,? extends U> searchFunction)
<U> U searchKeys(long parallelismThreshold, Function<? super K,? extends U> searchFunction)
<U> U searchValues(long parallelismThreshold, Function<? super V,? extends U> searchFunction)
<U> U searchEntries(long parallelismThreshold, Function<Map.Entry<K,V>,? extends U> searchFunction)

<U> U reduce(long parallelismThreshold, BiFunction<? super K,? super V,? extends U> transformer,
             BiFunction<? super U,? super U,? extends U> reducer)
long mappingCount()         // Returns the number of mappings (use instead of size() for large maps)

static <K> KeySetView<K,Boolean> newKeySet()  // Returns a Set backed by a ConcurrentHashMap

// All List methods work. Additional:
boolean addIfAbsent(E e);                           // Adds only if not present; returns true if added
int addAllAbsent(Collection<? extends E> c);        // Adds elements not already present; returns count added

// No additional methods beyond Queue

public interface BlockingQueue<E> extends Queue<E> {
    // Blocking insert
    void put(E e) throws InterruptedException;                     // Blocks until space available
    boolean offer(E e, long timeout, TimeUnit unit) throws InterruptedException; // Waits up to timeout

    // Blocking remove
    E take() throws InterruptedException;                          // Blocks until element available
    E poll(long timeout, TimeUnit unit) throws InterruptedException; // Waits up to timeout

    int remainingCapacity();  // Returns remaining space; Integer.MAX_VALUE if unbounded
    int drainTo(Collection<? super E> c);             // Removes all available elements into c
    int drainTo(Collection<? super E> c, int maxElements);
}

new ArrayBlockingQueue<>(int capacity)
new ArrayBlockingQueue<>(int capacity, boolean fair) // fair=true: FIFO access for blocked threads

new LinkedBlockingQueue<>()               // unbounded (Integer.MAX_VALUE capacity)
new LinkedBlockingQueue<>(int capacity)   // bounded

new PriorityBlockingQueue<>()
new PriorityBlockingQueue<>(int initialCapacity)
new PriorityBlockingQueue<>(int initialCapacity, Comparator<? super E> comparator)

void transfer(E e);                              // Blocks until a consumer takes the element
boolean tryTransfer(E e);                        // Returns false immediately if no consumer waiting
boolean tryTransfer(E e, long timeout, TimeUnit unit);
boolean hasWaitingConsumer();
int getWaitingConsumerCount();

// Sorting - uses merge sort, O(n log n), stable
static <T extends Comparable<? super T>> void sort(List<T> list)
static <T> void sort(List<T> list, Comparator<? super T> c)

// Binary search - list MUST be sorted first
// Returns index of key if found; otherwise (-(insertion point) - 1)
static <T> int binarySearch(List<? extends Comparable<? super T>> list, T key)
static <T> int binarySearch(List<? extends T> list, T key, Comparator<? super T> c)

// Reversing order
static void reverse(List<?> list)
static <T> Comparator<T> reverseOrder()                           // Returns natural order reversed
static <T> Comparator<T> reverseOrder(Comparator<T> cmp)         // Returns given comparator reversed

static <T extends Object & Comparable<? super T>> T min(Collection<? extends T> coll)
static <T> T min(Collection<? extends T> coll, Comparator<? super T> comp)
static <T extends Object & Comparable<? super T>> T max(Collection<? extends T> coll)
static <T> T max(Collection<? extends T> coll, Comparator<? super T> comp)

static int frequency(Collection<?> c, Object o)    // Number of occurrences of o in c
static boolean disjoint(Collection<?> c1, Collection<?> c2) // True if no common elements

static void shuffle(List<?> list)                           // Random shuffle
static void shuffle(List<?> list, Random rnd)               // Shuffle with given Random

static void swap(List<?> list, int i, int j)                // Swaps elements at i and j

static <T> void fill(List<? super T> list, T obj)           // Replaces all elements with obj

static <T> void copy(List<? super T> dest, List<? extends T> src)
    // Copies src into dest; dest must be at least as large as src

static <T> boolean replaceAll(List<T> list, T oldVal, T newVal) // Replaces all oldVal with newVal

static void rotate(List<?> list, int distance)
    // Rotates elements; element at index i moves to (i + distance) % size

static int indexOfSubList(List<?> source, List<?> target)   // First occurrence of target in source; -1 if none
static int lastIndexOfSubList(List<?> source, List<?> target)

static <T> List<T> nCopies(int n, T o)                      // Immutable list of n copies of o

static <T> Collection<T> unmodifiableCollection(Collection<? extends T> c)
static <T> List<T> unmodifiableList(List<? extends T> list)
static <T> Set<T> unmodifiableSet(Set<? extends T> s)
static <T> SortedSet<T> unmodifiableSortedSet(SortedSet<T> s)
static <T> NavigableSet<T> unmodifiableNavigableSet(NavigableSet<T> s)
static <K,V> Map<K,V> unmodifiableMap(Map<? extends K, ? extends V> m)
static <K,V> SortedMap<K,V> unmodifiableSortedMap(SortedMap<K, ? extends V> m)
static <K,V> NavigableMap<K,V> unmodifiableNavigableMap(NavigableMap<K, ? extends V> m)

static <T> Collection<T> synchronizedCollection(Collection<T> c)
static <T> List<T> synchronizedList(List<T> list)
static <T> Set<T> synchronizedSet(Set<T> s)
static <T> SortedSet<T> synchronizedSortedSet(SortedSet<T> s)
static <T> NavigableSet<T> synchronizedNavigableSet(NavigableSet<T> s)
static <K,V> Map<K,V> synchronizedMap(Map<K,V> m)
static <K,V> SortedMap<K,V> synchronizedSortedMap(SortedMap<K,V> m)
static <K,V> NavigableMap<K,V> synchronizedNavigableMap(NavigableMap<K,V> m)

List<String> syncList = Collections.synchronizedList(new ArrayList<>());
synchronized (syncList) {
    Iterator<String> it = syncList.iterator();
    while (it.hasNext()) { /* ... */ }
}

// Singleton (immutable, single-element)
static <T> Set<T> singleton(T o)
static <T> List<T> singletonList(T o)
static <K,V> Map<K,V> singletonMap(K key, V value)

// Empty (immutable, no elements)
static <T> List<T> emptyList()
static <T> Set<T> emptySet()
static <K,V> Map<K,V> emptyMap()
static <T> Iterator<T> emptyIterator()
static <T> ListIterator<T> emptyListIterator()
static <T> Enumeration<T> emptyEnumeration()

// Checked wrappers (throw ClassCastException at insertion if wrong type)
static <E> Collection<E> checkedCollection(Collection<E> c, Class<E> type)
static <E> List<E> checkedList(List<E> list, Class<E> type)
static <E> Set<E> checkedSet(Set<E> s, Class<E> type)
static <K,V> Map<K,V> checkedMap(Map<K,V> m, Class<K> keyType, Class<V> valueType)
// ... and variants for SortedSet, NavigableSet, SortedMap, NavigableMap, Queue

// Sorting (uses Dual-Pivot Quicksort for primitives; TimSort for objects)
static void sort(int[] a)
static void sort(int[] a, int fromIndex, int toIndex)
static <T> void sort(T[] a, Comparator<? super T> c)
static <T> void sort(T[] a, int fromIndex, int toIndex, Comparator<? super T> c)
static void parallelSort(int[] a)       // Java 8 - uses ForkJoinPool for large arrays
static <T extends Comparable<? super T>> void parallelSort(T[] a)

// Binary search (array must be sorted first)
static int binarySearch(int[] a, int key)
static int binarySearch(int[] a, int fromIndex, int toIndex, int key)
static <T> int binarySearch(T[] a, T key, Comparator<? super T> c)

// Copying
static int[] copyOf(int[] original, int newLength)          // truncates or pads with 0/null
static int[] copyOfRange(int[] original, int from, int to)  // [from, to)

// Filling
static void fill(int[] a, int val)
static void fill(int[] a, int fromIndex, int toIndex, int val)
static <T> void parallelSetAll(T[] array, IntFunction<? extends T> generator) // Java 8
static <T> void setAll(T[] array, IntFunction<? extends T> generator)         // Java 8

// Comparison
static boolean equals(int[] a, int[] a2)                    // Element-wise equality
static boolean deepEquals(Object[] a1, Object[] a2)         // Recursive equality for nested arrays
static int compare(int[] a, int[] b)                        // Java 9 - lexicographic comparison
static int mismatch(int[] a, int[] b)                       // Java 9 - index of first mismatch; -1 if equal

// String representation
static String toString(int[] a)                             // "[1, 2, 3]"
static String deepToString(Object[] a)                      // Works for multi-dimensional arrays

// Hash codes
static int hashCode(int[] a)
static int deepHashCode(Object[] a)

// Conversion
static <T> List<T> asList(T... a)
    // Returns a FIXED-SIZE List backed by the array.
    // set() works; add() and remove() throw UnsupportedOperationException.
    // Changes to the list write through to the array.

// Streams (Java 8)
static IntStream stream(int[] array)
static IntStream stream(int[] array, int startInclusive, int endExclusive)
static <T> Stream<T> stream(T[] array)
static <T> Stream<T> stream(T[] array, int startInclusive, int endExclusive)

// Parallel prefix (Java 8) - cumulative operation in-place
static void parallelPrefix(int[] array, IntBinaryOperator op) // e.g., cumulative sum

String[] arr = {"A", "B", "C"};

List<String> fixed = Arrays.asList(arr);
// fixed.add("D");    // UnsupportedOperationException
fixed.set(0, "Z");    // Works - reflected back in arr

List<String> mutable = new ArrayList<>(Arrays.asList(arr));
mutable.add("D");     // Works - independent copy

public interface Comparable<T> {
    int compareTo(T o);
    // Returns: negative if this < o
    //          zero     if this == o
    //          positive if this > o
}

public class Student implements Comparable<Student> {
    String name;
    int gpa;

    @Override
    public int compareTo(Student other) {
        return Integer.compare(this.gpa, other.gpa); // ascending by GPA
    }
}
// Now TreeSet<Student>, Collections.sort(list) work without explicit Comparator

public interface Comparator<T> {
    int compare(T o1, T o2);        // negative if o1 < o2, zero if equal, positive if o1 > o2
    boolean equals(Object obj);     // not commonly overridden

    // Default methods for chaining (Java 8)
    default Comparator<T> reversed();
    default Comparator<T> thenComparing(Comparator<? super T> other);    // secondary comparator
    default <U extends Comparable<? super U>> Comparator<T> thenComparing(Function<? super T, ? extends U> keyExtractor);
    default Comparator<T> thenComparingInt(ToIntFunction<? super T> keyExtractor);
    default Comparator<T> thenComparingLong(ToLongFunction<? super T> keyExtractor);
    default Comparator<T> thenComparingDouble(ToDoubleFunction<? super T> keyExtractor);

    // Static factory methods (Java 8)
    static <T, U extends Comparable<? super U>> Comparator<T>
        comparing(Function<? super T, ? extends U> keyExtractor);        // compare by extracted key
    static <T, U> Comparator<T>
        comparing(Function<? super T, ? extends U> keyExtractor, Comparator<? super U> keyComparator);
    static <T> Comparator<T> comparingInt(ToIntFunction<? super T> keyExtractor);
    static <T> Comparator<T> comparingLong(ToLongFunction<? super T> keyExtractor);
    static <T> Comparator<T> comparingDouble(ToDoubleFunction<? super T> keyExtractor);
    static <T extends Comparable<? super T>> Comparator<T> naturalOrder();
    static <T extends Comparable<? super T>> Comparator<T> reverseOrder();
    static <T> Comparator<T> nullsFirst(Comparator<? super T> comparator); // nulls sort first
    static <T> Comparator<T> nullsLast(Comparator<? super T> comparator);  // nulls sort last
}

List<Student> students = ...;
students.sort(
    Comparator.comparingDouble(Student::getGpa)
              .reversed()
              .thenComparing(Student::getName)
              .thenComparingInt(Student::getAge)
);

Need key-value mapping?
    Yes --> Map
        Need sorted keys?          --> TreeMap
        Need insertion order?      --> LinkedHashMap
        Need thread safety?        --> ConcurrentHashMap
        General purpose?           --> HashMap

    No --> Collection
        Allow duplicates?
            No --> Set
                Need sorted order? --> TreeSet
                Need insertion order? --> LinkedHashSet
                General purpose?   --> HashSet

            Yes --> Need ordering?
                FIFO queue?        --> ArrayDeque (or LinkedBlockingQueue if concurrent)
                Priority order?    --> PriorityQueue
                Stack (LIFO)?      --> ArrayDeque (push/pop)
                Indexed access?    --> ArrayList
                Frequent insert/remove at ends? --> ArrayDeque or LinkedList