Collection Framework

3 minute read 

Collection (Interface)
├── List (Interface)
│   ├── ArrayList
│   ├── LinkedList  ← also implements Deque
│   ├── Vector
│   │   └── Stack
│
├── Set (Interface)
│   ├── HashSet
│   │   └── LinkedHashSet
│   ├── TreeSet  ← implements NavigableSet
│   └── ConcurrentSkipListSet  ← implements NavigableSet
│
├── Queue (Interface)
│   ├── PriorityQueue
│   ├── BlockingQueue (Interface)
│   │   ├── LinkedBlockingQueue
│   │   └── LinkedBlockingDeque  ← also implements Deque
│   └── Deque (Interface)
│       ├── ArrayDeque
│       └── LinkedList  ← also implements List
│
Map (Interface)  ← Not a subtype of Collection
├── HashMap
│   └── LinkedHashMap
├── TreeMap  ← implements NavigableMap
├── WeakHashMap
├── ConcurrentHashMap
└── ConcurrentSkipListMap  ← implements NavigableMap

NavigableMap (Interface)
├── TreeMap
└── ConcurrentSkipListMap

NavigableSet (Interface)
├── TreeSet
└── ConcurrentSkipListSet

Collection Interface (core methods)

  • boolean add(E e)
  • boolean contains(Object o) — Returns true if this collection contains the specified element.
  • Iterator<E> iterator() — Returns an iterator over the elements in this collection.
  • boolean remove(Object o) — Removes a single instance of the specified element.
  • int size() — Returns the number of elements in this collection.
  • Object[] toArray() — Returns an array containing all of the elements in this collection.

List

ArrayList

  • boolean add(E e) — Appends the specified element to the end of this list.
  • void add(int index, E element) — Inserts at the specified position; shifts subsequent elements right.
  • int indexOf(Object o) — Index of the first occurrence, or -1 if not present.
  • E get(int index) — Returns the element at the specified position.
  • ListIterator<E> listIterator()
  • E set(int index, E element) — Replaces at position; returns previous element.
  • E remove(int index) — Removes at position; shifts subsequent elements left.
  • boolean remove(Object o) — Removes the first occurrence if present.

LinkedList

  • boolean add(E e) — Appends to the end of this list.
  • void add(int index, E element) — Inserts at the specified position.
  • void addFirst(E e) — Inserts at the beginning.
  • void addLast(E e) — Appends at the end.
  • E element() — Retrieves, but does not remove, the head (first element).
  • Iterator<E> descendingIterator() — Iterates in reverse order.
  • E getFirst() — Returns the first element.
  • E getLast() — Returns the last element.
  • boolean offer(E e) — Adds the specified element as the tail (last element).
  • E peek() — Retrieves, but does not remove, the head.
  • E poll() — Retrieves and removes the head.

Vector / Stack

  • boolean empty() — Tests if this stack is empty.
  • E peek() — Looks at the top element without removing it.
  • E pop() — Removes the top element and returns it.
  • E push(E item) — Pushes an item onto the stack.
  • int search(Object o) — Returns the 1-based position where an object is on this stack.

Set

HashSet / LinkedHashSet

  • Same as Collection interface methods.

TreeSet

  • E first() — Returns the first (lowest) element in this set.
  • E last() — Returns the last (highest) element in this set.

Queue

  • boolean add(E e) — Inserts the element, throwing IllegalStateException if no space.
  • boolean offer(E e) — Inserts the element; preferred over add when capacity-restricted.
  • E element() — Retrieves, but does not remove, the head.
  • E peek() — Retrieves, but does not remove, the head (or null if empty).
  • E poll() — Retrieves and removes the head (or null if empty).
  • E remove() — Retrieves and removes the head.

PriorityQueue (heap representation in Java)

  • boolean offer(E e) — Inserts the specified element.
  • E peek() — Retrieves, but does not remove, the head (or null if empty).
  • E poll() — Retrieves and removes the head (or null if empty).

BlockingQueue

  • add() internally just calls offer() and does nothing extra.

Map

  • boolean containsKey(Object key) — Returns true if this map contains a mapping for the specified key.
  • boolean containsValue(Object value) — Returns true if this map maps one or more keys to the specified value.
  • boolean isEmpty() — Returns true if this map contains no key-value mappings.
  • int size() — Returns the number of key-value mappings in this map.
  • Set<K> keySet() — Returns a Set view of the keys contained in this map.
  • V put(K key, V value) — Associates the specified value with the specified key.
  • V remove(Object key) — Removes the mapping for a key if present.

HashMap

  • boolean isEmpty() — Returns true if this map contains no key-value mappings.

TreeMap

  • Inherits core Map methods plus NavigableMap operations.

Collection Interface

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