Collections Framework

The Java Collections Framework (JCF) is a unified architecture for representing and manipulating groups of objects.

Iterable<E> is the root interface of the Java Collections hierarchy.

Collection<E> is the root interface of the main collection hierarchy, extending Iterable<E>.

ArrayList is a resizable-array implementation of the List interface.

LinkedList is a doubly-linked list implementation of both the List and Deque interfaces.

Vector is a legacy resizable-array implementation of the List interface that predates the Collections Framework.

Stack is a legacy class that extends Vector and represents a last-in, first-out (LIFO) stack of objects.

The Queue interface in java.util represents a collection designed for holding elements prior to processing, following a defined ordering policy.

PriorityQueue is an unbounded queue implementation backed by a binary heap that orders elements by priority rather than insertion order.

Deque (pronounced 'deck') stands for double-ended queue — a linear collection that supports element insertion and removal at both ends.

ArrayDeque is the recommended general-purpose Deque implementation in Java.

HashSet is the most commonly used Set implementation in Java.

LinkedHashSet is a Set implementation that combines the O(1) performance of a hash table with the predictable iteration order of a doubly-linked list.

TreeSet is a NavigableSet implementation backed by a red-black tree that stores elements in sorted order.

SortedSet is an interface that extends Set and adds the contract that all elements are maintained in ascending order, determined either by their natural ordering…

NavigableSet extends SortedSet with a rich set of navigation methods that go beyond the simple range-view operations of SortedSet.

HashMap is the most widely used Map implementation in Java.

LinkedHashMap extends HashMap and maintains a doubly-linked list running through all its entries, preserving the order in which entries were inserted.

TreeMap is a Red-Black tree implementation of the NavigableMap interface.

Hashtable is a legacy hash table implementation that predates the Collections Framework.

ConcurrentHashMap is a thread-safe, high-performance hash map optimised for concurrent access.

WeakHashMap is a hash map implementation where the keys are held with weak references.

IdentityHashMap is a Map implementation that uses reference equality (==) instead of object equality (equals()) to compare keys.

EnumMap is a specialised Map implementation for use with enum keys.

The Collections class in java.util is a utility class consisting entirely of static methods that operate on or return collections.

Comparable is a functional interface in java.lang that defines a natural ordering for a class.

Comparator is a functional interface in java.util that defines an ordering for objects that is separate from the objects themselves.

Iterator is the fundamental traversal interface in the Java Collections Framework.

ListIterator extends Iterator to provide bidirectional traversal of List elements, along with the ability to add, set, and remove elements during iteration.

Spliterator (Splittable Iterator) is Java 8's mechanism for parallel and sequential element traversal of sources such as collections, arrays, I/O channels, and…

Fail-fast and fail-safe are two competing philosophies for how Java iterators and collections respond to structural modifications made to a collection while it is being…

Immutable collections in Java are collections that cannot be structurally modified after creation — no elements can be added, removed, or replaced.

Synchronized collections wrap standard Java collections to make every individual method call thread-safe via an intrinsic monitor lock.