Collections Framework

The Java Collections Framework is a unified architecture for representing and manipulating collections, which are groups of objects. It provides a set of interfaces and classes to help programmers manage data more effectively. At the top of the hierarchy is the `Collection` interface, which defines the basic operations for all collections, such as adding, removing, and querying elements. From `Collection`, two main interfaces are derived: `List` and `Set`. The `List` interface represents an ordered collection, also known as a sequence. Lists allow duplicate elements and provide control over where each element is inserted. Elements can be accessed by their integer index. Common implementations include `ArrayList` (a dynamic array) and `LinkedList` (a doubly-linked list). The `Set` interface represents a collection that cannot contain duplicate elements. It models the mathematical set abstraction. Sets are unordered by default. Common implementations are `HashSet`, which stores elements in a hash table for fast access, and `TreeSet`, which stores elements in a sorted order. Separate from the `Collection` hierarchy is the `Map` interface. A `Map` is an object that maps keys to values, and it cannot contain duplicate keys. Each key can map to at most one value. It's useful for key-value association data, like a dictionary. The primary implementations are `HashMap`, which provides unsorted, fast access, and `TreeMap`, which keeps the keys in sorted order. Understanding these core interfaces is the first step to effectively using Java's powerful data structure library.