Collections Framework Overview

The Java Collections Framework is a unified architecture of interfaces, implementations, and algorithms for storing, retrieving, and manipulating groups of objects in a type-safe, resizable way.

Collections (with 's') is a utility class containing static methods for sorting, searching, reordering, wrapping (unmodifiable, synchronized, checked), and creating special-purpose collections.

Use List when you need ordered, indexed data that may contain duplicates; use Set when each element must be unique and you primarily need fast membership testing.

toArray() returns Object[] that cannot be cast to a typed array; toArray(new String[0]) returns String[]; toArray(String[]::new) (Java 11) is the cleanest form.