arraylist

ArrayList is a resizable-array implementation of the List interface in Java. It is part of the java.util package and provides a dynamic array that can grow as elements are added. ArrayList implements the List interface and is commonly used for storing an ordered collection of elements with fast random access by index.

Internally, an ArrayList uses an Object[] array to hold its elements. When the array becomes full, the list grows by creating a larger array and copying the elements over. This growth is typically by about 50 percent, yielding amortized constant time for adding elements at the end. The class supports null elements and is not synchronized by default, making itunsuited for concurrent access without external synchronization.

Constructors include a no-argument constructor, a constructor that specifies an initial capacity, and a constructor that creates a list containing the elements of a given collection. The list offers standard operations such as add, add at a specific index, get, set, remove both by index and by element, and bulk operations like addAll and removeAll. Other common methods include size, isEmpty, contains, indexOf, lastIndexOf, toArray, clear, and trimToSize.

Performance characteristics center on fast random access (O(1) time for get and set) and efficient appends at the end (amortized O(1)). Inserting or removing elements at arbitrary positions requires shifting subsequent elements, resulting in O(n) time in the worst case. Because it stores elements as references, using ArrayList with primitive types incurs boxing overhead unless used with specialized wrappers or streams.

ArrayList is often preferred when frequent index-based access is needed and insertions/removals are mostly at the end. For frequent insertions in the middle or memory-constrained scenarios, alternatives such as LinkedList or Vector (for synchronized contexts) may be considered.