Collection Frameworks Cheatsheet

Interface and Class Hierarchy

I = Interface AC = Abstract Class Underlying Data Structure mentioned beside the classes.

💡 Every Collection class is based on some standard data structure.

In order of hierarchy:

From ArrayList

	data.structure.primary.array (Private)	ArrayList
Definition	An array is a dynamically-created object. It serves as a container that holds the constant number of values of the same type. It has a contiguous memory location.	The ArrayList is a class of Java Collections framework.
Size	Static - fixed-length data structure	Dynamic - variable length data structure - can resize itself when needed
Size specification	Mandatory to provide size while initializing	Size can be left unspecified - default initial capacity will be used
Performance	Fixed size => Faster	Internally backed by array. Resize operation slows down the performance
Primitive/generic type	Can store both primitives and objects	Can't store primitives - automatically converts them to objects. For example, int is converted to Integer.
Getting the length	arr.length	arr.size()
Adding elements	Can add elements using assignment operator	add() function
Multiple dimensions	Can be multi-dimensional	CAN'T be multi-dimensional

	HashMap	Hashtable
Introduced in	1.2	1.0
Thread Safety	Not thread safe (not synchronized) => works with single thread	Thread safe (synchronized) => works with multiple threads
Speed	Faster	Slower
Null key	One null key allowed	Null key not allowed

Vector is thread-safe (most methods are synchronised) ArrayList is not.

💡 If something is thread-safe, it will inevitably have a lower performance in comparison to a corresponding non-synchronised something.