Set

• Set Implementations on Oracle Tutorials

Three general-purpose implementations of the Set interface:

• HashSet
• TreeSet
• LinkedHashSet

Use HashSet unless you need the elements to be in a certain fixed order.

List

• List interface

Tutorial on List implementations.

General-purpose List implementations:

• ArrayList
• LinkedList

Map

Note that Map does not extend Collection like all the other interafaces in this document.

• Map interface

Tutorial on Map implementations

General-purpose Map implementations:

• HashMap
• TreeMap
• LinkedHashMap

HashMap vs. Hashtable

HashMap is a member of the Java Collections Framework.

Hashtable is an older legacy class that has been retrofitted to fit in the Java Collections Framework.

The differences between HashMap and Hashtable are:

• HashMap permits a null key and null values, whereas Hashtable permits only non-null keys and values.
• HashMap is not syncrhonised, whereas Hashtable is synchronised (that is, thread-safe).

Hashtable can be regarded as obsolete. If you need a synchronised HashMap, it’s better to use ConcurrentHashMap than Hashtable (see here).

Queue

Queue interface.

General-purpose implementations of Queue:

• PriorityQueue
• LinkedList

Deque

Stands for double-ended queue.

Deque interface.

General-purpose implementations:

• ArrayDeque
• LinkedList

The Java Collections Framework

• Guide on Oracle
• All the interfaces and classes that are part of the Collections Framework are listed here

General-purpose Implementations

Interfaces	Hash table Implementations	Resizable array Implementations	Tree Implementations	Linked list Implementations	Hash table + Linked list Implementations
Set	HashSet		TreeSet		LinkedHashSet
List		ArrayList		LinkedList
Queue		PriorityQueue		LinkedList
Deque		ArrayDeque		LinkedList
Map	HashMap		TreeMap		LinkedHashMap

All the above interfaces (except Map) implement the Collection interface.

The Collections class contains static methods that operate on objects implementing the above interfaces.