Java in a Nutshell, 5th Edition

Collections

The Java Collections Framework is a set of important utility classes and interfaces in the java.util package for working with collections of objects. The Collections Framework defines two fundamental types of collections. A Collection is a group of objects while a Map is a set of mappings, or associations, between objects. A Set is a type of Collection with no duplicates, and a List is a Collection in which the elements are ordered. SortedSet and SortedMap are specialized sets and maps that maintain their elements in a sorted order. Collection , Set, List, Map, SortedSet, and SortedMap are all interfaces, but the java.util package also defines various concrete implementations, such as lists based on arrays and linked lists, and maps and sets based on hashtables or binary trees. Other important interfaces are Iterator and ListIterator, which allow you to loop through the objects in a collection. The Collections Framework was added in Java 1.2, but prior to that release you can use Vector and Hashtable, which are approximately the same as ArrayList and HashMap.

In Java 1.4, the Collections API added the RandomAccess marker interface, which is implemented by List implementations that support efficient random access (i.e., it is implemented by ArrayList and Vector but not by LinkedList). Java 1.4 also introduced LinkedHashMap and LinkedHashSet, which are hashtable-based maps and sets that preserve the insertion order of elements. Finally, IdentityHashMap ...