java.util

The Java Collections Framework has a data structure that should work for virtually anything you'll ever need to do. Want to keep a list that you can easily keep adding to? Want to find something by name? Want to create a list that automatically takes out all the duplicates? Sort your co-workers by the number of times they've stabbed you in the back? Sort your pets by number of tricks learned? It’s all here...

TreeSet

Keeps the elements sorted and prevents duplicates.

HashMap

Let's you store and access elements as name/value pairs.

LinkedList

Designed to give better performance when you insert or delete elements from the middle of the collection. (In practice, an ArrayList is still usually what you want.)

HashSet

Prevents duplicates in the collection, and given an element, can find that element in the collection quickly.

LinkedHashMap

Like a regular HashMap, except it can remember the order in which elements (name/value pairs) were Inserted, or it can be configured to remember the order in which elements were last

accessed.

The java.util package contains classes and interfaces that are useful for working with groups of objects.

The package also contains classes that are used to develop applications to manipulate date, time, and generating random numbers and so on.

It also contains a list of classes and interfaces to manage a collection of data in memory. 

               Collections Framework
               Collection Interface
               List Interface
               ArrayList class
               Vector class
               LinkedList class
               Set Interface
               HashSet class
               TreeSet class
               Map Interface
               HashTable class
               HashMap class
               TreeMap class
               Stack class