The Collections Framework
See the Sun Collections Framework overview,
which includes an annotated outline and a tutorial
path. See also the CFX
examples. See also the MageLang Institute tutorial
on the Collections Framework on Sun's site, to which there are links from
the notes below.
Background
Set theory gives us the mathematical concepts of a set being a unique collection
of items, with no duplicates. This is often illustrated in a Venn diagram
(taken from Sun's site) as:
where we can see intersections and unions of sets. Examples
of sets are: the set of reserved words in Java, the set of nonnegative prime
numbers. Sets contain only one instance of an item, sets may be finite or
infinite, and sets can define abstract concepts.
A map is a special kind of set. It is a set of pairs
with each pair representing a one-directional "mapping" from one
element to another. Examples of maps are: map of IP addresses to domain names,
map of keys to database records, map of conversions from base 2 to base 10.
Collection Interfaces and Classes
The Collections framework is made up of a set of Java interfaces
for working with groups of objects. The following image map diagram shows
the hierarchy. Click inside the diagram on the class name for the API/tutorial
documentation, or on the text inside the box for explanation and examples.
You may also select the CFX text in the box for links to examples.
CFX also has Iterators
and common algorithms.
You can also browse
the directory of CFX examples. (All CFX shortcut links in the above paragraph currently don't work correctly in FireFox browsers.)
Note also the Array
class, which is part of the Collections Framework, giving arrays with
built-in sorting and searching.
See the lecture notes describing sequence of exploring the above diagram.
You may also work through some practical examples involving storing Student test scores, using the above classes, with corresponding examples lecture notes.
Methods You Should Always Have
To easily use the Collections Framework, your classes should properly implement the following methods:
- equals()
Without it Set collections can't eliminate duplicates.
- hashCode()
Without it SortedSet and SortedMap can't map into the hashtable, and thus can't eliminate duplicates since hashCode() is called to find position in which equals() is then used.
- toString()
Without it we can't meaningfully print out an object using System.out.println( theObject).
- compareTo(), implementing the Comparable interface
Without it we can't sort objects.
See this pdf document (username:password required) describing these methods.