Java Programming ... From the Grounds Up
Part 4

by Mark C. Reynolds

Packaging up your code

Advanced Java Classes

The collection of Java classes is structured as a tree, in which each node has exactly one parent. Since Java classes can only inherit from one class at a time, it is said to support a single-class inheritance hierarchy.

FIGURE 2
Important components of the Java hierarchy.

java.lang
   Boolean	Character	Integer
   Long		Float		Double
   String	Class		Math
   Number	Object		Process
   System	Thread

java.awt
   Button	Canvas		Checkbox
   Choice	Color		Dimension
   Event	FileDialog	Font
   Graphics	Image		Insets
   Label	List		Menu
   MenuBar	MenuItem	Panel
   Point	Polygon		Rectangle
   Scrollbar	TextArea	TextField
   Window

java.io
   DataInput	    DataInputStream      InputStream
   DataOutput	    DataOutputStream     OutputStream
   File		    FileInputStream      FileOutputStream
   FilenameFilter   FilterInputStream    FilterOutputStream
   PrintStream	    RandomAccessFile     StreamTokenizer

java.net
   ContentHandler  InetAddress		ServerSocket
   URL		   URLConnection	URLStreamHandler

java.applet
   Applet          AudioClip

java.util
   BitSet	Date 		Dictionary
   Enumeration	Hashtable	Properties
   Random	Stack		StringTokenizer
   Vector

The top of the Java class hierarchy is organized into a set of related classes, called packages. The main Java packages are java.lang, java.net, java.io, java.awt, java.util, and java.applet. The java.lang package implements the basic components of the Java language, including string manipulation, mathematical functions, and conversion operations such as the simple string-to-integer one seen in Primes.java. The java.net package contains the network access functions, including those associated with opening a URL and getting its contents. The java.io package handles all kinds of input and output, implementing various forms of streaming (much like stdin and stdout in C or cin and cout in C++). Graphics are handled by java.awt. "AWT" is said to stand for "Another Window Toolkit" or "Advanced Window Toolkit," depending on who you ask. The AWT implements standard graphic components such as menus, panels, buttons, and checkboxes. The AWT also implements a number of sophisticated image manipulation functions within the java.awt.image subpackage. If you have ever programmed Windows or Macintosh graphics, used X or a higher level toolkit such as Tk, then AWT will seem very familiar. The java.util package has additional utility functions for string, vector and stack manipulation, hashing, date conversion, and a number of other goodies. Finally, java.applet is a very special package that actually creates a subclass of Applet whenever a new applet is produced.