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Java Programming
from the Grounds Up

Packaging up your code

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
Advanced Java Classes
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.

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Mark Reynolds is a network protocol designer, Java programmer, computer animator, and fanatic mountaineer. He currently consults to Adaptive Optics Associates, a United Technologies company.

Reprinted from Web Developer® magazine, Vol. 2 No.1 Spring 1996 (c) 1996 internet.com Corporation. All rights reserved.

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