1.3 The Java Base System

The complete Java system includes a number of libraries of utility classes and
methods of use to developers in creating multi-platform applications.

Very briefly, these libraries are:

java.lang—the collection of base types (language types) that are always
imported into any given compilation unit. This where you’ll find the
declarations of Object (the root of the class hierarchy) and Class, plus
threads, exceptions, wrappers for the primitive data types, and a variety of
other fundamental classes.

java.io—streams and random-access files. This is where you find the rough
equivalent of the Standard I/O Library you’re familiar with on most UNIX
systems. A further library is called java.net, and provides support for
sockets, telnet interfaces, and URLs.

java.util—container and utility classes. Here you’ll find classes such as
Dictionary, HashTable, and Stack, among others, plus encoder and
decoder techniques, and Date and Time classes.

java.awt—an Abstract Windowing Toolkit that provides an abstract layer
enabling you to port Java applications easily from one window system to
another. This library contains classes for basic interface components such as
events, colors, fonts, and controls such as buttons and scrollbars.