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Chapter 4. The Java Platform


Java Platform Overview
Strings and Characters
Numbers and Math
Dates and Times
Types, Reflection, and Dynamic Loading
Files and Directories
Input and Output Streams

Chapter 2, "Java Syntax from the Ground Up", and Chapter 3, "Object-Oriented Programming in Java", documented the Java programming language. This chapter switches gears and covers the Java platform, which is the vast collection of predefined classes available to every Java program, regardless of the underlying host system on which it is running. The classes of the Java platform are collected into related groups, known as packages. This chapter begins with an overview of the packages of the Java platform that are documented in this book. It then moves on to demonstrate, in the form of short examples, the most useful classes in these packages.

4.1. Java Platform Overview

Table 4-1 summarizes the key packages of the Java platform that are covered in this book.

Table 4-1. Key Packages of the Java Platform


The JavaBeans component model for reusable, embeddable software components.


Additional classes that define bean context objects that hold and provide services to the JavaBeans objects they contain.

Classes and interfaces for input and output. Although some of the classes in this package are for working directly with files, most are for working with streams of bytes or characters.


The core classes of the language, such as String, Math, System, Thread, and Exception.


Classes that define weak references to objects. A weak reference is one that does not prevent the referent object from being garbage-collected.


Classes and interfaces that allow Java programs to reflect on themselves by examining the constructors, methods, and fields of classes.


A small package that contains classes for arbitrary-precision integer and floating-point arithmetic.

Classes and interfaces for networking with other systems.

Classes and interfaces for access control and authentication. Supports cryptographic message digests and digital signatures.

A package that supports access control lists. Deprecated and unused as of Java 1.2.

Classes and interfaces for working with public key certificates.

Interfaces used with DSA and RSA public-key encryption.

Classes and interfaces for transparent representations of keys and parameters used in public-key cryptography.


Classes and interfaces for working with text in internationalized applications.


Various utility classes, including the powerful collections framework for working with collections of objects.


Classes for reading and writing JAR files.

Classes for reading and writing ZIP files.


Classes and interfaces for encryption and decryption of data.


Interfaces that represent the Diffie-Hellman public/private keys used in the Diffie-Hellman key agreement protocol.


Classes that define transparent representations of keys and parameters used in cryptography.

Table 4-1 does not list all the packages in the Java platform, only those documented in this book. Java also defines numerous packages for graphics and graphical user interface programming and for distributed, or enterprise, computing. The graphics and GUI packages are java.awt and javax.swing and their many subpackages. These packages, along with the java.applet package, are documented in Java Foundation Classes in a Nutshell (O'Reilly). The enterprise packages of Java include java.rmi, java.sql, javax.jndi, org.omg.CORBA, org.omg.CosNaming, and all of their subpackages. These packages, as well as several standard extensions to the Java platform, are documented in the book Java Enterprise in a Nutshell (O'Reilly).

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