The Java Class Library

One of the great strengths of Java is its large and powerful class library.

You can get an idea of the size and scope of the Java class library by going to the “javadoc” pages for the appropriate version of the JDK.

For example, click on this link: https://docs.oracle.com/javase/1.5.0/docs/api/ to go to the javadoc pages that describe the Java API for Java 5 (a.k.a. 1.5). (Of course, there are many later versions of Java than this.)

Browse around a little to see how much there is!

Of course, with any such rich and powerful class library, you have to spend a lot of time learning what is there, what to use for what you need, and how to use it.  But don’t worry!  You don’t have to know everything in the class library in order to use it.  (I know I don’t!)

Overview

What is the Java Class Library?

Java comes with a very large and useful library of classes and interfaces.

The Java class library supports a large number of capabilities, too numerous to mention at this point.

The Organization of the Library

The following gives an idea of the organization of the java class library :

java
  applet	  // applet support
  awt		  // Abstract Windowing Toolkit (AWT)
    datatransfer    // Generic inter-app data transfer
    event	    // Event handling
    image	    // Image processing support
    peer	    // GUI component peer interfaces
  beans		  // Java Beans (reusable components)
  io		  // I/O support
  lang		  // Java language support
    reflect	    // Java Reflection API
  math		  // Math support
  net		  // Network support
  rmi		  // Remote Method Invocation (RMI)
    dgc		    // Distributed Garbage Collection
    registry	    // Remote object registration
    server	    // Remote object creation
  security	  // Cryptographic security
    acl		    // Access Control List support
    interfaces	    // Java Security API interfaces
  sql		  // Java Database Connectivity (JDBC)
  text		  // Internationalization support
  util		  // "Utility" support (central)
    zip		    // Data [de]compression support

Note: Each version of Java adds considerably to the Java class library, so it is impractical to keep the above up to date.  If you wish to learn what is available in any given version of the Java class library, you should use your web browser to go to https://www.oracle.com/java/technologies/ and browse around, seeing how Java gets into lots of areas.

Class java.lang.Object

As the textbook says, the Object class is the “Cosmic Superclass” for Java.

Every class in Java ultimately extends from the Object class.  It is the top of the Java inheritance hierarchy, which means that there is only a single inheritance tree in Java.  (In C++, this is not true;  there are as many inheritance trees as you wish to invent — you get a forest, not a single tree.)

To get a good feel for what is in the Object class, click on this link: https://docs.oracle.com/javase/1.5.0/docs/api/java/lang/Object.html , which will show you the Object class for Java 5/1.5.  Browse over it, to see what it supplies.

Here, we will talk about some of the features it provides.

java.lang Classes

Here is a quick synopsis of some of the classes you’ll find in the java.lang package:

Object

The ultimate root class of all Java classes

Class

A class that describes a Java class.

Wrapper Classes

These are:

  • Boolean
  • Character
  • Number
    • Byte (JDK 1.1)
    • Short (JDK 1.1)
    • Integer
    • Long
    • Float
    • Double

These classes provide immutable wrappers for the primitive data types, when you wish to manipulate such types as objects — especially to allow for their use in polymorphism.

Number is an abstract class, from which the numeric wrappers all derive. All the other classes are final classes.

String, StringBuffer

Objects that represent strings.

String is an immutable type.

StringBuffer allows its string to be changed in place.

Runtime

Contains a number of low-level methods associated with the Java run-time system.

System

Contains a number of low-level methods associated with the system.

All its methods are class methods; it may not be instantiated.

Math

Provides support for the mathematical values e and Π (pi), and defines static methods for floating point trigonometry, exponentiation, minimum, maximum, and pseudo-random number generation.

All its methods are class methods; it may not be instantiated.

Thread

Provides support for multiple threads of control running within the same Java interpreter.

Process

Defines a platform-independent interface to platform-dependent processes running externally to the Java interpreter.

Throwable, Exception, Error, and Specific Exception Classes

Defines a set of objects that may be thrown as exceptions.

java.util Classes

The package java.util, as its name implies, provides a number of useful classes.

(Note: It should not be considered a “utility” package separate from the rest of the language. It is part and parcel of the library, and Java depends heavily on several of the classes in this package.)

The Old Collections Classes

HashTable and Properties

HashTable implements a hashtable or associative array. It allows abritrary objects to be stored and retrieved by arbitrary keys.

Properties, a subclass of HashTable, is used to store the Java system properties list.

Vector

Implements an array of objects that grows as needed when objects are added.

Stack

Implements a LIFO stack on which objects may be pushed and from which they may be popped.

Enumeration

Interface which provides a simple and consistent way to loop through all the elements contained within some kind of object or data structure.

The Collections Framework

Many interfaces and classes used to implement the powerful Java collections framework.  We discussed these classes in some detail, earlier.

Date, Calendar, and TimeZone

The Date class represents a date and time, using a millisecond representation.

The Calendar class manipulates dates using years, months, days, hours, minutes, etc.

The TimeZone class is used in conjunction with dates.

ResourceBundle, ListResourceBundle, PropertyResourceBundle

Represent a “bundle” of localized resources that are read in by an internationalized Java program at run-time.

BitSet

Implements an arbitrary-size array of bits.

Random

Generates and returns pseudo-random numbers in a variety of forms.

StringTokenizer

Parses a string into tokens.

Observer and Observable

Provide infrastructure for implementing the object-oriented model-view paradigm in Java.

java.math Classes

The java.math package contains classes for arbitrary-precision integer and floating-point arithmetic.

BigDecimal

Represents a floating-point number of arbitrary size and precision.

Its methods duplicate the functionality of the standard Java arithmetic operators.

BigDecimal is a subclass of java.lang.Number.

BigDecimal values can be useful for applications (such as financial applications) where rounding errors are problematic.

BigInteger

Represents integers that can be arbitrarily large.

It defines methods which duplicate the functionality of the standard Java arithmetic and bit-manipulation operators.

BigInteger is a subclass of java.lang.Number.

Arbitrary-length integers are used in cryptography, and BigInteger contains many methods designed for cryptographic applications.