jclass2

SCOUG Java Programming Class 2
Outline

Class 1 Review
Class1 Samples
Execution Environment
Program Types (applet, application)
(break)
Base Packages (lang, util, applet)
(questions, discussion)
Reference
Sample Programs

Class 1 Review

Classes, methods, fields
Interface, package
Operators mostly apply only to intrinsics (exceptions such as = +)
== versus equals()
null
try, catch (exception handling)

Class 1 Samples

Samples

Execution Environment

Based on Java virtual machine
Interpretive byte-codes generated by compiler conform to vm specification. Byte-code verification occurs when class is loaded
Memory mapping of classes, methods, fields, etc occurs dynamically by the runtime (pointers are not supported in the language)
Dynamic class loading when class is referenced ( reference can be to external class on some other machine, subject to existing security rules). CLASSPATH environment variable determines local machine's search path.
Performance improvements with "just in time compiler" which creates native code from interpretive code when class first accessed; the native code is cached
Native classes and methods can be implemented and accessed
Security with respect to local machine's resources is dependent on how the program is executed (ie application versus applet, more details below). Further restrictions can be imposed by subclassing the Security Manager class.
Exception handling can be explicitly performed by the program using the try...catch contruct (more detail in subsequent class)

Program Types

Application

A Java classfile can be executed via the interpreter as a standalone program provided it contains a main method. This method:

must be declared with the public and static modifiers
should define, as its parameter, a String array (which will contain the command line parameters passed to it)
normally will not have restrictions on its ability to access local machine resources (such as files and programs) as defined by the standard classes and methods

Typical definition:

   public static void main(String args[]) {
   ...
   }

Applet

A Java class can be embedded in an HTML document (or executed by the Applet viewer) provided it defines its public class to be an extension of the Applet class. Several methods will typically be subclassed:

init - invoked after the applet has been loaded

start - invoked after init and again (when in a browser environment) each time the containing webpage is re-displayed.

stop - invoked when the applet should stop processing; for example if in a browser environment, this method will be invoked when the containing webpage is exited

destroy - invoked when the applet's resources are about to be eliminated (the stop method is always called first).

Normally an applet will have restrictions on its ability to access local machine resources.

To invoke an applet from an HTML document, use the APPLET tag. Parameters can be made available to the applet by using the PARAM tag and can be accessed within the applet by using the getParameter method.

Sample HTML:

<applet code="myapplet.class" width=300 height=100>
<param name="First" value="this is the first parameter">
<param name="Font" value="TimesRoman, BOLD, 24">
</applet>

Sample code to reference the parameters:

String paramFirst, paramFont;
...
paramFirst = getParameter("First");
paramFont  = getParameter("Font");

It is possible to write a classfile that can be executed either as an application or as an applet. To do so, define the class as an extension of Applet but also include a main method. If the class attempts to access local machine resources, this will fail if it is being run as an applet.

Base Packages

lang

Modifier	Name	Extends
class	Boolean
class	Character
class	Class
abstract	ClassLoader
class	Compiler
class	Double	Number
class	Float	Number
class	Integer	Number
class	Long	Number
class	Math
class	Number
class	Object
abstract	Process
class	Runtime
abstract	SecurityManager
class	String
class	StringBuffer
class	System
class	Thread
class	ThreadGroup
class	Throwable
interface	Cloneable
interface	Runnable

util

Modifier	Name	Extends
class	BitSet
class	Date
abstract	Dictionary
class	Hashtable	Dictionary
class	Observable
class	Properties	Hashtable
class	Random
class	Stack	Vector
class	StringTokenizer	Enumeration
class	Vector	implements Cloneable
interface	Enumeration
interface	Observer

applet

Modifier	Name	Extends
class	Applet
interface	AppletContext
interface	AppletStub
interface	AudioClip

Reference

Class, Interface and Method detailed specs are in the API documentation included with the JDK toolkit

Java and JDK as referenced herein are protected by the following copyright notices:

The JDK is based in part on work of the Independent JPEG grouip.

The Java (tm) technology is owned and exclusively licensed by Sun Microsystems Inc. Java (tm) is a trademark of Sun Microsystems Inc in the U.S. and other countries.