CPS616 1996
	CPS 616
	January-April 1996
	Computational Science Track on base technologies for the Information Age:
	Module on Java Language and Applets
		Instructor: Geoffrey Fox
		teamed with Wojtek Furmanski, Nancy McCracken
		Syracuse University
		111 College Place
		Syracuse
		New York 13244-4100
	Abstract of CPS616 Java Presentatation 
		Overview including History and alpha versus beta issues
		Comparison of Java and Javascript
		Overall Java Philosophy and Features  including security etc.
		Java Programming Language
		Object Oriented and Class Structure
		Exceptions
		Applet Programming and Threads
		Abstract Windowing Toolkit
		Networking and I/O; native classes
		Futures and HPCC Implications
		Hot Java is not discussed as beta version not currently  available
	Overview -- What Is Java/HotJava?
		HotJava is a Web browser that supports dynamically downloadable interactive content. Alpha version of HotJava expected to be replaced by major rewrite
		Arbitrarily sophisticated dynamic multimedia applications inserts called Applets can be embedded in the regular HTML pages and activated on each exposure of a given page.
		Applet constructs are implemented in terms of a 
			special HTML tag: <APPLET codebase="URL directory path" code="Java class file name" width=".." height=".." > 
			where the URL and class file name points to a chunk of server side software that is to be downloaded and executed at the client side on each presentation of a page containing this applet which executes in window specified in size by width and height in picxels.
		Applets are written in Java -- a new general purpose object-oriented programming language from Sun Microsystems.
	History of Java Language and Team
		Starts in 1991 by Project Green --- a group in Sun that detaches from the main campus as a semi-autonomous task force focused on operating software for consumer electronic devices such as smart set-top boxes
		Gosling (creator of Sun NeWS which had major conceptual impact both on current Java and Telescript models) realizes that C++ is not adequate and initiates development of a new language Oak, later renamed as Java.
		A PDA (Personal Digital Assistant -- codename *7) based on Oak/Java ready in 1993. Green Team incorporates as FirstPerson, Inc.
		*7 proposal to Time-Warner rejected in 1993. 3DO deal falls through in 1994. FirstPerson, Inc. dissolves.
		Small group (~30 people, now Java Team) continues development and decides to adapt Oak as a Web technology.
	History of Java Language and Team (contd)
		An experimental web browser written in Java, called WebRunner and later renamed as HotJava, ready in 1994. 
		Java/HotJava published in April '95.
		Netscape licences Java in May '95 and builds Java into Netscape 2.0 -- This confuses ownership and open-ness of Java 
		Beta JDK (Java Development Kit) published in summer/fall '95. It is better software but lower functionality than Alpha.
		First alpha Java books appear in fall '95 such as a popular overview by SAMS and technical book "Java!" by Tim Ritchey, edited by New Riders.
		Dec 4 1995 Business Week cover story on "Software Revolution --- The Web Changes Everything" exposes Java as a breakthrough force in the expanding Web/Internet. Also points out that "Java as a business" is yet to be defined.
		In next week, SGI IBM Adobe Macromedia and finally Microsoft adopt/license Java. It appears that Java will be open and should be adopted by open Web community
	Java/HotJava Alpha vs JDK Beta Situation -- Alpha
		Alpha release of Java/Hotjava included:
			C source code for the Java interpreter (Virtual Machine)
			Java foundation classes (base language support, I/O, networking,  Internet protocols, windowing, graphics, utilities)
			HotJava browser package used to build HotJava in Java. Includes  'hotjava' class which 'is' HotJava application.
		In Short, Alpha Java/HotJava was a complete software release of all system components, free for research and subject to ($125K) licence for business use.
	Java/HotJava Alpha vs JDK Beta Situation -- Beta/1.0
		Version 1.0 of JDK released January 96 by JavaSoft
		Beta/1.0 JDK  should be the Internet standard and so compatible with ongoing Java implementations by all licensees, most notably Netscape. Beta/1.0 JDK includes:
			Java Compiler (.java to .class) for Sun Solaris and Windows NT/95 avaliable as .class itself (Javac written in Java) but no source
			Java source for the foundation classes (modified and incompatible with  alpha)
			appletviewer to run/preview applets
			tools for converting alpha applets to beta applets. 
			Source code for the interpreter
			There is also no browser package and no beta HotJava yet --- current users are given an option to use Netscape2 or just preview beta applets using appletviewer and wait for beta/1.0 HotJava (with unknown release date).
	Java/HotJava Alpha vs JDK Beta Situation -- HotJava/Books
		Release of Beta/1.0 HotJava expected soon and could be very interesting technically for certainly alpha HotJava was very helpful in building customized Graphical User Interfaces
			Also raises questions about Netscape's business model and Netscape specific HTML extensions. 
			HotJava allows others who do not like Netscape's GUI or HTML extensions to challenge with a base system (modular browser) that has better architecture  than current Netscape
		Hooked on Java by Java developers Arthur van Hoff, Sami Shaio, Orca Starbuck is (first) serious (but still rather high-level) book on beta Java
		Substantial tutorial and report material available on line http://www.javasoft.com
	Comparison of Java and JavaScript -- I
		Netscape renames Livescript as Javascript and this is an interesting variant of Java which is fully interpreted -- use for overall customization of client
		Use Java for detailed programming and JavaScript for overall integration of client interface and system
		JavaScript: Interpreted by client and NOT compiled
		Java: Compiled on Server before execution on client
			Note both are reasonably "pure" C/C++ like languages and do NOT have useful sh/awk text and system enhancements of Perl(5)
		JavaScript: Object based -- no classes or inheritance -- built in extensible objects
		Java: Object-oriented. Programs consist of object classes with inheritance
	Comparison of Java and JavaScript -- II
		JavaScript: Integrated with HTML as embedded ascii but of course HTML looks rather irrelevant at times!
		Java: Applets distinct from HTML but invoked from HTML Pages
		JavaScript: do not declare variables' datatypes -- Loose typing
		Java: MUST declare variables' datatypes -- Strong typing
		JavaScript -- Dynamic Binding -- object references computed at runtime
		Java -- Static Binding -- object references must exist at compile time
		Java and JavaScript are secure and cannot write to disk
	Some Key Java Features
		Document The Java: A White Paper  by Sun Microsystems -- October 1995 draft by James Gosling and Henry McGilton -- enumerates the following main features of Java:
		Simple and Familiar
		Object-oriented 
		Architecture-neutral 
		Portable 
		Somewhat Interpreted 
		Distributed 
		Robust 
		Secure 
		High performance 
		Multi Threaded
		Dynamic
	Java Features -- It's Simple and Familiar!
		Java omits several rarely used, poorly understood and confusing features of C++ including operator overloading, multiple inheritance, pointers and automatic type coercions. 
		It adds automatic garbage collection which makes dynamic programming easier in Java than in C or C++.
			No more mallocs!
		It also adds 'Interface' construct, similar to Objective C concept, which often compensates for the lack of multiple inheritance by allowing method calling syntax to be "inherited".
		The resulting language is familiar as it looks like C++ but is simpler and hence easier to program in. 
		It also results in a much smaller kernel which is suitable for planned Java ports to consumer electronic devices. Base (alpha) interpreter is ~40Kb, libraries and threads add additional 175Kb.
	Java Features -- It's Object-oriented
		Java model can be viewed as a C++ subset, with some dynamic elements inherited from Objective-C (method overloading, garbage collection).
		Structures, Unions and Functions are absorbed into data and methods of Java classes -- Java is Simple!
		The strength of Java object-oriented model is not is sophistication but in simplicity and the extensive class library associated with the system (some 250 public classes were released in both alpha and beta).
		Java class plays also a role of a communication atom in the Web embedding model. Applet classes identify themselves by names in the HTML applet tag. Applet downloads other classes, present in the applet source. Hence, the Java class names play the role of addressing mode for the distributed Java code database.
	Java Features -- It's  Architecture-Neutral
		C/C++ programming in a heterogeneous network environment requires use and compatibility across several vendor platforms and the corresponding compilers. This problem is solved in Java by designing platform-independent binary representation called Java bytecode (or opcode).
		Java compiler (written in Java and  platform-independent) reads Java source and generates Java bytecode. These bytecodes are shipped to client machines upon browser requests. 
		Each client machine must run Java interpreter which performs runtime execution of Java bytecodes. Java interpreter is written in POSIX compliant ANSI C and needs to be ported to and conventionally compiled (once) on each individual platform.
		Once the interpreter is ported, application developers don't need to worry at all about platform specificity and differences between native compilers. They write portable Java which is compiled by platform independent Java compiler and executed on Java interpreter, ported to various platforms.
	Java Features -- It's Portable
		Java language offers a uniform abstract (virtual) machine model which is identical for all platforms.
		SUN owns the Java Virtual Machine (see online report) -- it is universal while classes can be added by any user 
		Unlike in C/C++ where various integers match the architecture of a physical machine at hand, Java byte, char short, int and long are always of the same size, equal to 8, 16, 16(unicode), 32 and 64 bits, respectively.
			No header files, preprocessors,#define etc.
			floating point is always IEEE 754
		Differences between vendor specific windowing environments (X Windows, MS Windows, Macintosh) are removed in terms of the Abstract Windowing Toolkit (AWT) metaphor. 
		AWT is given by ~60 Java classes (alpha) which offer a universal GUI programming model, portable between UNIX, PC and Mac, and translated automatically to native windowing systems on individual platforms by Java interpreters. 
	Java Features -- It's Somewhat Interpreted
		Java represents a compromise between fully compiled (like C/C++) and fully interpreted (like Smalltalk or Perl) models.
		Java "compiler" produces a binary bytecode output which is portable and much smaller than the real binary for a specific machine (Typical bytecode size is of order of the original source code, within a factor of 2).
		Java "interpreter" executes this bytecode and is therefore less dynamic than e.g. Perl interpreter (which performs an equivalent bytecode construction internally and on-the-fly when reading the program source).
		In general, the compilation process is: a) time consuming and b) platform specific. Hence, interpreters are built and used to facilitate a) rapid prototyping and/or b) portability. Java model is focused on platform independence but the development throughput is also reasonable since the Java compiler is fast and generates  compact bytecode output.
	Java Features -- It's Distributed
		Popular TCP/IP based protocols such as FTP or HTTP are supported in terms of network protocol classes. This facilitates various forms of distributed processing. New protocols (e.g. PVM etc.) can added and dynamically installed.
		Distributed computing model of Java is mainly client-server, with Java compiler preparing the opcodes at the server side, and Java interpreter executing it at the client side.
		One can expect more dynamic uses of Java with Java threads on both Server and Client side communicating with each other. This is illustrated by Java based Collaboratory developed by Vishal Mehra as NPAC/Ece Master's Thesis
	Java Features -- It's Robust
		Java enforces compiler-time type checking and eliminates this way some error prone constructs of C/C++.
		Pointer arithmetic is fully eliminated which allows e.g. for runtime checking of array subscripts and enforces security of the Java model.
		Explicit declarations are always required, i.e. C-style implicit declarations are abandoned. This allows the Java complier to perform early error detection.
		Rapid prototyping in Java is less natural than in JavaScript,Lisp, Tcl, Smalltalk or Perl, but the software quality assurance of Java is higher than in these more dynamic and 'forgiving' languages.
	Java Features -- It's (Hopefully) Secure
		Java binaries are shipped across the network and executed on client machines. Security is therefore a critical issue and strongly enforced in Java. 
			Java contains its own networking classes which are designed to be secure
		Modifications of the C++ model such as eliminating pointer arithmetic and coercion were dictated mainly by the security requirements.
		Most viruses are based on acquiring access to private/protected sectors of computer memory which is impossible in Java.
		Java opcodes are executed at the client side by Java interpreter which operates exclusively on the virtual memory. Hence, unless there are security bugs in the Java interpreter itself, the model is safe and users cannot create security holes by incorrectly or maliciously written applets.
		The byte codes sent across network are verified at the client which prevents evil/corrupted classes from causing problems
	Java Features -- High Performance
		Java interpreter performs on-the-fly runtime execution of the Java bytecodes which results typically in a satisfactory performance. 
		Support for generating native machine code out of Java bytecodes, viewed as intermediate compiler form, is also provided and useful for performance demanding applications. 
		The performance of the machine code, generated from Java bytecodes, is comparable to that offered by typical C/C++ compilers on the same platform.
		Several of these concepts are in fact similar as in the OSF/ANDF project. Using ANDF terminology, we would call Java compiler a 'producer', and the machine code generator discussed here, an 'installer'. Default Java working mode doesn't use installers but directly interprets the intermediate form (this mode is supported in ANDF by GAI -- Generalized ANDF Interpreter). 
		ANDF model is in principle applicable to all languages. Java/HotJava system implements ANDF concepts for the Java language.
	Java Features -- It's Multithreaded
		Java model offers preemptive multithreading, implemented in terms of the Thread class. Thread methods offer a set of synchronization primitives based on monitor and conditional variable paradigm by C.A.R. Hoare. Java threads inherit some features from the pioneering Cedar/Mesa System by Xerox Park that gave birth to Macintosh and object-oriented programming.
		A typical use of Java multithreading in applet programming is to have several independent but related simulations (e.g. various sorting algorithms), running concurrently in an applet window. Multithreading is also used internally by the HotJava browser to handle multiple document dynamics.
		Another interesting application domain are multi-HotJava environments to come such as collaboratory or gaming. 
		Java threads don't have built-in point-to-point communication primitives. Various thread communication environments can be provided by coupling the thread and network protocol objects.
	Java Features -- It's Dynamic
		Java model is more dynamic than C++ and closer to Smalltalk or Perl. 
		Subclasses don't need to to be recompiled after superclass implementation is updated.
		Classes have runtime representation (implemented in terms of the Class class) which allows one to look up type of a given object instance at runtime (in C cannot know if pointer is to integer or browser!)
		C++ has "fragile superclass" problem where must recompile children if change anything (method/instance variable) in a superclass or referenced class -- Java resolves references at runtime and avoids this.
	Sun's Comparison of Language Features
		l Good      l  Fair        l  Poor
	The Java Programming Language
		One resource is the  The Java Language Specification by Sun Microsystems, Inc., March 1995 updated to October 1995.
		This and Java Tutorial can be found on http://www.javasoft.com web site with Java Virtual Machine and the White Paper described earlier
		There is also the Hooked on Java book, its CDROM and the updated online resources refered to in Book
		The CDROM is superceded in many ways by latest JDK 1.0 release
	Java Language Discussion -- Table of Contents
		Program Structure 
		Lexical Issues 
		Applet versus Application 
		Variables and Expressions
		Types and Array 
		Control Flow
		Exceptions
		Object Model and Classes
		Interfaces 
		Methods
		This is followed by discussion of Implementation in terms of overall Classes, Applet Programming, Threads, Graphics and the Abstract Windowing Toolkit, Networking and I/O, Native classes, HPCC implications
	Java Language -- Program Structure
		Source code of a Java program consists of one or more compilation units, implemented as files with .java extension.
		Each compilation unit can contain: 
			a package statement 
			import statements 
			class declarations 
			interface declarations
		Java compiler (called javac) reads java source and produces a set of binary bytecode files with .class extensions, one for each class declared in the source file. For example, if Foo.java implements Foo and Fred classes, then "javac Foo.java" will generate Foo.class and Fred.class files.
		Suppose that Foo implements an applet and Fred is an auxiliary class used by Foo. If HotJava/Netscape encounters a tag <APPLET  code="Foo.class">, it will download Foo.class and Fred.class files and it will start interpreting bytecodes in Foo.class.
	Java Language -- Lexical Issues I
		Lexical structure inherits a lot from C/C++. There are however some notable differences which are listed below.
		Java characters are based on 16--bit wide Unicode Worldwide Character Encoding rather than the usual 8--bit wide ASCII.
		This allows full support of all alphabets and hence all languages
		Three types of comments are supported: 
			// ignore all till the end of this line 
			/* ignore all between starts */
			 /** an insert into an  automatically generated software documentation */
		 for /** */ one inserts HTML documentation with some simple macros such as @see (to designate see also) BEFORE the method or class being documented
	Java Language -- Lexical Issues II
		Java reserves the following keywords: 
		abstract boolean break byte byvalue case cast
		catch class const continue default do double
		else extends final finally float for future generic
		goto if implements import inner instanceof int
		interface long native new null operator outer
		package private protected public rest return
		short static super switch synchronized this
		throw throws transient try var void volatile while
		Note goto is not allowed in Java but its still reserved!
	Applications Versus Applets - I
		Applications are .java files with a main class which reads arguments and is excuted first
		One uses javac to compile the application converting .java to .class
		Then run the interpreter java with the .class file as argument
		.java files are in Java; .class files are in universal bytecodes
		The resources javac and java are part of JDK and are not in Netscape and so are currently not broadly available. You must log in to our servers to use them
		So instead in this course we will use applets so all the Java threads are distributed around and don't grind our servers to a halt
	Applications Versus Applets - II
		Applets should NOT have main method but rather init, stop, paint etc.
		They should be run through javac compiler getting a .class file as before
		Create an HTML file (say HelloWorld.HTML in same directory as .class file) and include in this
		<applet code="Example.class" width=500 height=200 > </applet> in simplest case with no parameters where applet will run in window of given width and height (in pixels)
		If you have JDK on one's machine, one can run the applet with appletviewer HelloWorld.html
		Alternatively run Netscape 2.0 essentially anywhere and applet is interpreted and run by Java interpreter built into Netscape.
		This way we can compile on places with JDK installed but run almost anywhere!
	Java Language -- Variable/Expression Types
		Each Java variable or expression has a definite type. Simple types such as integers or booleans are built-in. New composite types can be constructed in terms of classes, arrays and interfaces.
			Note booleans are either TRUE or FALSE -- they are not 0, 1 ,-1 ...
		There are 4 integer types: byte, short, int, long of size 8, 16, 32 and 64 bits, respectively. 
		Float is 32 bits, double is 64 bits. Floating point arithmetic and data formats are defined by IEEE754.
		Characters are given by 16bit Unicode charset and represented as short integers.
		One can use casts such as longint = (long) i;
		Arrays are "true" or "first class" objects in Java and no pointer arithmetic is supported. Hence, arrays are handled as all other composite objects and its special role is due only to some syntactic sugar inherited from C/C++.
	Java Language -- Types: Array
		An Array is declared as:   
			int vec[]; 
		and created by:   
			vec = new int[128]; 
		or concisely:   
			int vec[] = new int[128];
		Subscripts are range checked in runtime and so vec[-1] and vec[128] will generate exceptions.
		Array length can be extracted via the length instance variable, e.g.   
			int len = vec.length will assign len = 128.
		Arrays of arbitrary objects can be constructed,
			e.g.   Thread myThreadList[] = new Thread[1024];
		Arrays can have dynamic sizing
			int sizeofarray = 67;
			int vec[] = new int[sizeofarray];
	Java Language -- Expressions
		Java's expressions are very similar to C and the following are valid:
		 2+3
		(2+3)*i
		i++   /* equivalent to i = i +1  */
		(i > 0 ) && (j>0)  /* Boolean */
		i <<1  /*  Left shift by 1 binary digit */
		(i>0) ? true:false  /* Boolean */
		i >>> 2  /* Signed right shift by 2 binary digits */
		"fred" + "jim" is "fredjim" 
		 /*  + Equivalent to . in Perl */
		(a instanceof B) /* True iff object a is of class B */
	Java Language -- Control Flow I
		if(some boolean expression)  {   .. }
		else if(another boolean)  { .. }
		else {   ...   } 
		while(any boolean) {  /* Do Stuff */  }
		do { /* What to do */ } while(another boolean);
		for(expression1; booleanexpression ; expression2) { ...}
		naturally starts with expression1, applies expression2 at end of each loop, and continues as long as booleanexpression true
		switch (expression)  {  /* Just as in C */
		case Constant1: /* Do following if expression=Constant1 */
		 /* Bunch of Stuff */
		 break;
		case Constant2: /* Do following if expression=Constant2 */
		 /* Bunch of Stuff */
		 break;  /* ....... */
		default:
		/* Bunch of Stuff */
		 break;
		}
	Java Language -- Control Flow II -- continue
		One can break out of (nested) for loops in fashion offered by Perl with different syntax
		outer: // label
		for( int j=0; j<10; j++) { /* Note j only defined in for loop */
		/* select course j */
		     for( int i=0; i<15; i++)   {
		        if(studentgrade[j][i] == 4.0)  {
		            /* Celebrate  */
		            continue outer; // go to next iteration of outer loop
		        }
		     }
		/* Continue jumps to here  to next iteration of loop labelled with outer */
		}
	Java Language -- Control Flow III -- break and for loop
		One can break out of (nested) for loops in fashion offered by Perl with different syntax
		outer: // label
		for( int j=0; j<10; j++) { /* Note j only defined in for loop */
		/* select course j */
		     for( int i=0; i<15; i++)   {
		        if(studentgrade[j][i] == 4.0)  {
		            /* Celebrate  */
		            break outer; // go to end of outer loop
		        }
		     }
		}
		/* break jumps to here out of loop labelled by outer  */
	Java Language -- Control Flow IV -- break and switch
		loopstart: // label
		for( int j=0; j <10; j++)  {
		 switch(j)  {
		  case 3:
		  break;
		  default:
		  if( studentgrade[j] == 2.5)
		      break loopstart;  /* go to end of loop */
		  /* do some stuff */
		  break;
		 }
		}
		/* break loopstart goes to here out of loopstart loop */
	Java Language -- Control Flow V -- continue and switch
		loopstart: // label of following for loop
		for( int j=0; j <10; j++)  {
		 switch(j)  {
		  case 3:
		  break;
		  default:
		  if( studentgrade[j] == 2.5)
		      continue loopstart;  /* go to next iteration of loop */
		  /* do some stuff */
		  break;
		 } // End Switch Block
		/* continue loopstart goes to here for next iteration of loopstart loop */
		} // End loopstart for loop
	Java Language -- Handling Errors Using Exceptions
		Java supports a hierarchical model of exceptions which allow and indeed require user to supply suitable handlers for any exception that can occur in a Java program
		Note exceptions that can occur in a method must either be caught (i.e. handled inside method) or thrown (i.e. returned to callee)
		Thrown exceptions are like returned arguments and are for instance part of interface to a method
		Exceptions are all (at some level in hierarchy) subclasses of Throwable class
		method1 {
			try {
				call method2;
			}  catch (Exception3 e) {
				doErrorProcessing(e);
			}
		}
		method2 throws Exception3  {
			call method3;
		}
		method3 throws Exception3 {
			call dividebyzeroorreadfileorsomething;
		}
	Examples of Exception Hierarchy
		As Examples of hierarchy:
		catch(InvalidIndexException e) { .. } would catch particular exception whereas
		catch(ArrayException e) { .. } would catch all Arrayexceptions
	The Java Object Model Overview
		Objects are instances of Classes which form a template to allow definition of objects of the same type -- Classes are not objects although one can define static or class variables and methods which are shared by all instances of a class
		The data of an object are called instance variables and they can be accessed for read or write via suitable methods which define the behavor of an object
		/* Movable Point Class */
		public class mPoint {   
			  public int x, y;   /* Instance Variable  */
			  public int dx = 1, dy = 1;  /* More Instance Variables */
		Both class and its instance variables can be accessed by all other java objects
	Use of Methods Defined in Parent
		Call to method in Object2 (message) from object1 is passed up the class hierarchy until a definition is found
	Use of Methods Defined in Parent but overridden in child class
		Call to method in Object2 (message) from object1 is passed up the class hierarchy until a definition is found
	Java Language -- Overview of Classes
		Class declaration in Java shares common aspects with C++ but there are also some syntactic and semantic differences.
		Only single inheritance is supported but aspects of multiple inheritance can be achieved in terms of the interface construct. Interface is similar to an abstract class with all methods being abstract and with all variables being static (global). Unlike classes, interfaces can be multiply-inherited.
		ClassModifiers class className [extends superClass] [implements interfaces] {
		e.g. public class Test extends Applet implements Runnable { 
		defines an applet that can use threads which have methods defined by Runnable interface
	Hello World Applet from Sun Tutorial
	Getting Started
		This prints in applet window, the classic Hello World string!
		import java.awt.Graphics
		public class HelloWorld extends java.applet.Applet {
			public void init()	{
				resize(150,25)
			}
			public void paint(Graphics g)   {
				g.drawString("Hello world!", 50, 25);
			}
		}
		g which is of class Graphics, is window in which applet runs (i.e. is displayed)
		
	Hello World Application from Sun Tutorial
	Getting Started
		This is "application" version of previous applet
		class HelloWorldApp   {
			public static void main (String args[]) {
				System.out.println("Hello World!");
			}
		}
		
		Note args are of class String and needed even though HelloWorldApp has no command line arguments
	Date Application from Sun Tutorial
	Anatomy of an Application
		This invokes the class Date and creates object today as an instance of this class
		Date() is Constructor of Date class which sets default value (current date) in today
		Date today declares today to be instance of this class
		import java.util.Date;
		class DateApp	{
			public static void main (String args[])  {
				Date today = new Date();
				System.out.println(today);
			}
		}
	Counting Application from Sun Tutorial
	Nuts and Bolts of Java
		This application reads from standard input and counts number of characters which are then printed
		class Count	{
			public static void main(String args[])
				throws java.io.IOException
			{
				int count = 0;
				while (System.in.read() != -1)
					count++;
				System.out.println("Input has " + count + " chars.");
			}
		}
	Java Language -- Types of Classes
		Possible ClassModifiers are:
		abstract -- Contains abstract methods without implementation -- typically such abstract classes have several subclasses that define implementation of methods
		final -- Cannot have a subclass
		public -- May be used by code outside the class package and (unix) file must be called ClassName.java where ClassName is unique public class in file
		private -- this class can only be used within current file
		friendly(i.e. empty ClassModifier) -- class can be used only within current package
		synchronizable -- Instances of this class can be made arguments of synchronization statements
	Java Language -- Methods
		MethodModifier ReturnType Name(argType1 arg1, ......)
		Returntypes are either simple types (int, byte etc.), arrays or class names
		Possible MethodModifiers are:
			public -- This method is accessible by all methods inside and outside class 
			protected -- This method is only accessible by a subclass
			private -- This method is only accessible to other methods in this class
			friendly(i.e. empty) -- This method is accessible by methods in classes that are in same package
			final -- a method that cannot be overriden
			static -- This method is shared by ALL instances of this class and can be invoked with <Class>.method syntax as well as <Instance>.method
			synchronized -- This method locks object on entry and unlocks it on exit. If the object is already locked, the method waits until the lock is released before executing
			native -- to declare methods implemented in a platform -- dependent language, e.g. C. 
	A little more complicated Paint Method for Applets
		paint is an important method in applets which is defined in parent component class as the null method
		It must be overwritten by any dynamic applets that need to update display.
		import java.awt.*; //  imports all classes in java.awt package -- this applet needs Graphics, Font, Color which could be listed separately
		public class HelloWorld extends java.applet.Applet {
		Font f = new Font("TimesRoman",Font.BOLD,36);
		public void paint(Graphics g)   {
		g.setFont(f);
		g.setColor(Color.red);
		g.drawString("Hello world!", 50, 25); }
		}
	Java Language -- Interfaces
		A class can only have one parent class but can implement several interfaces
		interfaces are essentially abstract classes and can have methods without impementation and static variables
		interface Printable {
			void print();
			void print(String str);
		}
		class Cookie implements Printable {
			void print()	{
				System.out.println("Cookie");
			}
		}
		class WastePaper implements Printable  {
			void print(String str)  {
				int i,len=str.length();
				for(i=0; i < len; i++)  {
					singlechar = str.charAt(i);
					System.out.println("This is character No." + i + " " + singlechar);
				}
			}
		}
	Cars as an Examples of Interfaces/ Multiple Inheritance
		We could imagine a Ford Mustang as inheriting from two major sources 
		Firstly a set of manufacturing companies -- make these interfaces as "qualitative"
		Secondly from a set of Vehicle types which we will make real classes as there are non trivial methods involved in constructing cars
	Picture of Interfaces and Classes for Cars and their Manufacture
		
		Cars MyFordMustang = new Cars(Lots of Options)
		is a particluar instance of  class Cars
	More on Interfaces -- II
		Note that Interfaces often play a software engineering as opposed to required functional role
		For instance, the printable interface (which is fictitious and not part of current Java release) establishs that any class implementing it can be "printed" with a standard method -- "print"
		Note that Interfaces are not significantly used in current Java release where perhaps there are 15 times as many class definitions as interface definitions
		Two examples are Runnable and Cloneable both of which extend Object class -- note interfaces like classes can extend existing classes: 
	More on Interfaces -III- Real Examples
		The Cloneable Interface has NO methods but is used to indicate that an instance of this class can be "cloned" i.e. that the clone method (defined for overarching Object class) can be used
		The Runnable Interface has one method run() which must always be overwritten and is used to indicate that class with this interface can use threads without being a subclass of Thread. Applets must use Runnable if they need explicit threads because they explicitly are a subclass of panel and as no multiple inheritance, one cannot be a subclass of two classes
	ReverseString Class from Sun Tutorial
	The String and StringBuffer Classes
		This class returns object of class String which reverses order of characters in input object source which is also an instance of class String
		class ReverseString  {
			public static String reverseIt(String source)   {
				int i, len= source.length();
				StringBuffer dest = new StringBuffer(len);
					for( i= (len-1); i >=0 ; i--)   {
						dest.append(source.charAt(i));
					}
					return dest.toString();
				}
			}
		length, CharAt and toString are methods of class String which is used for fixed character strings
		StringBuffer is constructor of class StringBuffer which is used for mutable character strings and has methods append and toString 
	Instantiating an Object from its Class
		Suppose we have defined a class called mPoint for a movable point which has position (x,y) Color color and can be moved by amount (dx,dy) at each step
		mPoint BunchofPoints[2]; /* defines BunchofPoints to be 2 dimensional array of mPoint objects -- it does not instantiate BunchofPoints */
		FirstPoint = new mPoint(); /* Allocates an instance of mPoint and invokes Constructor of mPoint to Initialize */ 
		FirstPoint.dx=5;
		FirstPoint.dy=5; /* Redefines values of instance variables dx,dy for object FirstPoint */
		mPoint could be defined by
		import java.awt.*;
		public class mPoint {
			int x, y;
			int dx = 1, dy = 1;
			Color color = Color.black;  
		}
	Summary of Object-Oriented Concepts - I
		Class: A template for an object which contains variables and methods which can be inherited from other superclasses and whose calling sequence and existence can be defined by interfaces
		Object or Instance: A particular realization of some class; different instances usually have different values for their variables or instances but the same methods
		Simple types: variables defined by int, char etc. are NOT objects but each has an associated wrapper class which can be used with greater power but lower efficiency
		Superclass: A class further up in the inheritance tree
		Subclass: A class further down in the inheritance tree
		Abstract classes:contain abstract methods which are not implemented and only define interfaces. Subclasses will provide implementations
	Summary of Object-Oriented Concepts - II
		Method or Instance Method: the usual type of method which is defined in a class and operates in instances of class or subclasses of defining class
		static or class method: A method defined in a class which can operate on the class itself or on any object
		static or class variable: A variable that is owned by the class and all its instances as a whole. It is stored in class and not in particular instances.
		Interface: A collection of abstract behavior(method) specifications that individual classes can then implement
		Package: A collection of classes and interfaces. Classes or interfaces from packages other than java.lang must be explicitly imported or referred to by full package name
	Constructors of Objects
		Methods of identical name to class are special -- they are constructors
		mPoint() {	/* Default Constructor */
			x=0;
			y=0;
		}
		We can overload method to define different versions with different numbers and types of arguments
		mPoint(int x, int y) { /* Constructor to set Specific Initial values */
			this.x = x;
			this.y = y;
		}
		Note this refers to current object
		mPoint can have other methods to set its instance variables such as:
		public void setDelta(int _dx, int _dy) {
			dx = _dx;
			dy = _dy;
		  } /* where we don't need this as used _dx, _dy */
	Class Finalizers
		Any class can have an optional finalizer that will perform any necessary clean-up needed when garbage collector (which is automatic in Java) tries to delete object and free-up resources it uses
		An example is given for an object that uses an I/O stream and needs to close it on termination
		protected void finalize()  {
			try   {
				file.close();
			}   catch  (Exception e)   { /* Catches ANY exception */
			}
		}
		Earlier one might have set this up with syntax such as that on next foil
	Handling Exceptions in Closing a File
		File file; /* defines file to be object of class File */
		try{
			file = new File("filenameyouwant");
			.......
			file.write("stuff put out");
		} catch (IOException e)  {
		/* This catches ALL I/O errors including read and write stuff */
		/* After Handling Some How */
		return;   /* but the finally clause will be executed whether or not code terminates normally */
		}  /* We must close file whatever happens */
		finally {
			file.close();
		}
	Some More Methods for mPoint
		import java.awt.*;   /* Imports Graphics Method for display  */
		public class mPoint {  /* Continue as before and add */
			public void setColor(Color color) { this.color = color;}
			public void checkBoundry(Rectangle rect) { /* check if object crosses boundary */
				int nx = x+dx; /* caculate new location */
				int ny = y+dy;
				if ( (nx < rect.x) || (nx >= rect.x+rect.width) ) dx = -dx;
				if ( (ny < rect.y) || (ny >= rect.y+rect.height) ) dy = -dy;
			}
			public void move(Graphics g) { /* move object */
				paint(g);   /* use XOR to hide object */
				x += dx;    /* update location */
				y += dy;
				paint(g);   /* draw object on new location */
			}
				public void paint(Graphics g) {}
		}
	Inheritance or Subclassing
		We can define a new class mRectangle that extends mPoint where (x,y,color) from mPoint are lower left hand corner/color and we add width and height
		public class mRectangle extends mPoint {
			int w, h;
			public mRectangle(int _x, int _y, int _w, int _h) {
				super(_x, _y); /* call mPoint's constructor */
				 w = _w;
				 h = _h;
			} /* End mRectangle Constructor  */
		/* overwrite the mPoint's checkBoundry method */
			public void checkBoundry(Rectangle rect) {
				int nx = x+dx;
				int ny = y+dy;
				if ( (nx < rect.x) || (nx+w >= rect.x+rect.width) ) dx = -dx;
				if ( (ny < rect.y) || (ny+h >= rect.y+rect.height) ) dy = -dy;
			}
			public void paint(Graphics g) { /* Override mPoint Method */
				g.setColor(color);
				g.drawRect(x, y, w, h);
			}
		}
	Overview of Packages
		The command 
		package cps616;
		can be used to group classes together. A package statement can be repeated -- e.g all students could use the above package statement in their Java files and the corresponding classes would be grouped in this one package
		Note classes are called package.Classname where packages in Java system are all termed java.subpackagename
		One uses files in a package by inserting
			import cps616.* at beginning of file that neededs classes from cps616 package
			In this case classes in cps616 package can be refered to by just using their Classname
			Without import command, one must explicitly say cps616.Classname
	Java System Packages
		java.lang  Contains essential Java classes and is by default imported into every Java file and so import java.lang.* is unnecessary. Thread, Math, Object and Type Wrappers are here
		java.io contains classes to do I/O. This is not necessary (or even allowed!) for applets which can't do I/O in Netscape!
		java.util contains various utility classes that didn't make it to java.lang. Date is here as are hashtables 
		java.net contains classes to do network applications. This will be important for any distributed applications
		java.applet has the classes needed to support applets 
		java.awt has the classes to support windowing -- The Abstract Windows Toolkit
		java.awt.peer is a secret set of classes with platform dependent details
	The Overarching Object Class
		public class Object is the root of the Class hierarchy. Every java class has Object as its ultimate parent and so any object (object with a small o is any instance of any class) can use methods of Object
		Methods of Object (used as object.Method()) include:
		clone() Creates a clone of the object
		equals(another object) compares two objects returning boolean answer 
		getClass() returns a descriptor of type Class (a child of Object) defining class of object
		toString() returns a String which represents "value" of object. It is expected that each subclass will override this method
		wait() in various forms is used to cause threads to wait
		finalize() contains code to perform when object is deleted by system (I.e. garbage collected)
	Determining and Testing Class of Object
		Suppose an object is called obj , then we get Class of obj by
		Class itsclass = obj.getClass(); // and the name of this class by:
		String name = itsclass.getName();
		One can also use instanceof in following fashion
		"foo" instanceof String // is evaluated to true but
		mPoint pt = new mPoint(x,y);
		pt instanceof String // is evaluated to false
	java.lang.Object Wrappers
		"types" such as int char float etc. are NOT classes
		Thus  one cannot use methods such as
		int var;
		var.toString
		Thus ALL types have associated wrappers used like
		Character wrappedchar = new Character(char);
		now wrappedchar has lots of good methods you can use such as:
		wrappedchar.equals(anotherobject);
		wrappedchar.toString();
		There are also static (class) functions such as:
		toUppercase(char ch);
		isUpperCase(char ch);
	Some Basic Methods for Applets -- I
		Some of These are in Applet and some in parent (in particular Component)
		public void init() is called ONCE and ONCE only when the applet is loaded or reloaded. Set initial parameters etc. here.
		public void destroy() is what you do when Applet is entirely finished and you need to clean up stray threads or connections to be closed.
		public void start() is called whenever the applet is started which can happen several times during an applet's life-cycle as it starts each time you revisit a page
		public void stop() is called when we temporarily leave Applet to visit another page. A wise move would be to suspend running of Threads to save system resources.
	Some Basic Methods for Applets -- II
		public void paint(Graphics g) actually has an argument and draws what you want on the screen
		There is somewhat confusing other methods update() and repaint() which need to used in advanced applications. You may need to call repaint() in a dynamic applet to change display but update() would not need to be called as invoked by repaint(). However update() is sometimes best overridden 
		public void repaint() is a request by you to the Java runtime to update screen
		public void update(Graphics g) is invoked by repaint() and performs the screen update which involves clearing screen and invoking paint()
	The details of the <applet> tag
		<applet code="fred.class" codebase="classes" width="widthvalue" height="heightvalue" >
		Put a bunch of text here to be displayed by browsers such as Netscape 2.0 on Windows 3.1 that do not support Java
		</applet>
		One can also include INSIDE <applet > tag, attributes available for <img > tag in conventional HTML. These attributes are
		Align=texttop which aligns top of applet with top of the tallest text in the line
		Align=top which aligns top of applet with top of tallest item in the line
		and align= absmiddle, middle, baseline, bottom, absbottom with similar meaning
		hspace=num1 allows num1 pixels to either side of applet and surrounding text
		vspace=num2 allows num2 pixels to top and bottom of applet and surrounding text
	<param> Tags and Applets
		<applet .... > can be followed by
		<param name=attributename1 value="attributevalue1" > .......
		<param name=attributenameN value="attributevalueN" >
		</applet>
		The Java program accesses this information by
		String attribute;
		attribute = getParamter("attributename1");
		if( attribute == null )   
			attribute = yourdefaultvalue; // null is Java way of saying unset
		Typically this processing would be in init() method of Applet
	The java.awt.Graphics Class
		java.awt has several important classes including Graphics, Color, Font and FontMetrics
		Graphics class has primitives to construct basic two dimensional images with methods drawString (text), drawLine, drawRect, fillRect, drawRoundRect (for a rectangle with rounded corners!), draw3DRect (to get shadow effect as in buttons), drawPolygon (general polygon), drawOval, fillOval
		There are also Image, Font, Color operations
	The java.awt.Font and FontMetrics Classes
		Graphicsinstance.setFont(particularFont) will set the current Font in the instance  Graphicsinstance of graphics class to the value particularFont of class Font. There are several other such Font related methods in the Graphics class
		The class Font has an important constructor used as in
		Font particularFont = new Font("TimesRoman",Font.PLAIN,36);
		where one can use Courier Helvetica etc. instead of Time Roman
		Font.PLAIN, Font.BOLD, Font.ITALIC are possible text styles
		FontMetrics fm = getFontMetrics(particularFont); // allows one to find out about the font
		fm.stringWidth("text"); // returns pixel width of string "text" 
		fm.getHeight(); // returns total height of one line of Font
	The java.awt.Color Classes
		Color c = new Color (redvalue, greenvalue, bluevalue); // red/green/bluevalue can be specified as integers in 0.....255 or floating point numbers from 0 to 1.
		c is generated as a Color in RGB format.
		graphicsobject.setColor(c); // sets current color in graphicsobject which is used for all subsequent operations
		graphicsobject.setFont(particularFont); // similarily sets font hereafter as on previous page
		There are particular Color instances already defined such as
		Color.white equivalent to Color(255,255,255)
		Color.black as equivalent to Color(0,0,0) 
		Color.pink as equivalent to Color(255,175,175)
	Threads as Discussed in Sun's Tutorial
		One can implement threads in two ways
			Firstly subclassing the Thread class and overriding its run() method
			Secondly by creating a Thread with a Runnable object (i. e. that implements Runnable interface) 
		Only the second way is possible for applets as these already extend Applet class and so cannot multiply inherit from the Thread class
		Threads must have a run method which is code Thread executes. 
		If you are using first way, this is written for this particular thread and overrides run() in Thread class
		In second way, the created Thread automatically takes its run() method to be the run method of the applet
	Sun's Digital Clock Applet to Illustrate Threads -- I
		import java.awt.Graphics;
		import java.util.Date;
		
		public class Clock extends java.applet.Applet
		 implements Runnable {
		 Thread clockThread;
		 public void start () {
		   if (clockThread == null) {
		  clockThread = new Thread (this, "Clock");
		  clockThread.start ();
		   }
		 }
	Sun's Digital Clock Applet to Illustrate Threads -- II
		 public void run () {
		   while (clockThread !=null) {
		  repaint ();
		  try { clockThread.sleep(1000);
		    } catch (InterruptedException e) { } 
		   }  // end while
		 }  // end run()
		 public void paint (Graphics g) {
		  Date now = new Date ();
		  g.drawString (now.getHours() + ":"+ now.getMinutes() + ":"+now.getSeconds(), 5, 10);
		 }
		 public void stop () {
		  clockThread.stop();
		  clockThread = null;     
		 }
		} // End Digital Clock Applet
	Flickering in Applets and its Solution
		Unless you are careful, dynamic applets will give flickering screens
		This is due to cycle
		paint(g)
		update(g) clearing screen
		paint(g) drawing new screen .....
		where flicker caused by rapid clear-paint cycle.
		There are two ways to solve this problem which involve changing update in different ways
			1: Change update() either not to clear screen at all (because you know paint() will write over parts that are to be changed) or to just clear the  parts of the screen that are changed
			or 2:Double Buffering
	Sun's Animator.class Applet
		Sun distributes a general purpose Animation applet with the Java Development Kit
		Advantages:
			1. quick: users don't have to write their own program in order to get interesting animation effects.
			2. flexibility: user can assign the directory and the file name for the image frame and sound files by using imagesource, soundsource, soundtrack, and sound parameters in applet tag.
		Disadvantage:
			1. Image blinking: This applet fail to use double buffering and one can see serious blinking on screen.
			2. Separating the frames into different files makes it easy to program but will slow down the image loading process.
	Details of Using the Sun animator Applet --I
		1. Prepare a directory. and cd to this directory.
		2. Copy Animator.class, ParseException.class,  and ImageNotFoundException.class into your directory.
		3. create a directory audio and put your audio file in it.
		Your audio files can be called 0.au, 1.au, etc.
			The background soundtrack could be any .au file. 
		4. create a directory images and put your image files in it.
			your image files can be called T1.gif, T2.gif, etc.
			the loading message can be called loading-msg.gif.
		5. prepare a HTML page which has the following type of applet tag:
		<applet code=Animator.class width=200 height=200>
		<param name=imagesource value="images">
		<param name=endimage value=10>
		<param name=soundsource value="audio">
		<param name=soundtrack value=spacemusic.au>
		<param name=sounds value="1.au|2.au|3.au|4.au|5.au|6.au|7.au|8.au|9.au|0.au">
		<param name=pause value=200>
		</applet>
	Details of Using the Sun animator Applet --II
		the imagesource attribute indicate you put your image files in directory images.
		the endimage value 10 indicate that you have 10 image files held in
			T1.gif, T2.gif, ..., to T10.gif
		The soundsource indicate that you put your sound files in audio directory.
		The sounds attribute lets you express your audio file sequence.
		the pause attribute is the pause (ms) between each image frame.
		There are other parameters which can be set such as:
		<param name=startup value="loading-msg.gif">
		This image will show up first to remind user that image is loading.
	The default Update(Graphics g) Method
		This sets background color and initializes applet bounding rectangle to this color
		public void update(Graphics g)	{
		g.setColor(getBackground());
		g.fillRect(0,0,width,height));
		g.setColor(getForeground();
		paint(g);
		}
		getBackground() and getForeground() are methods in component class
		fillRect() is a method in Graphics class
	ColorSwirl -- An Example from Teach Yourself Java in 21 Days - I
		public class ColorSwirl extends java.applet.Applet
		implements Runnable {
		Font f= new Font ("TimesRoman", Font.BOLD, 48);
		Color colors [] = new color [50]
		Thread runThread;
		public void start () {
		   if (RunThread == null) {
		 runThread = new Thread (this);
		 runThread.start ();
		    }
		}
	ColorSwirl -- An Example from Teach Yourself Java in 21 Days - run Method
		public void run () {  
		 float c = 0; // initialize the color array
		 for (int i = 0; i < colors.length; i++) {
		  colors [i] = Color.getHSBColor (c, (float) 1.0, (float) 1.0);
		  c +=.02;  } 
		 int i = 0; // cycle through the colors
		 while (true)  {   setForeground (colors [i]);
		 repaint ();
		i++;
		 try { Thread.sleep(50); }
		catch (InterruptedException e) { }
		if (i == color.length) i = 0;
		} // End while(true)
		 } // end run()
	ColorSwirl -- An Example from Teach Yourself Java in 21 Days - paint and update
		public void stop () {
		    if (runThread != null) {
		 runThread.stop();
		 runThread = null;
		    }
		}
		public void paint(Graphic g) {
		 g.setFont(f);
		 g.drawString ("All the Swirly Colors", 15,50);
		 }
		public void update(Graphics g)  {
		  paint(g); }  // No need to clear background as unchanged! -- only color of letters changes
		}  // end Applet ColorSwirl
	Double Buffering to Reduce Flicker - I
		Here you have two "graphics contexts" (frame buffers of the size of the applet), and you construct the next image for an animation "off-line" in the second frame buffer. 
		This frame buffer is then directly copied to the main applet Graphics object without clearing image as in default update()
		Image OffscreenImage; // Place to hold Image
		Graphics offscreenGraphics; // The second graphics context which contains offscreenImage
		offscreenImage = createImage(width,height);
		offscreenGraphics = offscreenImage.getGraphics();
	Double Buffering to Reduce Flicker - II
		The previous statements could be in init() while in paint(), one will construct applet image in offscreenGraphics as opposed to the argument g of paint(). So one would see statements such as:
		e.g. offscreenGraphics.drawImage(img,10,10,this);
		Finally at end of paint(), one could transfer the double buffered image to g by
		g.drawImage(offscreenImage,0,0,this);
		One would also need to override the update() method by
		public void update(Graphics g) {
		paint(g);
		}
	Events in the java.awt -- Mouse, Keyboard Interaction - I
		Events ( distinguish these from Exceptions!) are the way the AWT interacts with the user at mouse or keyboard.  
		The AWT calls particular event handlers (analogous to exception or interrupt handlers) when user interacts with system in particular ways.
		The handling is defined in packages java.awt and java.awt.peer (the machine dependent stuff) with method handleEvent() in class Component(peer)
		One could add additional capability here for systems with nifty virtual reality and other user interfaces but we won't cover this here!
		public boolean keyDown(Event evt, int key) { } method is called when you press keyboard.
	Events in the java.awt -- Mouse, Keyboard Interaction - II
		The Event class has various special class (static) variables defined including
		Event.F1 -- the F1 key
		Event.UP The Up arrow etc.
		The Component class (grandparent of Applet) has a rich set of Event handlers which you should override if you wish to process particular input
		public boolean mouseDown(Event evt, int x, int y) {
			anchor = new Point(x,y); // record position of mouse click
			return true; // must do this
		}
		Other handlers are mouseDrag, mouseEnter (enters current component), mouseExit, mouseMove (with its button up), keyUp, keyDown
	Structure of the java.awt GUI Components - I
		In Java, the GUI (Graphical User Interface) is built hierarchically in terms of Components -- one Component nested inside another starting with the smallest Buttons, including Menus, TextFields etc. and ending with full Window divided into Frames, MenuBars etc.
		Not all useful Classes are inherited from Component. For instance Menu inherits from MenuComponent (interface) --> MenuItem --> Menu
		One also needs a set of methods and classes to define the layout of the Components in a particular Panel
		LayoutManager is a java.awt interface with several particular layout strategies implemented as classes under this interface
		The Container class has methods to interact with LayoutManager classes
	Structure of the java.awt GUI Components - II
		In the simplest use of AWT, one could add a Button to an Applet (grandchild of Container) using in the init() method for Applet
		Button b = new Button("Are You Feeling well");
		add(b); // add() is a Container method
		The various added Components are put in the panel by the LayoutManager using order in this they were added
		A Final critical part of the AWT is the actions generated by these components which are processed by overriding the action() method in Component
		action(Event e, Object Anyargemightliketoreturn);
		We define extra events -- such as those connected with scrolling or selecting buttons to those of basic mouse/keyboard
	Picture of the AWT Component Class and its inheritance
		This is incomplete!
	Some Simple AWT Components -- label,button
		add(new Label("aligned left")); // default alignment
		produces a text string using constructor Label of Label class
		add(new Button("Grade is A"));
		add(new Button("Grade is B")); // surely this is lowest grade for a course on such an easy language?
	AWT Components -- Checkbox 
		Checkbox's are on-off toggles implemented as
		add(new Checkbox("Red"));  // defaulted to false as initial value which can be changed by user
		add(new Checkbox("Green"));  // defaulted to false
		add(new Checkbox("Blue"),null, true);  // set to true -- second argument required but ignored here
	AWT Components -- Radio Buttons , CheckboxGroup
		Radiobuttons are identical to Checkbox's but grouped so that only one in a group can be on at a time. They use same class for buttons but add CheckboxGroup class
		CheckboxGroup cbg = new CheckboxGroup(); // cbg is group for one set of radiobuttons
		add(new Checkbox("Red", cbg, false)); // cbg is second argument!! 
		add(new Checkbox("Green", cbg, false));
		add(new Checkbox("Blue", cbg, true)); // Only one in group cbg can be true
	Some Further AWT Components -- typical subunits of panels
		Choice is a class that gives a menu where you choose from various items
		TextField is a simple class where user can enter information into fields
		TextArea is a somewhat more sophisticated text entry area which are scrollable and so useful where amount of data to be entered is unlimited
		List is another child of Component that is similar in use to Choice but gives a fixed size list which can be scrolled and where you can select one or more entries
		Scrollbar is a class that defines a horizontal or vertical scrollbar. Note this is distinct from scrollbars that come with TextArea and List
	Some Further AWT Components -- Canvas, Window - I
		Canvas is a simple class which are used to draw on as in artist's canvas. They cannot contain other components
		Upto now, we have described how to build typical Applet panels inside browser window. There are classes that allow one to generate complete windows separately from the browser window
		Window Class has subclasses Frame and Dialog
		Frame("TitleofWindow"); // creates a window with memubar and given title
			Note Frame is a Container and can thereof be defined hierarchically with components that are laid out by LayoutManagers
	Some Further AWT Components -- Canvas, Window - II
		MenuBar(); // defines a menubar which can be used in a frame
		Menu("Type"); // defines a menu which can itself have hierarchically defined submemus
		Dialog(Frame, String Title, boolean mustbeansweredatonceornot); // defines a dialog box
		Dialog boxes are used for transient data 
			Issue warning to user or require (third argument true) user to verify some action etc.
	Actions associated with Components in AWT - I
		We already discussed handling Mouse and Keyboard Events. These AWT components come with new actions which need to be handled with an action() method in your applet
		Put action ( a method of class Component) in Container instance that is at lowest possible level so you can customize action to things in that Container
		action(Event evt, Object arg)'s are looked for in same fashion as exceptions. Scan up Containers looking for a method of this name. Scanning stops when you find an action and that method returns true
		evt.target holds the object that caused the Event
		Object Arg returned depends on particular Component invoked
		There are further evt.id's associated with the various peculiar Components -- see description of class Event for current detailed description.
	Actions associated with Components in AWT - II
		Suppose we have a bunch of buttons in a particular Container saying Red, Green, Blue as we illustrated earlier. Then a good action method would be
		public boolean action(Event evt, Object arg) {
		 if( evt.target instanceof Button)
		    changeColor((String) arg);  // changeColor Supplied by the user processes color defined by text string used in defining buttons
		  return true; // tell runtime that this event fully processed
		}
		void changeColor(String bname)  {  // A suitable user routine to be called by above action
		if( bname.equals("Red")) setBackground(Color.red);
		else if (bname.equals("Green")) setBackground(Color.green);
		else if (bname.equals("Blue")) setBackground(Color.blue);
		else  setBackground(Color.pink); // our favorite color
		}
	Layout of Components in a Panel
		The various panels in a container are laid out separately in terms of their subcomponents
		One can lay components out "by hand" with positioning in pixel space
		However this is very difficult to make machine independent. Thus one tends to use general strategies which are embodied in 5 LayoutManger's which all implement the LayoutManager Interface. One can expect further custom LayoutManager's to become available on the Web
		setLayout(new FlowLayout()); //  creates a basic flow layout in your panel -- actually unnecessary as default
		Other available LayoutManager's are GridLayout(), BorderLayout() (default for Frame's), CardLayout() (Used for dynamic layouts) and GridBagLayout() (the most flexible)
	Description and Example of BorderLayout
		BorderLayout has five cells called North South East West Center and components are assigned to these cells with the add method. As used in a window, one would naturally use:
			add("North", new TextField("Title",50));
			add("South", new TextField("Usuallyreservedforstatusmessage",50));
		Remember this is default for a Frame Container
	Brief Description of Four Other LayoutManager's
		FlowLayout is a one dimensional layout where components are "flowed" into panel in order they wre defined. When a row is full up, it is wrapped onto next row
		GridLayout is a two dimensional layout where you define a N by M set of cells and again the components are assigned sequentially to cells starting at top left hand corner -- one component is in each cell
		GridBagLayout uses a new class GridBagConstraints to customize positioning of individual components in one or more cells
		CardLayout lays out in time not space and each card (Displayed at one time) can be laid out with one of spatial layout schemes above
	Hierarchical use of LayoutManagers
		Layout's can be made very sophisticated using an hierarchical approach
		setLayout(new GridLayout(1,3,10,5)); // Number of cells in y, Number in x, Horizontal gap, Vertical Gap
		subpanel1 = new MysubpanelClass(); // Add arguments to make subpanel1 special
		subpanel2 = new MysubpanelClass();
		add(Some Simple Component such as a Button);
		add(subpanel1);
		add(subpanel2);
		..........
		Class MysubpanelClass extends panel { // has constructor
		MysubpanelClass() { // that includes another layout such as
		setLayout(new GridLayout(7,7,5,5); // etc.
	Networking, Web Access and I/O in Java
		This area will evolve rapidly as existing I/O systems get linked to Java with special classes such as those needed to link MPI (HPCC Message Passing) or Nexus (Well known distributed memory thread package)
		One can establish Web Connection with URL class and access documents there
		One can set up a more general URLConnection for more general input/output operations through Web(HTTP) protocol
		One can set up a Socket link which is permanent rather than on again off again Web client-server interaction
		One can send messages and so transfer information between different Applets
		One can (outside Netscape) access same functionality as usual UNIX stdio library
	Accessing URL's in Java -- URL, showDocument
		First you set the URL in various ways using something like
		String npacurl = "http://www.npac.syr.edu/index.html";
		try { theinputurl = new URL(npacurl);  } // URL class is in java.net
		catch ( MalformedURLException e)  {
		    System.out.println("Bad URL: " + npacurl); }
		where you are meant to test to see if URL is legal!
		The simplest thing to do now is to see this page with
		getAppletContext().showDocument(theinputurl, Frame targetdisplayframe); 
	Accessing URL's in Java -- URLConnection
		More interesting is to access a URL and process the information there!
		This is done using a stdio like stream (generalizes pipe) which is supported in java.io package with usual buffering capabilities
		There are methods in class URL -- InputStream in = url.openStream(); // opens an InputStream associated with given url 
		More general is instanceofURL.openConnection() establishes a general connection and returns an instance of class URLConnection 
			This provides most general HTTP connectivity and indeed has some advantages over sockets as these are subject to special security restrictions in Netscape's current implementation 
		Note that one can connect not just to HTML files but also to CGI scripts i.e. programs at server and so obtain flexible connectivity
	Input/Output in Java -- InputStreams -- I
		In java.io, there are several classes that throw IOExceptions -- please catch them!
		The class InputStream is basic and it has methods such as read() skip() (bytes in a stream) available() (bytes remaining in stream) close().
		InputStreams can be created from memory
		inputStream s = new ByteArrayInputStream(buffer, 100, 300); // creates a stream of 300 bytes in locations 100 to 399
		More usefully, they can be created from a UNIX file
		inputStream s = new FileInputStream("/usr/gcf/greatidea");
	Input/Output in Java -- (Filter etc.)InputStreams -- II
		There is a clever class called FilterInputStream which can be used to add value to a raw InputStream. You can define your own filters but important ones provided are:
			BufferedInputStream  -- establishs an intermediate buffer to service stream 
			DataInputStream -- allows one to address stream in terms of higher level constructs -- namely read a line, read a long integer etc.
			LineNumberInputStream -- adds line numbers to a stream
			PushbackInputStream -- allows one to "unread" a character and put it back on the input stream
	Accessing URL's in Java -- InputStreams and URLConnections
		String line;  // Here is a useful example
		try { URLConnection conn = npacurl.openConnection();
		  conn.connect(); // could set options after creation of conn in above line and before connect method invoked
		  InputStream in = conn.getInputStream(); // establish an InputStream on this connection
		  DataInputStream data = new DataInputStream(new BufferedInputSream(in)); // set up two filters allowing both buffering and line access to InputStream
		  while ((line = data.readline()) != null )  {
		    System.out.println("line");   } // print out line -- obviously put your favorite line processing here
		} catch (IOException e)  {  process error }
		Note one useful exception EOFException which can be caught and remove testing in read loops
	Use of Native Classes to Speed Up Execution
		One can essentially augment supplied Java Runtime by supplying your own C or other code implementing a particular functionality in a higher performance mode
		This of course generates machine dependence and should only be used if really needed
		First for classes you wish to implement in native fashion, put in your java code lines like:
		public native mustgofast(arguments); // default functions
		static { System.loadLibrary("faststuffhere"); } // static and in class and so runs only ONCE! and loads the native code
	HPCC and Java -- High Performance HPjava -- I
		The activities that has gone into defining High Performance Fortran (HPF) and HPC++ can be reexamined for Java which is quite well suited for parallelism as
			It essentially has task parallelism built in with Applet mechanism
			As there are no pointers, it is easier to define data and other implicit parallelism so it can be efficiently implemented
		Interesting Compiler challenge independent of parallelism, is to produce efficient code from bytecodes. Here a technique called "just in time" compilation does compilation at runtime and can probably increase performance of Java code by a factor of 10 or so to become comparable with performance of C
		Surely many will produce Fortran C PASCAL to Java translators so you can webify your existing applications
			This is an alternative to wrapper technology as in native classes
	HPCC and Java -- HPjava -- II
		Current runtime (java Threads) assume shared memory but there are interesting possible distributed memory and distributed shared memory implementations
		One can imagine a dataparallel interface where methods would implement array and other data parallel operations with distributions specified by annotations as in HPF
			Here one has a HPJava translator that produces java "plus message passing" code
		Important implication for parallel CORBA using Java classes to interface with CORBA
		Java based servers will allow data and task parallel Java to implement World Wide compute webs