cis 5930-04 - spring 2001 part 2: overview of java
DESCRIPTION
CIS 5930-04 - Spring 2001 Part 2: Overview of Java. http://aspen.csit.fsu.edu/it1spring01/ Instructors: Geoffrey Fox, Bryan Carpenter Computational Science and Information Technology Florida State University Acknowledgements: Nancy McCracken Syracuse University. Plan of Java Tutorial. - PowerPoint PPT PresentationTRANSCRIPT
CIS 5930-04 - Spring 2001
Part 2: Overview of Java
http://aspen.csit.fsu.edu/it1spring01/
Instructors: Geoffrey Fox, Bryan CarpenterComputational Science and Information
TechnologyFlorida State University
Acknowledgements: Nancy McCrackenSyracuse University
Plan of Java Tutorial
Part 1:– Overview including History of Java Development– Overall Java Philosophy and Features including
security etc. Part 2:
– Java Programming Language– Object Oriented and Class Structure– Exceptions
What is Java, in a Nutshell? What is Java?
– A simple, object oriented, distributed, interpreted, robust, safe, architecture neutral, portable, high performance, multithreaded, dynamic, programming language.
Java is interesting because– It is both a general purpose object-oriented language along
the lines of C++, and– It is particularly designed to interface with Web pages and
to enable distributed applications over the Internet. The Web is becoming the dominant software
development arena; this will drive Java as the best supported, most widely taught language.– Particularly good as a language for K-12 teaching.– Even outside the Web, e.g. in scientific computing, Java is
as good and in some respects better than other languages.
Java is an important language in the World
The Java language has several good design features– secure, safe (w.r.t. bugs), object-oriented, familiar
(to C, C++ and even Fortran programmers) Java has a very good set of libraries covering
everything from commerce, multimedia, images to math functions (see http://math.nist.gov/javanumerics)
Java has best available electronic and paper training and support resources, growing labor force trained in Java.
Java is rapidly getting best integrated program development environments.
Java naturally integrated with network and universal machine supports powerful “write once-run anywhere” model.
Java is also important in computer science
Increasingly, Java is the language used for important computing paradigms that support applications:– object-oriented computing, event-driven computing,
distributed objects, linkage to databases, visual/component computing, client/servers, networking, multimedia computing . . .
So Java is an important language to include at an advanced computer science level, along with other languages like C++ that help students to get jobs.
But the good design features of Java also make it suitable for teaching basic computer science concepts: algorithms, data structures, software design, . . .– See the (old, but still relevant) discussion by Doug Lea at
http://gee.cs.oswego.edu/dl/html/javaInCS.html
Architecture of Java Applications
Java applications are compiled and run on a machine just like any other general programming language such as C/C++. No web server or network are required although Java applications may also use network connections for distributed computing.
Java code
is compiledto produce
byte code
run by Javainterpreterto produceresults
Java code
is compiledto produce
native code
run directlyon machinefor betterperformance
Java Applications in a Nutshell Java programs written in a file with extension
“.java”.
Applications are .java files with a main() method. This is called by the Java system.
Compile and run a Java application (using bytecodes):– Run the compiler on a .java file: javac MyProgram.java producing a file of Java byte code,
MyProgram.class
– Run the interpreter on a .class file: java MyProgram which executes the byte code
The tools javac and java are part of JDK.
The Simplest Java Application: Hello,World!
Since Java is object-oriented, programs are organized into modules called classes, which may have data in variables called fields, and subroutines called methods.
class HelloWorld { public static void main (String[] args) { System.out.println(“Hello World!”); }}
Each program is enclosed in a class definition.Each program is enclosed in a class definition.
main() is the first method that is run.main() is the first method that is run.main() is the first method that is run.main() is the first method that is run.
The notation class.method or The notation class.method or package.class.method is how package.class.method is how to refer to a public method to refer to a public method (with some exceptions).(with some exceptions).
Syntax is similar to C - Syntax is similar to C - braces for blocks, semicolon braces for blocks, semicolon after each statement.after each statement.
Java Applets Java applets are classes written in Java that are not
intended to run as stand-alone programs (like applications) but as subprograms of a browser that is already managing a window.
Applets are not trusted by default, so they have several restrictions in running on the client machine
Applets should NOT have a main() method. Instead they have init(), start(), paint(), etc. for displaying on the browser window– no printing or file I/O– cannot connect through the network to any machine but its
own server– any new windows created by the applet have a warning
label
Architecture of Java Applets Browsers (HotJava, Netscape 2.0/3.0/4.0,
Microsoft IE ...) supporting Java allow arbitrarily sophisticated dynamic multimedia applications inserts called Applets, written in Java, to be embedded in the regular HTML pages and activated on each exposure of a given page.
web serverJava code
is compiledto produce
applet codes,part of webdocumentcollection
web client, running browsersuch as Netscape or IE
executes (restricted)applet code to displayin browser window
Internet
The Simplest Java Applet: Hello, World!
Java applets can call methods to display on a screen (within the browser window). One way is to call the method drawString() from the standard method paint().
import java.awt.Graphics;
public class HelloApplet extends java.applet.Applet { public void paint (Graphics g) { g.drawString(“Hello World!”, 5, 25); }}
The The importimport statement (similar to an statement (similar to an includeinclude) allows the use of methods from the ) allows the use of methods from the GraphicsGraphics class . class .
The The paint()paint() method displays a graphics method displays a graphics object on the screen - one of the standard object on the screen - one of the standard methods that takes the place of methods that takes the place of main()main() for for applets.applets.
Makes this a subclass of Makes this a subclass of Applet.Applet.
Displaying your applet from a Web page.
Name the file HelloWorldApplet.java. Run the compiler, javac, to get a byte code file HelloWorldApplet.class. Put this in a web directory.
<html><head><title>Simple Hello Page</title></head><body>My Java applet says:<applet code=“HelloWorldApplet.class” width=150 height=25></applet></body></html>
Name of your applet class.Name of your applet class.
The browser will use a rectangle of width 150 pixels The browser will use a rectangle of width 150 pixels and height 25 pixels to display the applet within the and height 25 pixels to display the applet within the other html.other html.
Java vs. JavaScript
Despite the name, JavaScript is a different language from Java, albeit with some similarities.
A JavaScript program is written directly in the HTML page, and executed by the JavaScript interpreter, so also allows dynamic web page content in the browser window.
JavaScript is special purpose - it is an object-based language that deals directly with browser entities like windows, text fields, forms, frames and documents.
JavaScript can respond to browser events like mouse clicks and user-typed text.
JavaScript is fast to write, but not as powerful as Java.
Multi-tier Architecture Distributed applications on the web naturally have a multi-
tier architecture. Java plays a role at all three levels:
– Graphical User Interface and client side analysis systems, including visualization
– Middle layer servers and software integration, including web servers, distributed object servers and other application servers.
– Less important for back end client software, which may be legacy code.
Middle level servers
Client user interface running through browser
Internet
Internet or proprietary network
Backend computing or databases
History of Java Language and Team
Starts in 1991 as Project Green—semi-autonomous task force in Sun focusing on operating software for consumer electronic devices, e.g. smart set-top boxes.
Gosling (creator of Sun NeWS) considers C++ too complex and initiates development of Oak, later renamed to Java. Similarities to UCSD P-system.
A PDA (Personal Digital Assistant), codename “*7”, based on Oak/Java ready in 1993. Green Team becomes FirstPerson, Inc.
*7 proposal to Time-Warner rejected in 1993. 3DO deal falls through in 1994. FirstPerson, Inc. dissolves.
Small group (~30 people, now the Java Team) continues and decides to adapt Oak as a Web technology.
History of Java Language and Team to Dec. 95
Experimental Web browser written in Java, called WebRunner, later renamed HotJava, ready in 1994.
Alpha release of Java and HotJava April '95. Netscape licences Java in May '95 and builds Java
into Netscape 2.0 Beta Java Development Kit (JDK) and first Java
books Summer/Fall '95. Dec 4, 1995 Business Week cover story:
"Software Revolution—The Web Changes Everything" presents Java as a breakthrough force in the expanding Web/Internet.
In next week, SGI, IBM, Adobe, Macromedia and finally Microsoft adopt/license Java. Java is adopted by Web community.
More Recent Java History
1996 JavaOne Developer’s Conference (now held annually in June) focused on applets. Attendance of 5000 people.
1997 JavaOne focused on JavaBeans and new version of the language JDK1.1, both designed to put Java in line with other developments in component and secure distributed web object computing and use of databases.
1998 JavaOne featured attendance of 14,000+ and focused on Enterprise JavaBeans and other Enterprise classes, developing more on server-side applications.
1999 JavaOne had over 20,000 attendees. Continued development of Jini for dynamic networking, embedded devices such as Palm Pilots.
Some Key Java Features
Finally we mention additional features added to Java in more recent versions.
Object-oriented Architecture-neutral Portable Somewhat Interpreted Simple and FamiliarDistributed
Robust Secure High performance Multi ThreadedDynamic
First we discuss original Java base language features as discussed in Java: A White Paper by Sun Microsystems—October 1995 draft by James Gosling and Henry McGilton—enumerates the original design goals of Java:
Java Features—It's Simple and Familiar
Familiar as it looks like C++, but simpler to program.– omits several confusing features of C++ including
operator overloading, multiple inheritance, pointers and automatic type coercions
Adds automatic garbage collection to make dynamic memory management much easier than in C or C++.– No more frees or deletes. No more memory
leaks.
Adds Interface construct, similar to Objective C concept, to compensate for the lack of multiple inheritance.
Small kernel is suitable for Java ports to consumer electronic devices.
Java Features—It's Object-oriented
Java model is sometimes viewed as a C++ subset, with some elements imported from other languages.– This is arguable. In many ways Java and C++ are very
different, and many of the similarities that do exist are at a fairly superficial syntactic level.
Structures, Unions and Functions are absorbed into data and methods of Java classes—Java is simple.
The strength of Java object-oriented model is in simplicity and the extensive class library associated with the system.
Java Features—It's Architecture-Neutral
C/C++ programming in a heterogeneous network environment demands compatibility across several vendor platforms and their compilers.
Solved in Java by designing platform-independent binary representation called Java bytecode—comparable to P-code in UCSD Pascal.
Java compiler reads Java source and generates Java bytecode, which is shipped to user—e.g. on browser request, Jini lookup, etc.
Each client must have a Java Virtual Machine program, which interprets (“runs”) Java bytecodes.
Java Features—It's Portable
Java Virtual Machine model is identical for all platforms.
Sun “owns” the Java Virtual Machine specification—while classes can be added by any user, JVM is Universal.
In C/C++ various integer types match the architecture of machine at hand. Java byte, char, short, int and long are always 8, 16 (unicode), 16, 32 and 64 bits, respectively.– No header files, preprocessors, #define etc.– floating point is always IEEE 754
Java Features—It's Somewhat Interpreted
Java represents a compromise between fully compiled (e.g. C/C++) and fully interpreted (e.g. typical scripting languages) models.
Java “compiler” produces a binary bytecode output which is portable and typically 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”—the JVM—executes this bytecode.
Java Features—It's Robust
Java enforces compile-time type checking and this eliminates some error prone constructs of C/C++.
Pointer arithmetic is eliminated which allows for, e.g., runtime checking of array subscripts, and enforces security of the Java model.
Explicit declarations are always required; argument types of methods are always checked (unlike C). This allows the Java complier to perform early error detection.
Java Features—It's (Hopefully) Secure
Java bytecodes may be shipped across the network and executed on client machines. Security is therefore a critical issue and strongly enforced in Java.
The bytecodes sent across network are verified at the client which prevents evil/corrupted classes from causing problems
Java Features—High Performance
Early Java interpreters performed on-the-fly execution of the Java bytecodes, which gave “moderate” performance. – Initial software was often 100 times slower than C
Performance is improved in newer “just-in-time” JVMs, which compile methods after some number of executions, and save machine code to give compiled-code efficiency thereafter.
Support for generating native machine code out of Java bytecodes also exists (e.g. TowerJ).
The performance of the machine code, generated from Java bytecodes, may eventually be comparable to that offered by typical C/C++ compilers on the same platform.
Java Features—It's Multithreaded
Java model offers multithreading, implemented in terms of the Thread class.
Thread methods offer a set of synchronization primitives based on monitor and condition variable paradigm of C.A.R. Hoare.– One use of Java multithreading in applet programming,
for example, is having several independent but related simulations running concurrently in an applet window. Multithreading is also used internally by the browser to handle multiple document dynamics.
Java Features—It's Dynamic
Java model is more dynamic than C++, closer to Smalltalk or Perl.
Classes (often) don’t need to to be recompiled after implementation of a superclass (or other used class) is updated—binary compatibility.
Classes have runtime representation (available through the Class class) that allows one, e.g., to discover and execute methods of a given object at runtime– In C, can’t even distinguish at run-time whether a
given pointer references, say, an integer or a browser!
Sun's Comparison of Language Features
Good Fair Poor
Java Smalltalk TCL Perl Shells C C++PerformanceSimpleObject-OrientedRobustSecureInterpretedDynamicPortableNeutralThreadsGarbageCollectionExceptions
The Java 2 Platform
Sun are now offering 3 “editions”:
Java 2 platform, Standard Edition (J2SE)
– Develops earlier JDKs
– Available in V 1.3.
Java 2 platform, Enterprise Edition (J2EE)
– Incorporates multiple technologies for server-side and multi-tier applications.
Java 2 platform, Micro Edition (J2ME)
– Optimized run-time environment for consumer products.
Java Books—I
Core Java, by Gary Cornell and Cay S. Horstmann, offers detailed coverage of the whole language and packages for advanced programmers, including the Swing Set. Also Volume 2 gives good coverage of advanced topics such as JDBC, RMI, JavaBeans and security.
Java, How to Program, by Deitel and Deitel, Prentice-Hall, starts with beginning programming concepts and progresses rapidly through Java language. It has the most programming exercises and also has companion teaching multimedia books. The third edition has Swing Set and also the advanced API’s.
Java Books—II
The Java Programming Language, by Ken Arnold and James Gosling, David Holmes, 3rd Edition, Addison-Wesley, 2000, is the classic on the language basics for intermediate and advanced programmers. It covers threads and I/O packages, but not applets or windowing packages.
Java in a Nutshell, by David Flanagan, is the language reference book in the familiar O'Reilly series. One of the better references. Also Java Examples in a Nutshell.
Java Development Environments
These range from simple tools that give a windowing interface to the edit/compile/run or view cycle, e.g. JavaEdit from Dick Chase, on PC’s, . . .
. . . to the elaborate commercial development environments that can also track projects and help generate code for user interface components– Microsoft Visual J++– Symantec Visual Café– Java Workshop from Sun– Borland Jbuilder– Kawa from Tek-Tools
Resources for the Java Programming Language
The Java Language Specification, second edition. James Gosling, Bill Joy, Guy Steele, Gilad Bracha, April 2000. The ultimate reference for hardened computer scientists/compiler writers.
http://www.javasoft.com web site has plenty of references including– Tutorial:
http://www.javasoft.com/books/Series/Tutorial/index.html– Books:
http://www.javasoft.com/java.sun.com/aboutJavaSoft/book-news.html
– Collection of Applets:
http://www.gamelan.com