All Databases MacTech Vol 12-1996

Java Intro

Volume Number: 12

Issue Number: 3

Column Tag: Internet Tools

Writing Java 102

Moving beyond “Hello, World” - way, way beyond

By Richard Cardona, TradeWave Corporation

Note: Source code files accompanying article are located on MacTech CD-ROM or

source code disks.

In an attempt to carve a path through the forest of current announcements, discussion,

and interest in Java, let me lay out the components of Java and then proceed to a

discussion in detail [see also MacTech Magazine 12.1 (January, 1996) 51-57 -

man].

Java consists of a programming language, a virtual machine

architecture, and a runtime environment developed by Sun Microsystems. The

most common instance of Java technology is a Web browser running applets with

their own interface. These applets are downloaded in-line on a web page, just like

images. Thus applets can be served by all standard web servers, including those on the

Mac, as just another MIME type.

HotJava was the first Java-enabled Web browser. It was authored by Sun

completely in Java, to prove that complex applications could be written in Java. A

Java-aware browser runs applets with the help of the Java Virtual Machine, an

interpreter of bytecodes produced by a Java compiler. Although bytecodes are

currently interpreted, Sun expects to provide just-in-time compilation to improve

speed and rival native applications.

The most compelling reason for applets is that they let dynamic web media be

targeted for content processing by the client. Unlike today’s Common Gateway

Interface programs (CGI), Java applets execute on the client machine in the browser.

This relieves the burden on many servers that are far too busy already, and reduces

server cost, in a sense by introducing distributed computing over the net.

Lest the idea of horrible viruses loom in your mind, consider that applets run

under a layered, well-defined environment with user-controlled security

restrictions. Applets could be authenticated with public-key technology and encrypted

in transit (this is not implemented yet).

The built-in Java classes are very net-savvy, and provide for firewalls,

proxies, and default access control (for accessing the originating server only).

Depending on security, applets can seamlessly access other program components and

data, such as images and databases, across the Internet. These features are why Java

has been dubbed the Internet programming language.

As of this writing, Sun is working toward implementing the Java Virtual Machine

under the MacOS, and is also porting its HotJava browser to the Mac. Netscape is also

implementing a plug-in that allows Java applets to run in Navigator 2.0. On the

development side, Metrowerks and Symantec both announced Java support by adding a

new compiler to their suite of compiler plug-ins [and Borland recently made a similar

announcement - jaw]. Natural Intelligence of Cambridge, MA, was first to bring Java

to the Mac, by delivering their Java IDE, Roaster, at MacWorld San Francisco in

January; they implemented their Java environment working entirely from the

published Java specification. All compiler vendors are pledged to support applet and

stand-alone application development, integrated debugging, and a standard Java Virtual

Machine environment.

Another related component, JavaScript, became necessary to bridge the

technical boundary between an HTML author with scripting skills and a full-fledged

programmer. The Java language is too unwieldy for a scripting-level webmaster,

partially because it also requires an external development environment. JavaScript is

the result of Sun and Netscape collaborating on an object-based end-user scripting

language capable of generating Java applets transparently. To minimize the need for

learning several scripting languages on several platforms, and to leverage the name

recognition of Java, Sun and Netscape agreed on enhancing Navigator 2.0’s

LiveScript. JavaScript and the Java language share no syntax or similarities.

JavaScript will most likely appeal to AppleScript-savvy users as a way to script

Netscape.

History of Java

Originally named Oak at Sun Microsystems, Java was intended to be the simple,

object-oriented, distributed language for the much-hyped $500 Internet set-top

boxes. At first, C++ was the language of choice for these consumer devices; but Sun

decided that constant recompilation of code, version control, and problems with

pointers, memory leaks, and security, to name a few, were defeating the development

effort. James Gostling, a principal architect of Java, decided he needed a new small,

reliable, portable, distributed, interactive language - one that would facilitate

distribution of updates, would attend to network security, and would not be as fragile

about changes in source code of classes.

The fragility of compiled code, even with shared libraries, was a primary

influence in Oak’s design. Oak needed the functionality of binary-compatible libraries

similar to IBM’s System Object Model (SOM) in a small footprint. Through its

simplicity (and real-world shipping schedule!), Oak fulfilled its intended purpose for

coding small, distributed, robust, platform-independent agents.

Unfortunately, the market and network infrastructure for Sun’s appliances did

not materialize. In late 1994, Oak was renamed and redirected towards the Internet. A

large part of the success of Java has been Sun’s brilliant positioning for the Web.

After brainstorming for the name “Java”, Sun sold the idea of applets to Netscape, and

devised the plan to write the HotJava browser to prove the language.

Amidst significant press coverage and extravagant predictions, Sun’s strategy had

worked. All the talk was of how to develop desktop applications that always load the

latest code from a server, and how applications could even be used on a metered basis

vs. purchased and licensed, while Java itself was still in beta. Even Microsoft, aware

of the momentum of Java, and at least a few months behind in defining a similar

language, redirected Blackbird, its Visual Basic/OLE-centric paradigm, and embraced

Java in its Internet strategy. (Technically, Blackbird lacked the cross-platform

momentum of Java because of the dependency on OLE; Blackbird was renamed to

Internet Studio and now focuses on implementing custom publishing needs with OLE

objects and Visual Basic support.)

Some Mac enthusiasts feared that Java was in direct competition with OpenDoc

and Cyberdog. In reality, though, Java represents another type of OpenDoc data that

Cyberdog interfaces with over the Internet. Cyberdog [see MacTech 12.2 (February,

1996) 35-39 - man] is a collection of OpenDoc parts Apple is designing to show how

document-centric computing works with the Internet. Apple or a third party will

simply implement an OpenDoc part that knows how to interpret Java bytecodes, and

presto, Java will be accessible with OpenDoc. Parts for OpenDoc can be written in a

variety of programming languages because there is a language-neutral binary

interface for linking and binding with a part’s interface. Since Java is a programming

language, it’s likely that a proper interface for developing OpenDoc parts in Java will

be defined; Java allows for external routines to be implemented on a specific platform,

and this would be the facility necessary to interface with OpenDoc.

Language Overview

The Java language was still in beta specification at the time of this writing. Java is

undergoing peer review on the Internet with enhancements made to usability and

security. It is unlikely that any major changes to the language will occur in the

interval before this article appears, though. Most of the enhancements and changes are

in the large number of built-in classes the Java runtime environment comes with.

Java as a language is difficult to describe because it resembles so many languages

including SmallTalk, Objective C, C++ and Eiffel. Fortunately, the similarities go a

long way toward learning Java for most programmers. Java was developed in the same

time frame as Apple Dylan, and represents a trend of reversal against the size and

complexity of C++. In this regard, Java is a simpler, strongly typed, strongly

object-oriented, memory management-safe, exception- and thread-aware language.

I’m going to start by using C++ as a reference and describing what is not in Java.

From there I will list some features Java adds to help you program more efficiently.

Of course, nothing replaces actually coding in the language itself, but this is a good

start.

Java, as opposed to C++, has:

• no preprocessor and no typedefs

• no stand-alone procedures and functions

• no structures or unions

• no unsigned numeric types

• no automatic type coercions, e.g. long to short (with data loss)

• no multiple inheritance and operator overloading

• no goto statement (goto is reserved)

• no pointers

• no multiple expression comma operator

This may sound a little limited and constrained, but let me describe what Java

does offer. Because Java doesn’t allow #define or typedef, when you look at a data

member, you don’t have to guess what it is; it’s either a built-in type or a class.

Without structs and unions, you must organize your data structures by using classes.

By disallowing stand-alone functions, Java forces you to organize your access to your

data and provide a public interface to class members. This means you must play by

true object-oriented rules.

Another nice feature to help you (my favorite) is labels on continue and break

statements. When you’re deep in several embedded loops, you no longer have to set

flags to let you know when to break out of each one; you can break out to the exact level

you want. Many routines have program states, graphics states or other conditions that

must be restored before exiting. If you use try/catch blocks you will see code

Referenced by (3):