Sun's language for building Web applets may be a trendy item, but in many ways it's a strange brew
Andrew Singleton
Java, a language created to build applications that will leap across the World Wide Web to the far corners of the globe, is a bona fide phenomenon (see
"Wired on the Web,"
January BYTE). But can anything live up to the hype surrounding Java? We tried building a simple Java program to see just how far beyond Hypertext Markup Language (HTML) this new language can carry the Web.
The Language of Java
Java is based loosely on C++. Java programs come in the form of applets that load into a Java-enabled Web browser. The term
applet
comes from the
<applet>
tag that appears in an HTML document and tells the browser
to download the appropriate Java code.
The Java source files are compiled into .CLASS files that contain portable byte code. Client machines, such as Web browsers, run the Java byte code using a virtual-machine interpreter or emulator. The virtual machine is a simple stack machine, and the programs are structured to allow clients to verify that downloaded code contains no illegal references or grammatical errors.
The client side will get even more interesting with the promised arrival of a just-in-time compiler, which will convert the verified byte code into native machine code as it loads. This should greatly improve applet performance.
The goal for Java is to create a language that's completely portable, running correctly on every computer that downloads it. But most languages have some ambiguity in their specification. For instance, a C++ compiler can implement an
int
data type as a 16-, 32-, or 64-bit binary number, depending on the host machine. This ambiguity causes porting
problems, since Java defines an int as 32 bits on all platforms. Other sources of ambiguity in data types, expression evaluation, and syntax will be ironed out as the language specification is completed.
Java Is Not C++ Lite
There are important, intentional differences between Java and C++. To attach libraries, base classes, and referenced code, Java uses
import <
packagename
>
. This single statement performs both an include (to define the classes at compile time) and a load/link at run time. The imported classes can import additional classes, and so on.
Java loads all code dynamically at run time and loads the code class by class. When the Java compiler compiles a source file, each class comes out in a separate .CLASS file. All functions must be methods of some class. For instance, math functions, such as sine, are implemented as methods of the class
Math
.
Java acts somewhat like a BASIC or Lisp interpreter when handling objects. To ge
t a new object, you simply type the statement
<
variable
> = new <
object
>
. This gives you a new object off the Java heap. There is no delete operation; a garbage collector cleans up after you. Nor is there any direct access to memory regions--you cannot allocate memory in-line, and you cannot use C-style pointers.
Inheritance in Java is implemented with the
extends
keyword (see the listing
"Using
extends
to Implement Inheritance"
), but there is no support for multiple inheritance. To enable algorithms that operate on multiple types, Java uses
interfaces
, or enumerated sets of methods. Interfaces are a looser link than C++ inheritance or templates, and they provide a higher likelihood of successfully implementing polymorphic classes.
The standard Java class library started out small, but it's getting bigger by the day. It implements the math and I/O functions in the standard C library; a number of data types, suc
h as Integer, String, and Hash Table, with their attendant methods; threads; sockets; and a GUI system called AWT.
JDK
We downloaded the preliminary language specification and the Java Development Kit from the Sun Microsystems server at
http://java.sun.com
. The JDK is available free from Sun, and Win32 and Solaris versions are available. We also downloaded the Netscape Navigator Web browser, version 2 Beta-4, from the Netscape Communications serv-er at
http://ftp1.netscape.com
. Netscape Navigator 2.0 is the first major Web browser to contain an interpreter for Java applets.
The
JDK comes with
a Java compiler (called javac), an experimental alpha release of the Java debugger, a Java applications interpreter, and an applet viewer that can run applets without a Java-enabled browser. The JDK is a primitive, character-oriented environment. Fortunately, Borland, Symantec, and several other companies have announced plans to produce more sophisticated graphical development environments, hopefully with vastly improved debuggers.
The experimental debugger is a command-line program with a single output console. The user has to keep track of threads by number and keep track of objects with a 32-bit ID. The debugger includes breakpoints, but not single-stepping. When we tried the Windows version, it left hanging threads and windows. We found this debugger useless and soon resorted to the age-old debugging method of including print statements at strategic points in our code. Even when running graphical applications, Java provides a line-mode console that's convenient for debugg
ing.
The javac compiler performed well, turning .JAVA source files into .CLASS byte-code files suitable for the Java interpreter, the applet viewer, or the beta Netscape Navigator 2.0 Java-enabled browser. It generated useful error messages about the innumerable mistakes we made in the source code, and it caught some problems with unitialized variables.
The applet viewer runs applets as they would be called in a Web browser. It needs an HTML file that includes the
<applet>
tag and parameters. The applet viewer is a more complete implementation of Java than the Netscape 2.0 beta version, and it's integrated with the debugger.
The documentation available from Sun includes the Java language specification and a complete listing of the Java library classes and methods in PostScript form. It does not include a tutorial explaining what those methods do or how they might be strung together, so we spent a lot of time looking at sample applications provided by Sun and programmers aro
und the Internet.
The Program
We ran the Win32 version of the JDK on a Windows NT machine. We implemented a simple pop-up help applet, with the intention of using Java's object features to subclass these pop-up boxes for more-involved future applications.
To begin, we selected a list component from the Java AWT GUI library to hold the pop-up help and made a floating frame class to wrap around this component as a pop-up window (see the listing
"Using
extends
to Implement Inheritance"
). Rather than starting from the sometimes-restrictive confines of a browser applet, we created a short application to run under the Java command-line interpreter and call the
FramedTextList
for debugging. As an application, the frame pops up conveniently over the command-line console.
Then we ran into a snag. The size of the listbox (for setting the frame size) was coming out all wrong, and there was no indication of how it was calculated. On our Wind
ows NT machine, the AWT components were clumsy. The floating frames just piled up in the upper-left corner of the screen. Like many sample applets, ours looked more like a high-school homework project than the work of a highly trained professional.
On the positive side, however, the Java memory manager lets you dispose of a frame without going back to clean up the list object that constructed it. This would be a sticky situation in C++.
Next we added a
FramedURL
class to retrieve the text of an http uniform resource locator (URL) and insert it into the box (see the listing
"Creating an Object from URL Text"
). Java includes native support for socket-based networking, including a class for
URLconnection
. Java requires a "try" and "catch" operation to surround any action that might generate a run-time error. Then we built an applet, called
PopNetHelp
, to run from inside a browser, and disguised as a button (see the listing
"The
Po
pNetHelp
Applet
"). We then created a test HTML file with the following tag:
<applet code="PopNetHelp" width=40
height=27>
<param name="prompt" value="HELP">
<param name="helpurl" value="http://www.money.com/Java/
example.txt">
[Help Button]
</applet>
[Help Button]
is for non-Java browsers to display instead of the Java applet.
When we ran this assembly in the applet viewer, the Help button came up on the screen as planned. But when we pushed the button, a bunch of error messages flew by on the console. One of them was "SecurityException": The program had violated security by attempting to load a URL from outside the applet domain. When we copied all the files to the same Web server, the help text loaded properly.
We then entered the URL of the test page into a beta version of the Netscape 2.0 Java-enabled browser. After a long pause as Netscape loaded and initialized the applet, th
e button appeared. We pushed the button but nothing happened, except for a message at the bottom of the browser window saying, "Retrieving example.txt--0 bytes per second." The help popped up immediately after a second push of the button, however. Conclusion: A few gremlins lurk in Netscape 2.0's beta code.
After many ups and downs, Java allowed us to build a better Web front end than we could have built with HTML, and it lives up to its billing as a friendlier language than C++. However, the resulting applications are still slow and unattractive compared to more conventional software. The JDK and related documentation are crude and unfinished. Rapid improvement in all areas seems inevitable--and certainly worth waiting for.
PRODUCT INFORMATION
Java Development Kit..........free
(downloadable from Web site
at address shown below)
Sun Microsystems Computer Co.
Mountain View,
CA
Phone: (800) 821-4643; (800) 821-4642 (in California) or (415) 960-1300
Fax: (415) 969-9131
Internet:
http://www.javasoft.com
Circle 1154 on Inquiry Card.
HotBYTEs
- information on products covered or advertised in BYTE
-- Browsers allow Java applets
limited access to the local disk.
Forget having a local configuration or a persistent database.
-- Browsers do not allow
applets to print.
-- The Java security model allows
applets to open socket connections
only to their source domains. Free-ranging network software must
run as a local application.
-- Looks don't port.
The AWT graphics class must be tailored
to a particular machine and OS, and user-interface results vary widely.
import Java.awt.*
public class FramedComponent extends Frame
{
protected Component cmp;
// Constructor:
public FramedComponent(Component c)
{
cmp = c;
// add the component at the top (north)
add("North",cmp);
}
....
// Make a URL object from String u
try {URL url = new URL(u);}
catch (MalformedURLException e) {};
// Console message for debugging
System.out.println("Retrieving " + u);
// Open the URL and assign it to a new variable
try {URLconnection uc = url.openConnection();
// Read the result as a stream
InputStream instream = uc.getInputStream();
// make a buffer of the correct size
byte[] buf = new byte[instream.available
()];
if (instream.read(buf)) > 0
{
init(buf); // load the FramedTextList
}
} catch (IOException e) {}
...
public class PopNetHelp extends Applet
{
String prompt;
String helpurl;
// the browser calls the init() method on loading
public void init()
{
// get the prompt & URL from the HTML
prompt=getParameter("prompt");
prompt=getParameter("helpurl");
add(new Button(prompt));
}
public boolean handleEvent(Event evt)
{
if (Event.id==ACTION_EVENT)
// was a button pressed?
if (evt.target instanceof Button)
{ // Pop up a frame
try { popup = new FramedURL(helpurl);}
catch(IOException e) {};
return true;
}
else return false;
}
}
Will Java grow into a complete applications development environment?
Java supporters have positioned it as an improvement over C++ for
object-oriented development and as a competitor to Visual Basic for
graphical development. But Java needs a native-code compiler and a
graphical development environment to really challenge C++ and Visual
Basic.
FEATURE JAVA C++ VISUAL BASIC
=====================================================================
Components plus scripting X O X
Extensible objects X X O
Portable source code X X O
Portable object code X O O
Native-code compiler N/A X O
Dynamic memory manager,
garbage collector X O X
Secure mode X O O
Learning time Medium Long Short
Mature tools and libraries O X X
KEY:
X = yes; O = n
o; N/A = not yet available.
screen_link (54 Kbytes)
Among the Java development tools are a source code editor (
upper left
), technical support on one of Sun's Web sites (
upper right
), a sample browser page with an embedded Java applet (
lower right
), and the command line (
lower left
) for running the compiler.
The applet itself appears in the rectangular window in the middle of the screen. The smaller window below is the applet viewer, which lets developers preview the final product.
Andrew Singleton is p
resident of Cambridge Interactive (Cambridge, MA), a developer of on-line services on the Internet. You can reach him on the Internet at
andy@money.com
or on BIX c/o "editors."
A Hot Cup of Java
