TweetFollow Us on Twitter

Java Break 2
Volume Number:12
Issue Number:6
Column Tag:Getting Started

More Java Basics

By Dave Mark

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

Last month, we started exploring Java, Sun’s object-programming language that has exploded in popularity over the last year. As a reminder, Java is similar to C++, but with some very important differences. The Java syntax is just like C++, but without the pointer syntax. The Java environment that runs on your machine is multi-threaded, with a low-priority thread that does automatic garbage collection.

Your Java source code will reside in a file ending with the .java extension. Your Java source code will implement a class, or a series of classes. Depending on your development environment, you will compile the classes into a stream of Java byte-code, which will get stored in a .class file, in a series of .class files, in a .zip file, or, in some cases, in a double-clickable application file.

The .class file is designed to hold a single class. If you build a bunch of .class files, you could pay a severe penalty in wasted hard drive space. For example, my 1-gig hard drive has a minimal file size of 17K. If I build 100 .class files, each of which is filled with 100 bytes of byte-code, I’ll end up using 1700K of hard drive space to store 10,000 bytes of byte-code. Not very efficient, eh?

The solution to this problem is the .zip file. The .zip lets you combine a set of classes into a single file of byte-code. Since the .zip format is based on the .zip compression format from the PC universe, you can use a zip utility (like UnZip) to peruse the classes in a .zip file.

To run your byte code, you’ll need a byte-code interpreter. There are several options for doing this. Your development environment most likely came with some form of Java virtual machine. It might be called “javai”, “Applet Runner”, “Applet Viewer”, or something else. The point is, the virtual machine knows how to turn your generic Java byte-code into the equivalent machine code specific to your platform. There are Java virtual machines written specifically for the PowerMac, and Java virtual machines written for 680x0, x86, and Unix platforms as well. The first virtual machines came from Sun. Other vendors are writing their own. Check with your development environment for details on your virtual machine.

Java Reference Basics

Our first Java program this month demonstrates an important and potentially confusing difference between Java and C++. As stated earlier, Java doesn’t support pointers. When you create an object, the variable you associate with the object instance is known as a reference. This example should make this clear.

If you are using CodeWarrior or Caffeine, create a new project file using the appropriate Java stationery. I’m using the CodeWarrior droplet stationery, so my project was named reference.µ.

The CodeWarrior environment ships with a bunch of stationery, including one for Java applets and one for Java droplets. The Java applets stationery lets you create an applet designed to be launched from an HTML file. This is pretty standard stuff and will be supported by most every Java development environment you run into.

The droplet stationery is both cool and different. It lets you turn your Java code into a standalone application, complete with 4-byte creator code and its associated 'BNDL' resources. And if you drop a file or set of files onto the droplet, the names of the files get passed to main() via the argv parameter. I’ll present a small example of this later in the column.

Once your project file is set up, create a new source code file named reference.java and type this source code into the file:

public class reference
{
 public static void main( String argv[] )
 {
 String s1 = "Sample String";
 String s2 = "Sample String";
 String s3 = new String(s1); 

 if ( s1 == s2 )
 System.out.println( "s1 is the same object as s2" );
 else
 System.out.println("s1 is not the same object as s2");

 if ( s1 == s3 )
 System.out.println( "s1 is the same object as s3" );
 else
 System.out.println("s1 is not the same object as s3");
 }
}

Compile and run the program. Here’s the results you should see in your stdout window:

s1 is the same object as s2
s1 is not the same object as s3

Let’s take a walk through the source. These three lines declare references to String objects:

 String s1 = "Sample String";
 String s2 = "Sample String";
 String s3 = new String(s1); 

The first line creates a reference named s1 and also creates a new String object, initializing it with the literal “Sample String”. The second line also creates a reference, this one named s2. The question here is, was a new object created? The answer is no. The compiler checks to see if a literal with the value “Sample String” already exists. Since such a literal does exist, the compiler doesn’t bother creating a new one. It just creates the new object reference (s2) and makes it refer to the original literal. Since there is no way to modify a literal, this strategy is pretty sure-fire. Since pointers don’t exist in Java, the compiler has more freedom in allocating memory for objects.

The third line of the set uses new to force the allocation of a new String object. The String reference s1 is passed as a parameter to the String constructor. So we end up with three String references: s1 and s2 both refer to the same String object, and s3 points to a second String object.

To verify this theory, the first if-else statement uses the == operator to test if s1 is the same as s2. Note that this tests whether s1 and s2 refer to the same object. As proof, the result of the first if-else is:

s1 is the same object as s2

The second if-else compares s1 to s3. Since we used new to force the allocation of a new String object, it is no surprise that the second if-else produces this result:

s1 is not the same object as s3

Take a few minutes to review the String methods; you’ll find them in the API Documentation folder in the file java.lang.String.html. Pay specific attention to the compareTo(), equals(), and equalsIgnoreCase() methods.

Copying an Object

Before we move on to our second example, let’s talk about copying objects for a moment. I was reading through my pile of Java books when I noticed an interesting discrepancy. Several of the books specified that to duplicate an object, you should use the copy() method, inherited from java.lang.Object. Alternatively, some sources recommended that you use the clone() method, also inherited from java.lang.Object.

Being a curious son-of-a-gun, I wheeled over to Netscape and opened up java.lang.Object.html (in the folder API Documentation) to look for clone() and copy(). As it turns out, copy() isn’t there and clone() is there, but marked as protected and couldn’t be called from our main() class above.

So what the heck was going on here? After a few phone calls and emails to my Java buddies, I found out that copy() was dropped from the Java API between beta 1 and beta 2 of Java. I also found out that the clone() method was changed to protected and that a new interface (we’ll talk about Java interfaces in a future column) was created, called the cloneable interface. Basically, if you want your objects to be cloneable, they need to implement the cloneable interface. To learn more about this, check out the file CloneNotSupportedException.html and this URL: http://java.sun.com/JDK-beta2/changes.html

The point of all this isn’t to push the cloneable interface. I was just trying to save you from going through the head-banging exercise I just went through trying to figure out why copy() and clone() weren’t working as they were described in the books. But if you want to learn how to make your objects cloneable, well, go right ahead...

Figure 1: The Preferences dialog from the Hello droplet

A Quick Droplet

Our second example is a droplet, built using CodeWarrior. Basically, the droplet is an application that sends the embedded classes to the virtual machine. If any files are dropped on the droplet, their names are sent to main() in the argv parameter.

To create a droplet, create your project using the droplet stationery. Copy the resource file from the example “HelloWorld” droplet and edit it to change the creator signature and signature resource. Next, edit the project preferences to reflect the application’s file name, the name of your class, and your creator (Figure 1).

Here’s the droplet source code for the Hello droplet (note that the class is called HelloWorld but the droplet is called Hello):

public class HelloWorld
{
 public static void main(String argv[])
 {
 if (argv.length == 0 )
 System.out.println("You launched Hello " +
 "without dropping anything on it.");
 else
 {
 System.out.println("You launched Hello " +
 "dropping the following things:");

 for(int i = 0; i<argv.length; i++)
 System.out.println("Arg[" + 
 i + "]=" + argv[i]);
 }
 }
}

Basically, this code prints one message if the droplet is launched without any files dropped on it, or else prints the list of files dropped on the droplet.

Here’s the result of launching the Hello droplet without any files dropped on it:

You launched Hello without dropping anything on it.

Here’s the result when I dropped three files on the droplet:

You launched Hello dropping the following things:
Arg[0]=/Macintosh%20HD/Test%20Files/File1
Arg[1]=/Macintosh%20HD/Test%20Files/File2
Arg[2]=/Macintosh%20HD/Test%20Files/File3

Note that the %20 in the string represents ASCII character 32, which is the space character. 20 in hex is 32.

Our First Applet

Before we go, here’s a taste of things to come: our first official applet. As mentioned in last month’s column, a Java applet is a Java class that is derived from the class java.applet.Applet. The java.applet.Applet class is described in the file java.applet.Applet.html. Take a few minutes to look this page over.

Our first applet takes advantage of the packages java.awt.Graphics and java.awt.Font. Take a few minutes to look over the files java.awt.Graphics.html and java.awt.Font.html. In fact, it is probably a good idea to look through the files in the API Documentation just to get an idea of what is in there.

Our applet will consist of a single class, called hello, and a single method, called paint(). Our paint() overrides the standard paint() method that is part of the standard applet. The default paint() method does nothing. Ours will use a sequence of AWT (the Java equivalent to the Mac Toolbox) calls to draw the string “Hello, world!” in a pane or in a window (depending on the browser).

Here’s the source code:

import java.awt.Font;
import java.awt.Graphics;

public class hello extends java.applet.Applet
{
 public void paint( Graphics g )
 {
 Font f = new Font( "Chicago", Font.PLAIN, 36 );
 
 g.setFont( f );
 g.drawString( "Hello, world!", 0, 30 );
 }
}

Notice that we don’t have a main() in our class. Instead, our class follows the standard established for applets. The paint() method will be called when it is time to draw our applet. The Font object will be created using the Chicago font and is set to plain 36 point. The Font object is passed to the setFont() method, making that font, style, and size current for the Graphics object g. Next, the string “Hello, world!” is drawn in g at the coordinates (0, 30) using the method drawString().

To run this applet, you’ll need to first compile the source code into a class file (I called my class file hello.class) and then build a bit of HTML to launch the applet. Here’s my HTML code:

<title>My test applet...</title>
<hr>
<applet code="hello.class" width=250 height=35></applet>
<hr>

Of course, you might want to add more to your HTML, but this should do the trick. Save the code as hello.html, and be sure hello.html and hello.class are in the same folder. Now drag hello.html onto your applet runner. Theoretically, you should see something like the window shown in Figure 2.

Figure 2. Running the applet using CodeWarrior

Till Next Month...

To me, having the advanced windowing toolkit (AWT) is like having a copy of PowerPlant or the TCL. The framework takes care of all the administrative detail so I can concentrate on filling in the details. In next month’s column, we’ll do just that. We’ll poke around the nooks and crannies, exploring the AWT. See you then...

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Chromium 75.0.3770.142 - Fast and stable...
Chromium is an open-source browser project that aims to build a safer, faster, and more stable way for all Internet users to experience the web. Version 75.0.3770.142: Release notes were... Read more
Viber 11.1.0 - Send messages and make fr...
Viber lets you send free messages and make free calls to other Viber users, on any device and network, in any country! Viber syncs your contacts, messages and call history with your mobile device, so... Read more
Wireshark 3.0.3 - Network protocol analy...
Wireshark is one of the world's foremost network protocol analyzers, and is the standard in many parts of the industry. It is the continuation of a project that started in 1998. Hundreds of... Read more
DEVONthink Pro 3.0beta4 - Knowledge base...
DEVONthink Pro is your essential assistant for today's world, where almost everything is digital. From shopping receipts to important research papers, your life often fills your hard drive in the... Read more
Adobe Creative Cloud 4.9.0.512 - Access...
Adobe Creative Cloud costs $20.99/month for a single app, or $52.99/month for the entire suite. Introducing Adobe Creative Cloud desktop applications, including Adobe Photoshop CC and Illustrator CC... Read more
SketchUp 19.1.174 - Create 3D design con...
SketchUp is an easy-to-learn 3D modeling program that enables you to explore the world in 3D. With just a few simple tools, you can create 3D models of houses, sheds, decks, home additions,... Read more
ClamXav 3.0.12 - Virus checker based on...
ClamXav is a popular virus checker for OS X. Time to take control ClamXAV keeps threats at bay and puts you firmly in charge of your Mac’s security. Scan a specific file or your entire hard drive.... Read more
BetterTouchTool 3.151 - Customize multi-...
BetterTouchTool adds many new, fully customizable gestures to the Magic Mouse, Multi-Touch MacBook trackpad, and Magic Trackpad. These gestures are customizable: Magic Mouse: Pinch in / out (zoom)... Read more
FontExplorer X Pro 6.0.9 - Font manageme...
FontExplorer X Pro is optimized for professional use; it's the solution that gives you the power you need to manage all your fonts. Now you can more easily manage, activate and organize your... Read more
Dropbox 77.4.131 - Cloud backup and sync...
Dropbox is an application that creates a special Finder folder that automatically syncs online and between your computers. It allows you to both backup files and keeps them up-to-date between systems... Read more

Latest Forum Discussions

See All

Upcoming visual novel Arranged shines a...
If you’re in the market for a new type of visual novel designed to inform and make you think deeply about its subject matter, then Arranged by Kabuk Games could be exactly what you’re looking for. It’s a wholly unique take on marital traditions in... | Read more »
TEPPEN guide - The three best decks in T...
TEPPEN’s unique take on the collectible card game genre is exciting. It’s just over a week old, but that isn’t stopping lots of folks from speculating about the long-term viability of the game, as well as changes and additions that will happen over... | Read more »
Intergalactic puzzler Silly Memory serve...
Recently released matching puzzler Silly Memory is helping its fans with their intergalactic journeys this month with some very special offers on in-app purchases. In case you missed it, Silly Memory is the debut title of French based indie... | Read more »
TEPPEN guide - Tips and tricks for new p...
TEPPEN is a wild game that nobody asked for, but I’m sure glad it exists. Who would’ve thought that a CCG featuring Capcom characters could be so cool and weird? In case you’re not completely sure what TEPPEN is, make sure to check out our review... | Read more »
Dr. Mario World guide - Other games that...
We now live in a post-Dr. Mario World world, and I gotta say, things don’t feel too different. Nintendo continues to squirt out bad games on phones, causing all but the most stalwart fans of mobile games to question why they even bother... | Read more »
Strategy RPG Brown Dust introduces its b...
Epic turn-based RPG Brown Dust is set to turn 500 days old next week, and to celebrate, Neowiz has just unveiled its biggest and most exciting update yet, offering a host of new rewards, increased gacha rates, and a brand new feature that will... | Read more »
Dr. Mario World is yet another disappoin...
As soon as I booted up Dr. Mario World, I knew I wasn’t going to have fun with it. Nintendo’s record on phones thus far has been pretty spotty, with things trending downward as of late. [Read more] | Read more »
Retro Space Shooter P.3 is now available...
Shoot-em-ups tend to be a dime a dozen on the App Store, but every so often you come across one gem that aims to shake up the genre in a unique way. Developer Devjgame’s P.3 is the latest game seeking to do so this, working as a love letter to the... | Read more »
Void Tyrant guide - Guildins guide
I’ve still been putting a lot of time into Void Tyrant since it officially released last week, and it’s surprising how much stuff there is to uncover in such a simple-looking game. Just toray, I finished spending my Guildins on all available... | Read more »
Tactical RPG Brown Dust celebrates the s...
Neowiz is set to celebrate the summer by launching a 2-month long festival in its smash-hit RPG Brown Dust. The event kicks off today, and it’s divided into 4 parts, each of which will last two weeks. Brown Dust is all about collecting, upgrading,... | Read more »

Price Scanner via MacPrices.net

Verizon is offering a 50% discount on iPhone...
Verizon is offering 50% discounts on Apple iPhone 8 and iPhone 8 Plus models though July 24th, plus save 50% on activation fees. New line required. The fine print: “New device payment & new... Read more
Get a new 21″ iMac for under $1000 today at t...
B&H Photo has new 21″ Apple iMacs on sale for up to $100 off MSRP with models available starting at $999. These are the same iMacs offered by Apple in their retail and online stores. Shipping is... Read more
Clearance 2017 15″ 2.8GHz Touch Bar MacBook P...
Apple has Certified Refurbished 2017 15″ 2.8GHz Space Gray Touch Bar MacBook Pros available for $1809. Apple’s refurbished price is currently the lowest available for a 15″ MacBook Pro. An standard... Read more
Clearance 12″ 1.2GHz MacBook on sale for $899...
Focus Camera has clearance 12″ 1.2GHz Space Gray MacBooks available for $899.99 shipped. That’s $400 off Apple’s original MSRP. Focus charges sales tax for NY & NJ residents only. Read more
Get a new 2019 13″ 2.4GHz 4-Core MacBook Pro...
B&H Photo has new 2019 13″ 2.4GHz MacBook Pros on sale for up to $150 off Apple’s MSRP. Overnight shipping is free to many addresses in the US: – 2019 13″ 2.4GHz/256GB 6-Core MacBook Pro Silver... Read more
AirPods with Wireless Charging Case now on sa...
Amazon has extended their Prime Day savings on Apple AirPods by offering AirPods with the Wireless Charging case for $169.99. That’s $30 off Apple’s MSRP, and it’s the cheapest price available for... Read more
New 2019 15″ MacBook Pros on sale for $200 of...
B&H Photo has the new 2019 15″ 6-Core and 8-Core MacBook Pros on sale for $200 off Apple’s MSRP. Overnight shipping is free to many addresses in the US: – 2019 15″ 2.6GHz 6-Core MacBook Pro Space... Read more
Amazon drops prices, now offers clearance 13″...
Amazon has new dropped prices on clearance 13″ 2.3GHz Dual-Core non-Touch Bar MacBook Pros by $200 off Apple’s original MSRP, with prices now available starting at $1099. Shipping is free. Be sure to... Read more
2018 15″ MacBook Pros now on sale for $500 of...
Amazon has dropped prices on select clearance 2018 15″ 6-Core MacBook Pros to $500 off Apple’s original MSRP. Prices now start at $1899 shipped: – 2018 15″ 2.2GHz Touch Bar MacBook Pro Silver: $1899.... Read more
Price drop! Clearance 12″ 1.2GHz Silver MacBo...
Amazon has dropped their price on the recently-discontinued 12″ 1.2GHz Silver MacBook to $849.99 shipped. That’s $450 off Apple’s original MSRP for this model, and it’s the cheapest price available... Read more

Jobs Board

Best Buy *Apple* Computing Master - Best Bu...
**696259BR** **Job Title:** Best Buy Apple Computing Master **Job Category:** Store Associates **Location Number:** 001076-Temecula-Store **Job Description:** The Read more
Business Development Manager, *Apple* Globa...
Business Development Manager, Apple Global Tampa, FL, US Requisition Number:73805 As a Global Apple Business Development Manager at Insight, you proactively Read more
*Apple* Systems Architect/Engineer, Vice Pre...
…its vision to be the world's most trusted financial group. **Summary:** Apple Systems Architect/Engineer with strong knowledge of products and services related to Read more
*Apple* IOS Systems Engineer - Randstad (Uni...
Apple IOS Systems Engineer **job details:** + location:Irvine, CA + salary:$45 - $55 per hour + date posted:Tuesday, July 16, 2019 + job type:Temp to Perm + Read more
Business Development Manager, *Apple* Globa...
Business Development Manager, Apple Global Tampa, FL, US Requisition Number:73805 As a Global Apple Business Development Manager at Insight, you proactively Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.