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

Adobe Animate CC 2018 18.0.1.115 - Anima...
Animate CC 2018 is available as part of Adobe Creative Cloud for as little as $19.99/month (or $9.99/month if you're a previous Flash Professional customer). Animate CC 2018 (was Flash CC) lets you... Read more
Postbox 5.0.22 - Powerful and flexible e...
Postbox is a new email application that helps you organize your work life and get stuff done. It has all the elegance and simplicity of Apple Mail, but with more power and flexibility to manage even... Read more
Tunnelblick 3.7.4b - GUI for OpenVPN.
Tunnelblick is a free, open source graphic user interface for OpenVPN on OS X. It provides easy control of OpenVPN client and/or server connections. It comes as a ready-to-use application with all... Read more
Carbon Copy Cloner 5.0.5 - Easy-to-use b...
Carbon Copy Cloner backups are better than ordinary backups. Suppose the unthinkable happens while you're under deadline to finish a project: your Mac is unresponsive and all you hear is an ominous,... Read more
Bartender 3.0.32 - Organize your menu-ba...
Bartender lets you organize your menu-bar apps by hiding them, rearranging them, or moving them to Bartender's Bar. You can display the full menu bar, set options to have menu-bar items show in the... Read more
Adobe Lightroom Classic CC 7.1 - Import,...
Adobe Lightroom is available as part of Adobe Creative Cloud for as little as $9.99/month bundled with Photoshop CC as part of the photography package. Lightroom 6 is also available for purchase as a... Read more
Ortelius 2.0.8 - Vector drawing app espe...
Ortelius is a full-featured vector drawing application especially for map design. Draw directly with features such as roads, rivers, coastlines, buildings, symbols and contours. Ortelius is known for... Read more
Tunnelblick 3.7.4b - GUI for OpenVPN.
Tunnelblick is a free, open source graphic user interface for OpenVPN on OS X. It provides easy control of OpenVPN client and/or server connections. It comes as a ready-to-use application with all... Read more
Carbon Copy Cloner 5.0.5 - Easy-to-use b...
Carbon Copy Cloner backups are better than ordinary backups. Suppose the unthinkable happens while you're under deadline to finish a project: your Mac is unresponsive and all you hear is an ominous,... Read more
Postbox 5.0.22 - Powerful and flexible e...
Postbox is a new email application that helps you organize your work life and get stuff done. It has all the elegance and simplicity of Apple Mail, but with more power and flexibility to manage even... Read more

Latest Forum Discussions

See All

Amazing Katamari Damacy guide - beginner...
Amazing Katamari Damacy brings the bizarro world of the original games to mobile and shifts them into an endless format that's just as addictive as the PlayStation entries. Your goal is still to roll as much random stuff as you possibly can, though... | Read more »
Portal Knights guide - crafting tips and...
In Portal Knights, you're only as strong as the items you have at your disposal. This sandbox adventure is all about crafting and building up the next big thing. Whether you're an avid explorer or collector, crafting will likely play a large part... | Read more »
The best deals on the App Store this wee...
A new week means new discounts on the App Store. This week's deals run the gamut of action-adventure titles, puzzle games, and one of the best narrative adventure series out there. If you're looking to fill out your mobile gaming library on a... | Read more »
What you need to know about Animal Cross...
We hope you've been hard at work on collecting all of those holiday items in Animal Crossing: Pocket Camp, because you're about to get a whole new list of fun things to do as the game receives its first big update sometime soon. There are a lot of... | Read more »
Reigns: Her Majesty guide - how to use e...
Ruling a kingdom isn't easy--doubly so for a queen whose every decision is questioned by the other factions seeking a slice of power. Reigns: Her Majesty builds on the original game's swipey tactics, adding items that you can use to move the story... | Read more »
The best new games we played this week -...
Friday has crept up on us once again, so it's time to honor the best new games we've played over the past few days. This past week was a pretty exciting one, with the debut of lots of beautiful new indies and some familiar faces returning to the... | Read more »
Portal Knights guide- beginner tips and...
Portal Knights is finally making the jump to iOS and Android, and it's already climbing the ranks to become the next big MMO experience on mobile. This sprawling sandbox game will let you pursue any adventure you wish, whether you want to sling... | Read more »
Reigns: Her Majesty guide - how to swipe...
Reigns: Her Majesty is storming the App Store this week, bringing more tinder-esque kingdom building to eager players everywhere. If you've played the original Reigns, you'll know that leading a kingdom is never easy. It's a careful balancing act... | Read more »
Getting Over It (Games)
Getting Over It 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: A game I madeFor a certain kind of person To hurt them. • Climb up an enormous mountain with nothing but a hammer and a pot.•... | Read more »
Reigns: Her Majesty (Games)
Reigns: Her Majesty 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: | Read more »

Price Scanner via MacPrices.net

Apple Watch Series 2, Certified Refurbished,...
Apple has Certified Refurbished Apple Watch Nike+ Series 2s, 42mm Space Gray Aluminum Case with Anthracite/Black Nike Sport Bands, available for $249 (38mm) or $279 (42mm). The 38mm model was out of... Read more
Apple offers Certified Refurbished 2016 12″ R...
Apple has Certified Refurbished 2016 12″ Retina MacBooks available starting at $949. Apple will include a standard one-year warranty with each MacBook, and shipping is free. The following... Read more
B&H drops price on 13″ 256GB MacBook Air...
B&H has the 13″ 1.8GHz/256GB Apple MacBook Air (MQD42LL/A) now on sale for $1079 including free shipping plus NY & NJ sales tax only. Their price is $120 off MSRP, and it’s the lowest price... Read more
Holiday sale: 9″ iPads starting at $299, take...
MacMall has 9″ WiFi iPads on sale for $30 off including free shipping: – 9″ 32GB WiFi iPad: $299 – 9″ 128GB WiFi iPad: $399 Read more
Green Monday deal: 15″ 2.8GHz MacBook Pro on...
B&H Photo has the 15″ 2.8GHz Space Gray MacBook Pro on sale for $250 off MSRP for today only as part of their Green Monday/Holiday sale. Shipping is free, and B&H charges sales tax for NY... Read more
Green Monday sale: B&H offers 12″ Apple i...
B&H Photo has 12″ iPad Pros on sale for up to $150 off MSRP as part of their Green Monday/Holiday sale. Shipping is free, and B&H charges sales tax in NY & NJ only: – 12″ 64GB WiFi iPad... Read more
Holiday deal: 21″ and 27″ Apple iMacs on sale...
MacMall has 2017 21″ and 27″ Apple iMacs on sale for up to $200 off MSRP. Shipping is free: – 21″ 2.3GHz iMac: $999 $100 off MSRP – 21″ 3.0GHz iMac: $1199 $100 off MSRP – 21″ 3.4GHz iMac: $1379 $120... Read more
Holiday deal: Apple Mac minis for up to $150...
MacMall has Mac minis on sale for up to $100 off MSRP, each including free shipping: – 1.4GHz Mac mini: $399 $100 off MSRP – 2.6GHz Mac mini: $599 $100 off MSRP – 2.8GHz Mac mini: $949 $50 off MSRP... Read more
Beats by Dr. Dre – BeatsX Earphones on sale f...
Best Buy has BeatsX Earphones on sale for $109, $40 off, on their online store. Sale price for online orders only. Choose free store pickup, if available, or choose free shipping. Read more
10″ 64GB WiFi Apple iPad Pros on sale for $59...
MacMall has 10.5″ 64GB Apple iPad Pros on sale for $599 including free shipping. That’s $50 off MSRP and among the lowest prices available for these iPads from any Apple reseller. Read more

Jobs Board

QA Automation Engineer, *Apple* Pay - Apple...
# QA Automation Engineer, Apple Pay Job Number: 113202642 Santa Clara Valley, California, United States Posted: 11-Dec-2017 Weekly Hours: 40.00 **Job Summary** At Read more
*Apple* Retail - Multiple Positions - Apple,...
Job Description:SalesSpecialist - Retail Customer Service and SalesTransform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
*Apple* Retail - Multiple Positions - Apple,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
*Apple* Retail - Multiple Positions - Apple,...
Job Description:SalesSpecialist - Retail Customer Service and SalesTransform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
*Apple* Information Security - Security Data...
# Apple Information Security - Security Data Analyst Job Number: 113119545 Austin, Texas, United States Posted: 10-Nov-2017 Weekly Hours: 40.00 **Job Summary** This Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.