Preparing for Java Programming Language Certification

We still sometimes receive questions on the tutorial feedback alias asking how to prepare for the Java Programmer Certification test. The book strongly recommended by our Java instructors and course developers is the Complete Java 2 Certification Study Guide by Philip Heller.

You can follow this thread on the Java Tutorials Community forum.

-- Sharon Zakhour

Posted at 07:48AM May 20, 2008 by The Java Tutorial Team  |  Comments[4]

DefaultTreeModel -- Adding, Removing and Updating Nodes

Collin wasted no time creating such an example. Unfortunately, resources being what they are, I wasn't able to get engineering cycles for reviewing the example. Because we generally don't include material that hasn't been reviewed by engineering, it was backburnered.

Meanwhile, Collin posted his example and some accompanying text, The Secret Life of a DefaultTreeModel on JavaLobby.

Collin has now posted his example on the JavaTutorials forum!

When you compile and run the example (it will run under JDK 5) it brings up a panel containing a tree with a root node. When you open the root node you'll see three child nodes. At this point you can right-click any of the nodes (control-click on a Mac) and you can add 50 children to the current selection, remove the current selection, or modify the text on the current selection.

Thanks for this very useful example, Collin!

-- Sharon Zakhour

Posted at 02:46PM May 16, 2008 by The Java Tutorial Team  |  Comments[0]

JTable: Freezing a column while scrolling the others

Over on the javatutorials portal Collin Fagan has posted an example that shows how to do just that. Check out Row Labels/Frozen Columns. (You'll have to scroll down through several posts to see it.)

Thanks Collin!

-- Sharon Zakhour

Posted at 02:51PM May 13, 2008 by The Java Tutorial Team  |  Comments[2]

JTable Examples

Please check it out!

-- Sharon Zakhour

Posted at 12:38PM May 07, 2008 by The Java Tutorial Team  |  Comments[0]

Announcing the Java Tutorials Community Portal

This is the place to talk about the tutorials — how you use them, how you extend them (or would like to), new tutorials you would like to see or even contribute. You can share ideas, curriculum, code examples (either new or modified from existing examples).

To participate on the wiki and the forum you need to register on java.net (it's free). That's all you need to do.

Hope to see you there!

-- Sharon Zakhour

Posted at 08:18AM May 05, 2008 by The Java Tutorial Team  |  Comments[2]

SwingSet3

This new version, written by Swing expert, Amy Fowler, boasts a slick GUI showcasing the Nimbus look and feel. This demo is visually beautiful and extensible, but what I really love is the new feature that allows you to view the code used to create and display the selected component. One half of a split pane displays the selected component and the other half shows the code. For some complex components, such as JTable and JButton, a combo box allows you to choose exactly what snippet of code produces a particular effect.

You can obtain SwingSet3 from swingset3.dev.java.net.

(If, like me, are you a Mac OS X user, you need to be running the latest version of JDK 6 from Apple that runs only under Leopard.)

Check it out!

-- Sharon Zakhour

Posted at 02:51PM Apr 22, 2008 by The Java Tutorial Team  |  Comments[3]

Animation, the Easy Way!

If you've ever programmed animations and thought "there must be an easier way", then today's blog entry is sure to bring welcomed relief. The JavaFX^TM Script programming language now supports key frame animation, which allows you to declare your animations using a simple syntax that resembles plain English. All graphics rendering now happens automatically, behind the scenes, allowing you to focus on the animation at hand rather than the "plumbing" to make it all work.

To demonstrate this functionality we will re-create the classic "tumbling duke" animation. Since this may be your first exposure to the language, we'll break this discussion up into two separate entries: today we will cover the basics; early next week we'll cover everything else. You'll want to try this code for yourself, but fear not, the development environment is easy to set up. The edit-compile-run process is similar to that of the Java^TM programming language, but the commands are javafxc and javafx instead of javac and java. There is also a JavaFX Script compiler plug-in available for the NetBeans IDE, for those who prefer not to work from the command line. For instructions on setting up any of these environments, visit the openjfx project page at http://openjfx.dev.java.net.

Having said that, let's get animating!

As you may already know, the tumbling duke animation is comprised of 17 separate images. Traditionally, creating this animation has required an intimate understanding of the AWT and Swing painting mechanisms (if you haven't seen it before, take a look at the original Tumbling Duke source code.) As you will soon learn, accomplishing the same end result using JavaFX Script technology is a simple matter once you've mastered its syntax.

import javafx.ui.*;
import javafx.ui.canvas.*;
import javafx.animation.*;

// Load image data
var images = for (i in [1..17]) {
     Image { url: "file:images/T{i}.gif"};
}

...

Our first task is to load the images into memory. If your initial instinct is to place these images into an array, you're right, although here that data structure is known as a sequence. Sequences differ from Java programming language arrays, but are similar in that they also hold a list of objects. Sequences are generally declared with square brackets (the code [1..17] is a sequence), but the system will also infer this based on context when needed. Such is the case with our var images declaration. The for loop that follows returns one javafx.ui.Image object per iteration; when it's done, you end up with an images variable that contains all of the image data. (If you're wondering where the constructor call is, the JavaFX Script programming language uses object literals; the code Image{} is enough to return a new Image object.)

... // Create timeline and key animation frames var t = Timeline { keyFrames: for (image in images) { KeyFrame { time: 100ms* indexof image action: function(){currDuke=image;} } } } ...

Listing 2: Creating the Key Frames and Animation Timeline

Next, we'll create the animation's key frames and place them on a timeline. These constructs are represented by the javafx.animation.KeyFrame and javafx.animation.Timeline classes, respectively. In the interest of saving space we've rolled a lot of functionality into the above code, so let's take a minute to make sure you understand what's going on.

In the JavaFX Script programming language, an object's state is stored in its attributes. Above we initialize the keyFrames attribute of the Timeline class, followed by the time and action attributes of the KeyFrame class. So far as the animation itself is concerned, the important concepts are that a "key frame" (a KeyFrame instance) represents one specific point in time along your animation, and a Timeline instance holds a sequence of KeyFrame objects. It's up to you to decide where the key frames of your animation should be; in this case we consider each flip of the image to be one frame in the animation. It seems reasonable that the images should change 100 milliseconds apart, so we've initialized each key frame with a time value that is 100ms greater than the previous frame.

The action attribute of the KeyFrame class accepts a function; that function can contain any code that you want. Keep in mind that we're looping through each of the images where this occurs, so the only action that we care about is assigning the current image of the current key frame to a separate variable named currDuke. As you will see below, this currDuke variable is where the GUI will look to find the current image.

 ... 

// Set first image to be displayed
var currDuke = images[0];

// Create the GUI application frame
Frame {
     title: "Tumbling Duke"
     width: 400
     height: 150
     background: Color.WHITE
     visible: true
     content: FlowPanel {
          content: Canvas {
               onMouseClicked: function(e) {t.start();}
               content: ImageView {
                    image: bind currDuke;
               }
          }
     }
}

At this point, all that's left to do is to construct the GUI. This is actually the easiest part; all we need to do is create a few objects and we're done. The image attribute of the ImageView class is the actual image that will appear on screen. The code "image: bind currDuke" is the magic that keeps the GUI in sync with the images used in our timeline. By "binding" the image attribute to the currDuke variable, the on-screen image will automatically change whenever the value of currDuke changes.

In this early version of the code, we'll tell the animation timeline to begin only when the user clicks somewhere on the GUI's drawing surface. In the next blog entry we'll change that behavior to an infinite loop that begins automatically, thereby replicating the behavior of the original animation.

import javafx.ui.*;
import javafx.ui.canvas.*;
import javafx.animation.*;

// Load image data
var images = for (i in [1..17]) {
     Image { url: "file:images/T{i}.gif"};
}

// Create timeline and key animation frames
var t = Timeline {
     keyFrames: for (image in images) {
          KeyFrame {
               time: 100ms* indexof image
               action: function(){currDuke=image;}
          }
     }
}

// Set first image
var currDuke = images[0];

// Create the GUI application frame
Frame {
     title: "Tumbling Duke"
     width: 400 
     height: 150
     background: Color.WHITE
     visible: true
     content: FlowPanel {
          content: Canvas {
               onMouseClicked: function(e) {t.start();}
               content: ImageView {
                    image: bind currDuke;
               }
          }
     }
}

-- Scott Hommel

Posted at 10:38AM Apr 09, 2008 by The Java Tutorial Team  |  Comments[6]

Searching the Java Tutorials

Another easy way to find a subject in the tutorials is to use the Really Big Index. This index has always been available, but not everyone has been aware of it. For this most recent release of the tutorial, we have made it easier to find the Really Big Index. If you look at the tutorial's main page, you'll see that there is now a Tutorial Contents box on the upper right and a button in that box will lead you directly to the Really Big Index.

We hope you find this to be a useful feature for searching the contents of the tutorials.

- Sharon Zakhour

Posted at 04:39AM Apr 04, 2008 by The Java Tutorial Team  |  Comments[2]

Web Push of the Java Tutorials

The biggest change, content-wise, is the newly re-worked Drag and Drop and Data Transfer lesson.

The other big change is that we have moved the download of the tutorials to the Sun Download Center. You can download the tutorials in their entirety from that location.

As part of this process, we have updated the license used to distribute the tutorials. (You click-to-accept the license when you perform the download.) The updated license is now tailored to encourage academic and non-commercial use without any special permission from Sun.

Sharon Zakhour

Posted at 12:10PM Mar 19, 2008 by The Java Tutorial Team  |  Comments[1]

Superpackages

As you probably know, you can organize your code into a package. In the Swing tutorial, we put the examples into packages. The drag and drop examples are in the dnd package, the button, slider, and menu examples are in the components package, the SpringLayout and GridLayout examples are in the layout package, and so on. Even when you don't specify a package, your code goes into the default package.

But superpackages allow you to specify a much more sophisticated hierarchy to your code. You can nest superpackages: a superpackage may contain one or more packages and one or more superpackages. You can choose how much you want to expose of the nested superpackages.

For more information about this upcoming feature, see

• Strawman Proposal for JSR 294 Superpackages
• JSR 294 - Improved Modularity Support in the Java Programming Language

-- Sharon Zakhour

Posted at 02:48PM Jan 07, 2008 by The Java Tutorial Team  |  Comments[4]

Confusion about the Class Variable Q&E Page

1. Consider the following class:

        public class IdentifyMyParts {
            public static int x = 7; 
            public int y = 3; 
        }

        a. What are the class variables?

        b. What are the instance variables?

        c. What is the output from the following code:

            IdentifyMyParts a = new IdentifyMyParts();
            IdentifyMyParts b = new IdentifyMyParts();
            a.y = 5;
            b.y = 6;
            IdentifyMyParts.x = 1;
            b.x = 2;
            System.out.println("a.y = " + a.y);
            System.out.println("b.y = " + b.y);
            System.out.println("IdentifyMyParts.x = " + a.x);
            System.out.println("b.x = " + b.x);

             a.y = 5 
             b.y = 6 
             IdentifyMyParts.x = 2 
             b.x = 2

Actually the answer is correct, though it doesn't offer any explanation. The point of this question was to have you think about class variables. (In fact, the writer wrote IdentifyMyParts.x rather than a.x to give you a clue that this is a class variable.) What makes x a class variable is defining it as a public static int. This means that there is a single instance of x that is shared across every instance of IdentifyMyParts. When the first line of the following code snippet is executed:

    IdentifyMyParts.x = 1;
    b.x = 2;

As a result of your feedback, we have improved the question and the answer.

Your continued feedback helps us improve the tutorial. Thanks!

-- Sharon Zakhour

Posted at 08:16AM Dec 21, 2007 by The Java Tutorial Team  |  Comments[1]

Java Equivalent for sprintf

---

I was reading your tutorial on format. You can do something like

    System.out.format ("a = %05d\n", 32 )

which should print

    a = 00032

This is equivalent to printf( "a = %5.5d\n", a ); in C++. I want to simulate sprintf in Java, like:

    char b[20];

    sprintf( b, "%5.5d", 22);

then b would contain the string 00022.

That is, I want a way in Java to do an Integer.toString() but be able to specify a width and have it insert leading 0's. If g contained the integer value 22, I would want something like g.toString(5) to return 00022 instead of just "22" or " 22".

Any way to do that?

---

Why, yes, there are a couple ways to accomplish this!

If you are writing an application that must be localized and internationalized, you may want to use DecimalFormat, like this:

        DecimalFormat myFormatter = new DecimalFormat("00000");
        String s2 = myFormatter.format(22);

You can refer to the Internationalization tutorial for more information.

DecimalFormat uses the rather arcane text APIs. If you don't need something this heavy-duty, you may want to use the java.util.Formatter class, introduced in JDK 5. The String.format methods provide a front-end to the Formatter class. To accomplish the above task using String.format, the code would look like this:

        String s1 = String.format("'%05d'%n", 22);

For further information, check out these pages in the tutorial:

• Formatting Numeric Print Output
• Strings - see the section "Creating Format Strings"

If you are using an earlier release than JDK 5, you might be interested in this Formatted Printing for Java (sprintf) article.

As a result of this question, I've tweaked the tutorial a bit, so thanks for the feedback!

-- Sharon Zakhour

Posted at 12:48PM Dec 18, 2007 by The Java Tutorial Team  |  Comments[3]

Interested in Writing about Java SE?

We are looking for a recent college grad who wants to develop as a technical writer and who can work out of our Santa Clara campus.

For more information (and to submit your resume), see our job posting.

-- Sharon Zakhour

Posted at 11:24AM Nov 13, 2007 by The Java Tutorial Team  |  Comments[9]

The Inner Class Example Page

For those of you who enjoyed the stack example, don't worry. In addition to the example we mentioned earlier, we updated the one about stacks as well. It uses a stack based on an array, and push and pop methods to implement the FILO method and an inner class. To show the inner class in action, we 'expand' the stack functionality by stepping through it and picking its even values (this is not allowed for 'standard' stacks, as you probably remember). In brief: we push, pick even, and then pop. See the following link to view the updated stack example.

In conclusion, we would like to ask your opinion about this tutorial change. What do you think? Let's make our tutorial the most effective and useful that it can be. After all, isn't that the most important thing about tutorials? :)

-- Dmitry Bondarenko

Posted at 09:25AM Nov 09, 2007 by The Java Tutorial Team  |  Comments[2]

Java Beans and the NetBeans IDE

Many releases of NetBeans contained the Beans module - a robust visual tool to create a custom bean, add a property with the Bean Pattern, and edit bean info with the BeanInfo editor. Those features were covered in the Java Beans trail of the Java Tutorial, and some features were discussed in this blog.

Unfortunately, due to a variety of circumstances (see the NetBeans project page for more information) the Beans module was dropped from the 6.0 release of NetBeans. This may impede bean application development. Many typical bean operations are not available through the NetBeans 6.0 GUI, which makes creating a custom bean more than a five-minute task. Additionally, the newly introduced binding functionality will be cruelly limited in the IDE, because even to simply add a bound property you will have to code the getter and setter methods by hand.

So this blog offers you a chance to vote for bringing back the Beans module in the upcoming update release of NetBeans. Please leave your comments in support of the Java Beans functionality.

-- Alla Redko

Posted at 05:37AM Oct 29, 2007 by The Java Tutorial Team  |  Comments[47]