XML Reference Guide

Mar 14, 2003

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In our last installment, we talked about the Google Web Toolkit, and using it to easily create sophisticated web applications using Java that ultimately gets translated to Javascript. In that entry, we gave you just the basic idea of how to create an application, and what the parts of the application ultimately do.

Now it's time to get down to business and actually build an application.

Start by creating a new application called com.informit.client.Authors. This application will be a simple page that lists a group of authors and their relative comfort levels with specific technical areas. Ultimately, the application will use AJAX to pull information from a remote server, but to keep things simple, for now we'll create the Author objects within the application itself.

Let's start with Listing 1, the basic application:

Listing 1:

package com.informit.client;

import com.google.gwt.core.client.EntryPoint;
import com.google.gwt.user.client.ui.*;

public class Authors implements EntryPoint {

  class Author {
    String authorName;
    int javaSkill;
    int rubySkill;
    int htmlSkill;
  }

  Author[] authors;

  public Author[] getAuthors (){
     Author thisAuthor = new Author();
     thisAuthor.authorName = "Nick Chase";
     thisAuthor.javaSkill = 2;
     thisAuthor.rubySkill = 2;
     thisAuthor.htmlSkill = 3;

     Author thatAuthor = new Author();
     thatAuthor.authorName = "Joe Schmoe";
     thatAuthor.javaSkill = 1;
     thatAuthor.rubySkill = 1;
     thatAuthor.htmlSkill = 3;

     Author otherAuthor = new Author();
     otherAuthor.authorName = "Frank Black";
     otherAuthor.javaSkill = 1;
     otherAuthor.rubySkill = 3;
     otherAuthor.htmlSkill = 1;

     Author[] returnArray = {thisAuthor, thatAuthor, otherAuthor};

     return returnArray;
  }

  String getAuthorAndSkillsHTML(int index){
     Author thisAuthor = authors[index];
     String returnStr = thisAuthor.authorName + 
           "<ul><li>Java: " + thisAuthor.javaSkill + "</li>"
           + "<li>Ruby: " + thisAuthor.rubySkill + "</li>"
           + "<li>HTML: " + thisAuthor.htmlSkill + "</li></ul>";

     return returnStr;
  }


  public void onModuleLoad() {

    authors = getAuthors();

    for (int i = 0; i < authors.length; i++){
       HTML thisAuthor = new HTML(getAuthorAndSkillsHTML(i));
       RootPanel.get().add(thisAuthor);
    }

  }
}

Starting at the top, this class, com.informit.client.Authors, must implement the EntryPoint interface, so we'll import it to start off with. We'll also import all of the com.google.gwt.user.client.ui package to simplify things, though of course in your production applications you'll want to import only the classes you're actually using.

The next step, in this case, is to create the data objects we're going to display. Again, ultimately we'll retrieve this data live, but for now we'll create a simple internal class, Author, which includes the name and three skill levels for each one. Once we've created the class, we can create an array of Author objects and the getAuthors() method, which loads that array with arbitrary information. We're also creating the getAuthorAndSkillsHTML() method, which provides us an easy way to retrieve a text version of each Author object.

Now we can get on to the actual application, as represented by the onModuleLoad() method. In this case, the method is fairly straightforward. For each Author in the authors array, we create a new HTML widget that consists of the data for that author, and then we add that widget to the page.

As generated, the HTML page that displays all of this has a lot of extraneous information, but if you look in the <GWT_HOME>/src/com/informit/public folder, you'll see the Authors.html file, which acts as a "host" for the information. In Listing 2, I've cleared out all but the absolutely necessary information:

Listing 2:

<html>
    <head>
        <title>Author Listing</title>
        <meta name='gwt:module' content='com.informit.Authors'>

        
    </head>
    <body>
        <script language="javascript" src="gwt.js"></script>

        <h1>Authors</h1>

    </body>
</html>

Really, there are only two indications in this file that we're even dealing with the Google Web toolkit, and those are the meta tag that identifies the actual module, and the gwt.js script, which is the main Google Web Toolkit script that loads and runs the appropriate generated code.

If we then execute the Authors-shell.cmd script, we'll see something similar to Figure 1:

Figure 1:

Simple enough, I suppose. We told it to display three HTML widgets, and it did so.

So we know it's working, and we can move on to something a little more interesting. Our first task will be to resolve a problem that isn't obvious unless you're something close to a Java guru.

The beauty of the Google Web Toolkit is how easy it makes interactivity, so our goal will ultimately be to create a page that performs actions like popping up a window with more information when you click on an author's name, and so on. It's this kind of thing that got me really excited about the toolkit. While it's possible to create handlers that react to actions such as clicks in Javascript, trying to track what you've clicked and react to the right object can be downright irritating.

So I was very excited at the prospect of being able to create code that looked like Listing 3:

package com.informit.client;

import com.google.gwt.core.client.EntryPoint;
import com.google.gwt.user.client.ui.*;
import com.google.gwt.user.client.Window;

public class Authors implements EntryPoint {

  class Author {
    String authorName;
    int javaSkill;
    int rubySkill;
    int htmlSkill;
  }
...
  String getAuthorAndSkillsHTML(int index){
     Author thisAuthor = authors[index];
     String returnStr = thisAuthor.authorName + 
                "<ul><li>Java: " + thisAuthor.javaSkill + "</li>"
                + "<li>Ruby: " + thisAuthor.rubySkill + "</li>"
                + "<li>HTML: " + thisAuthor.htmlSkill + "</li></ul>";
     return returnStr;
  }

  String getSkillsHTML(int index){
     Author thisAuthor = authors[index];
     String returnStr =  "Java: " + thisAuthor.javaSkill + "<br />"
              + "Ruby: " + thisAuthor.rubySkill + "<br />"
              + "HTML: " + thisAuthor.htmlSkill;
     return returnStr;
  }


  public void onModuleLoad() {

    authors = getAuthors();

    for (int i = 0; i < authors.length; i++){
       Label thisAuthor = new Label(authors[i].authorName);

       thisAuthor.addClickListener(new ClickListener() {
         public void onClick(Widget sender) {
           Window.alert(getSkillsHTML(i));
         }
       });

       RootPanel.get().add(thisAuthor);

    }

  }
}

This seems pretty simple, right? For each Author object in the authors array, create a Label with the author's name, and then add a ClickListener that pops open an alert message with the skills for that author. It looks like an easy way to generate interactive content based on what the user clicks.

Well, unfortunately, it's a little too simple. If you refresh the Google window browser, you'll get an error that points you to the shell:

Cannot refer to a non-final variable i inside an inner class defined in a different method

Here, I'm afraid, we've run up against the structure of Java itself. Because the variable i is not final, we can't reference it from within the ClickListener class. And because the variable changes as we loop through the array, we can't label it final.

Fortunately, that doesn't mean that we're sunk. If you look at the ClickListener object itself, you'll see that we have easy access to the object sending the event, which is, conveniently, the object the user has clicked:

...
    for (int i = 0; i < authors.length; i++){
       Label thisAuthor = new Label(authors[i].authorName);

       thisAuthor.addClickListener(new ClickListener() {
         public void onClick(Widget sender) {

           Label clickedLabel = (Label)sender;

           Window.alert(clickedLabel.getText());

         }
       });

       RootPanel.get().add(thisAuthor);

    }
...

This enables us to grab the object, turn it back into a Label, and then access its text. If we run the application, we can see the names pop up in a normal alert box, as you can see in Figure 3:

Figure 3:

Popping up an alert box

The Window object represents the actual browser window, just as it does in JavaScript.

OK, so let's take stock of where we are. We can loop through each of the objects and display them on the page, complete with listeners that generate content based on the object we clicked. Now we just need a way to refer that back to the object itself.

package com.informit.client;

import com.google.gwt.core.client.EntryPoint;
import com.google.gwt.user.client.ui.*;
import com.google.gwt.user.client.Window;

import java.util.ArrayList;

public class Authors implements EntryPoint {

  class Author {
    String authorName;
    int javaSkill;
    int rubySkill;
    int htmlSkill;
  }

  Author[] authors;
  ArrayList authorIndexes = new ArrayList();

  public Author[] getAuthors (){
     Author thisAuthor = new Author();
     thisAuthor.authorName = "Nick Chase";
     thisAuthor.javaSkill = 2;
     thisAuthor.rubySkill = 2;
     thisAuthor.htmlSkill= 3;
     authorIndexes.add("Nick Chase");

     Author thatAuthor = new Author();
     thatAuthor.authorName = "Joe Schmoe";
     thatAuthor.javaSkill = 1;
     thatAuthor.rubySkill = 1;
     thatAuthor.htmlSkill = 3;
     authorIndexes.add("Joe Schmoe");

     Author otherAuthor = new Author();
     otherAuthor.authorName = "Frank Black";
     otherAuthor.javaSkill = 1;
     otherAuthor.rubySkill = 3;
     otherAuthor.htmlSkill = 1;
     authorIndexes.add("Frank Black");

     Author[] returnArray = {thisAuthor, thatAuthor, otherAuthor};

     return returnArray;
  }

  String getAuthorAndSkillsHTML(int index){
…
  }

  String getSkillsHTML(int index){
…
  }


  public void onModuleLoad() {

    authors = getAuthors();

    for (int i = 0; i < authors.length; i++){
       Label thisAuthor = new Label(authors[i].authorName);

       thisAuthor.addClickListener(new ClickListener() {
         public void onClick(Widget sender) {

           Label clickedLabel = (Label)sender;
           int index = authorIndexes.indexOf(clickedLabel.getText());

           Window.alert(getSkillsHTML(index));
         }
       });

       RootPanel.get().add(thisAuthor);

    }

  }
}

What we're doing here is creating an ArrayList that parallels the actual Array of Author objects. This enables us to use the ArrayList's indexOf() method to locate a specific item based on the author name, and then use that information to get the relevant skills in the alert box. (Yes, there are more efficient ways to do this, such as an ArrayList using generics that holds all of the information. But the point is that you can create something that links back to the original data based on the information that you have, in this case the text of a label.)

The result is an alert box that displays the raw HTML:

Of course, that's not terribly attractive, now is it? We'll look at some of the different UI classes the Google Web Toolkit provides in future installments, but for now let's take a quick look at converting this alert box to a popup.

If we were to do this in raw Javascript, we'd need to do all kinds of work setting up hidden divs, event handlers, figuring out positioning, and so on. Creating a popup in the GWT's Java, however, isn't nearly that complicated:

...
  String getSkillsHTML(int index){
     Author thisAuthor = authors[index];
     String returnStr =  "Java: " + thisAuthor.javaSkill + "<br/>"
                         + "Ruby: " + thisAuthor.rubySkill + "<br/>"
                         + "HTML: " + thisAuthor.htmlSkill + "";

     return returnStr;
  }


  private static class AuthorPopup extends PopupPanel {

    public AuthorPopup(HTML contents) {
      super(true);
      add(contents);
    }
  }

  public void onModuleLoad() {

    authors = getAuthors();

    for (int i = 0; i < authors.length; i++){
       Label thisAuthor = new Label(authors[i].authorName);

       thisAuthor.addClickListener(new ClickListener() {
         public void onClick(Widget sender) {

           Label clickedLabel = (Label)sender;
           int index = authorIndexes.indexOf(clickedLabel.getText());

           HTML popupText = new HTML(getSkillsHTML(index));
           AuthorPopup popup = new AuthorPopup(popupText);

           int left = sender.getAbsoluteLeft() + 70;
           int top = sender.getAbsoluteTop() + 10;
           popup.setPopupPosition(left, top);
           popup.setStyleName("popupStyle");
           popup.show();

         }
       });

       RootPanel.get().add(thisAuthor);

    }

  }
}

A popup is a HTML div that appears on the page in response to an event, and can optionally disappear when the user clicks outside of that div. We start by creating a subclass of the PopupPanel class, AuthorPopup, and creating a constructor that enables us to perform two tasks. The first is to set the panel to disappear when the user clicks outside of it (using super(true), since this is an attribute of the parent PopupPanel class) and the second is to set the contents to an HTML widget.

Once we've created the class, we can use it to create a new popup when the user clicks one of the author names. To do that, we first define the contents, and then use them to create the new AuthorPopup object. From there, we set the position of the popup relative to the name the user clicked, set the CSS style (which we'll see in a moment) and show the actual popup. (Remember, this code only gets executed when the user clicks a name, so it's appropriate to show it right away.)

The style determines what the popup actually looks like. Using CSS, you can add background images, font styles, and all sorts of decorative elements. I'm more technical than visual, however, so we'll stick to the simple things. Define the CSS class in the Authors.html file, just as you would do with any other CSS styles:

<html>
	<head>
		<title>Author Listing</title>

		<meta name='gwt:module' content='com.informit.Authors'>
 
            <style type="text/css">
               .popupStyle {
                                  background-color: white;
                                  border: 1px solid red;
                                  padding: 10px;
               }
            </style>
		
	</head>
	<body>

		<script language="javascript" src="gwt.js"></script>

		<h1>Authors</h1>

	</body>
</html>

The result is a simple div that appears when you click on one of the author names.

Now that we've got the basics of creating interactive interfaces down, we can look at GWT's other capabilities.

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