Java(TM) Boutique - Tutorial - Building Easy Java GUIs with Thinlet, Part 1

Tutorials : Building Easy Java GUIs with Thinlet, Part 1 :

Making Errors

Making deliberate errors in your code to see how the system reacts helps you better understand any error messages that show up during your work. This is especially true when working with Thinlet—the missing object model can cause some funny error messages if you make mistakes in your code. For example, suppose you try to fetch a property that does not exist, say, a text field does not have a selected property like a combo box:

getInteger(textField, "selected")

You'll get this message:

java.lang.IllegalArgumentException: unknown selected integer for textfield

If you use an incorrect data type, say, String instead of Integer:

getString(comboBox, "selected");

You get this message:

java.lang.IllegalArgumentException: integer

Or, you try to get a property from a non-Thinlet object:

String s = ""; getInteger(s, "selected");

It results in this message:

java.lang.ClassCastException: java.lang.String

If you accidentally declare a method defined in the configuration file as private, you may get something like this:

java.lang.IllegalArgumentException: load(this) dk.hansen.TreeTest.load(java.lang.Object)

You'll see the same message if the method is not present at all.

You'll probably see a few more errors during your work, but the above examples are the most common ones.

The Thinlet Hierarchy

Thinlet provides a nice, tree-like hierarchy for all the components within an application. At its root is a desktop object that typically contains a panel with widgets. Since a panel may contain other panels and all objects are of type java.lang.Object, you can use a recursive algorithm like this to print out the tree:

private void componentInfo(Object component, String indent) {
  String type = getClass(component);
  String name = getString(component, "name");
  System.out.println(indent + "Component '" + name + 
                            "' is of type " + type);
  for (int i = 0; i < getCount(component); i++) {
    Object child = getItem(component, i);
    componentInfo(child, indent+" ");
  }
}

Now, try this method on Example 1, above. Example 2 simply adds a call to componentInfo, and at the same time, also replaces the textfield with a label, to show another form of the display area:

public Example2() throws Exception {
  add(parse("example2.xml"));
  componentInfo(this.getDesktop(), "");
}

. . .
  <button text=" . " name="." action="display(this, result)" /> 
  <label text="" name="result" colspan="3" />
</panel>

Figure 3 shows the GUI with a label widget as display area.

Figure 3. The GUI:
This image shows the GUI with a label widget as its display area.

The printout of the tree becomes:

Component 'null' is of type desktop
  Component 'null' is of type panel
    Component '7' is of type button
    Component '8' is of type button
    Component '9' is of type button
. . .    
    Component '.' is of type button
    Component 'result' is of type label

The reason for the null names of the desktop and the panel is simply that you haven't given them any names.

More details on the hierarchy may be found here.

The hierarchy is important to understand if you want to add or remove widgets—or entire panels from the tree. Menus must also be placed correctly in the hierarchy.

The Thinlet Widgets

Thinlet offers you a wide selection of GUI controls, the most important of which are:

Text Field
Text Area
Button
Label
Check Box
Radio Button
Drop down
List

The complete widget list may be found here.

Table 3 shows how these widgets are defined in the XML file and how you retrieve the data entered or choices selected by the user.

Table 3. The widgets are defined in the XML file.

Widget XML element Data is retrieved by...

Text Field <textfield name="textField"/> getString(textField, "text")

Text Area <textarea name="textArea" wrap="true" columns="30" rows="2"/> getString(textArea, "text")

Button <button text="Go" action="showResults()" /> N/A

Label <label text="Thinlet Widgets"/> N/A

Check Box <checkbox name="checkbox"/> getBoolean(checkBox, "selected")

Radio Button <label text="choice a" /> <checkbox name="radioButton1" group="group" /> <label text="choice b" /> <checkbox name="radioButton2" group="group" /> getBoolean(radioButton1, "selected")

Drop Down with input field <combobox name="comboBox"> <choice text="USA"/> <choice text="Canada"/> </combobox> getInteger(comboBox, "selected")
or
getSelectedIndex(comboBox)The text entered or selected: getString(comboBox, "text")(see also note below)

Drop Down w/o input field <combobox name="comboBox2" editable="false"> <choice text="Europe"/> <choice text="Asia"/> <choice text="Africa"/> </combobox> as above

List <list name="listBox" selection="multiple"> <item text="Denmark" selected="true" /> <item text="Sweden"/> <item text="Norway"/> </list> Object[] items = getSelectedItems(listBox)

There are many more parameters for each widget than shown here. Find them all on Thinlet Widgets. But the ones shown are those that you'd always need. This small Java program displays these widgets (and a spinbox in addition) and displays the data entered. Figure 4 shows the GUI.

Figure 4. The GUI:
This is the GUI for the Java program displaying the widgets and the data entered.

This layout uses rowspan on the text area to the right and colspan on the button. Click here to view the XML configuration file for the GUI.

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