Java Programming, Lecture Notes # 1015
August 14, 2000
Recommended supplementary reading
It is recommended that in addition to studying this set of lessons, you also study my earlier lessons on Swing. A list of some of my Swing lessons can be found in an earlier lesson in this series. The lessons themselves can be found at Baldwin's Java Programming Tutorials.
The earlier lessons will introduce you to the use of Swing while avoiding much of the detail included in this series.
In an earlier lesson, I provided lists of properties, events, and methods that are defined in JComponent and its superclasses: Container, Component, and Object.
Default appearance and behavior
I explained that because most Swing components extend JComponent either directly or indirectly, the properties, events, and methods defined in these classes provide the default appearance and behavior of most of the Swing components.
Understanding common properties, events, and methods
I also explained that the next few lessons would concentrate on an understanding of these common properties, events, and methods in order to provide an overall knowledge of the appearance and behavior of Swing components.
Will discuss specialized appearance and behavior later
After I have illustrated this common appearance and behavior of Swing components, I will embark on a study of the additional specialized appearance and behavior associated with individual components.
What's in this lesson?
This lesson emphasizes an understanding of the opaque and preferredSize properties that are common to most Swing components.
I will also illustrate several other properties, including the following:
All but contentPane should be familiar
Except for contentPane, none of these properties are peculiar to Swing. They are also commonly used with the AWT.
A screen shot
A screen shot of the GUI that is produced when the program is started is shown below.
When the program is started, a JFrame object, about 330 pixels wide, appears on the screen. It contains a JButton component and a JLabel component in a container being managed by a FlowLayout manager.
One of the characteristics of the FlowLayout manager is that it attempts to honor the preferredSize of each component in both the horizontal and vertical dimensions.
Other layout managers don't necessarily honor the preferredSize in either or both dimensions. Some honor one dimension, others honor the other dimension, and some (such as GridLayout) don't honor either dimension.
I discuss the behavior of several layout managers in some of my other Tutorials.
Initial states of the label and button
Initially at startup, the label contains blue text on an opaque yellow background as shown in the above screen shot.
The text showing initially on the face of the button reads "Make Label Transparent."
Click the button
When the button is clicked, the appearance of the GUI changes to that shown below.
Basically, two things happen when the button is clicked.
If the button is clicked again, the appearance of the GUI reverts back to its original state.
Successively clicking the button causes the GUI to toggle between these two states.
The controlling class
Figure 1 shows the beginning of the controlling class, along with the declaration and population of two reference variables, one for the button and one for the label.
Note that the controlling class extends JFrame. Therefore, an object of the controlling class is a GUI that can be displayed directly on the desktop.
The main() method
Figure 2 shows the main() method for the application. This method simply instantiates an object of the controlling class, causing a JFrame object to appear on the screen.
Figure 3 shows the beginning of the constructor. This fragment sets the values of three properties.
The layout property
First, the code in Figure 3 sets the layout property of the content pane of the JFrame object to FlowLayout.
This layout manager places components in the container from left to right, top to bottom, while attempting to honor the preferredSize value of each component.
The default preferred size
Each component has a default preferred size, and it differs from one type of component to another.
The default preferred size for both a JButton and a JLabel is a size that reasonably accommodates the text showing on the button or on the label.
The preferredSize property is easy to control in Swing
Unlike the AWT, it is possible to modify the preferredSize property of a Swing component without a requirement to extend the component.
Overriding getPreferredSize() in the AWT
With the AWT, the only way that I have found to control the preferred size of a component is to extend the component and override the getPreferredSize() method.
By doing that, it is possible to return a value for preferred size that is different from the default value.
Swing has a preferredSize property
Swing has a setter method for the preferred size property that makes it easy to change the value of the property.
Changing value may not result in a change in size
Be aware, however, that many layout managers will ignore the value of the preferredSize property when placing the component in a container.
Transparency is controlled by the opaque property
The transparency, or lack thereof, of the background of a Swing component is controlled by the value of the opaque property.
If this property has a value of true, the background is opaque (not transparent).
If the value of the property is false, the background is transparent, allowing whatever is behind the component to show through.
Default values for opaque property
The default value of the opaque property of a Swing JLabel is false.
By default, the background of the label is transparent allowing whatever is behind the JLabel object to show through.
Text in the label is opaque
Only the text characters are opaque by default.
If the opaque property is set to true, the default background color of the JLabel appears to be gray, so it still looks to be transparent against a gray container background.
Default transparency for a JButton
On the other hand, the default value of the opaque property of a Swing JButton is true.
When instantiated, the default color of the JButton is gray.
JButton can become transparent
If the value of the opaque property of a JButton is set to false, the background becomes transparent allowing whatever is behind the button to show through. Only the text and the border of the button remain opaque.
Make it happen
Figure 4 shows the statement that overrides the default and causes the background of the label to be opaque when the program starts running.
Typical JavaBeans setter method
This is a typical setter method for a JavaBean property.
The name of the property
Note that the name of the property in this case is opaque with a lower-case "o". (I explained the property naming conventions of JavaBean properties in an earlier lesson.)
Some ordinary code
The code in Figure 5 is rather ordinary,
I explained the requirement to use getContentPane() in an earlier lesson on Swing. You must invoke this method whenever you need to add a component to a JFrame, or whenever you need to set the layout manager for a JFrame.
The preferredSize setter method
As I mentioned earlier, unlike the AWT, Swing makes it possible to control the value of the preferredSize property using an ordinary setter method, as shown in Figure 6.
I planned to change the text property value
In this case, I planned to change the value of the text property for the button at runtime. (The text property controls the string value displayed on the face of the JButton).
Didn't want the size of the button to change
I wanted to make certain that the size of the button did not change when I changed the value of the text property.
I knew that the initial value of the text property (the text showing on the button at startup) required more physical space for display than would be required for the display of later values of the text property.
Used startup button size as preferred size
Therefore, I used the getter method shown in Figure 6 to get the current value of the size property of the button after the GUI first became visible.
I used that value to set the preferredSize property for the button, causing the actual size of the button to remain constant from that point forward, even though the value of the text property changed to a value requiring less physical space for display.
Order of these operations is important
The order of operations was important here. It was necessary to set the visible property of the JFrame object to true before getting the value of the size property of the button. Until that time, the getter method for the size property of the button returned zero for both width and height. Apparently the size property is not set until the GUI is actually rendered on the screen.
An anonymous inner class action listener
This program uses an anonymous inner class, containing an if/else statement, as an action listener registered on the button. This action listener toggles the GUI between its two states.
The if clause
The first half of that action listener, including the if clause, is shown in Figure 7.
The else clause is shown later in Figure 8.
Use a getter method of type is...
Now please refer back to the if clause of Figure 7. The code in the actionPerformed() method of Figure 7 uses label.isOpaque() as the conditional expression in the if statement. This is an optional form of a getter method that can be used for boolean properties.
Toggle the value of the opaque property
If the conditional expression returns true, the value of the opaque property of the label is set to false, causing the background of the label to become transparent (not opaque).
Request a repaint for the label
The repaint() method is used to send a message to the operating system asking it to render the new transparent representation of the area of the screen occupied by the label.
Get a reference to the button
Then the getSource() method of the incoming ActionEvent object is used to get a reference to the JButton component that fired the event.
Modify the value of the text property
This reference is used to modify the value of the text property of the button.
Downcast is necessary
Note that it is necessary to downcast the reference to the source of the event before accessing the text property. The getSource() method returns a reference to the source of the event as type Object.
Text on button is automatically repainted
It is interesting to note that the text on the face of the button is automatically repainted without the necessity of a call to repaint(). However, it is necessary to call repaint() to cause the background of the label to be repainted.
I'm sure that there is a rule for this somewhere, but I haven't found it yet.
The else clause
Figure 8 shows the else clause of the if/else statement. This clause is executed if the value of the opaque property is false (meaning that the background of the label is transparent).
Reverse the previous action
The code in Figure 8 reverses the action of the if clause in Figure 7, returning the values of the opaque property of the label and the text property of the button to their original values.
Toggle each time the button is clicked
The effect is to cause the GUI to toggle back and forth between two states each time the button is clicked.
The remaining code
The remaining code, which you can view in Figure 9, is uninteresting. It consists simply of an anonymous inner class used to terminate the program when the user closes the JFrame object.
The primary purpose of this lesson was to illustrate the use of the opaque and preferredSize properties of Swing components.
Using the opaque property
I have illustrated how to use the opaque property to control the background transparency of a JLabel object.
The approach used here should work with any Swing component that supports transparency.
Using the preferredSize property
I have also illustrated how to use the preferredSize property to control the size of a JButton object.
This approach should also work with any Swing component that supports the preferredSize property.
Layout manager may not cooperate
Note, however, that even though a Swing component may support the preferredSize property, the layout property of the container that contains that component may specify a layout manager that won't honor the preferred size even if the component supports it.
For example, the GridLayout manager doesn't support the preferred size in either dimension. Rather, all components in a GridLayout are rendered the same size regardless of their preferred size.
Several other properties were illustrated
In the process of illustrating the use of the opaque and preferredSize properties, I have illustrated several other properties as well.
Copyright 2000, Richard G. Baldwin. Reproduction in whole or in part in any form or medium without express written permission from Richard Baldwin is prohibited.
Richard has participated in numerous consulting projects involving Java, XML, or a combination of the two. He frequently provides onsite Java and/or XML training at the high-tech companies located in and around Austin, Texas. He is the author of Baldwin's Java Programming Tutorials, which has gained a worldwide following among experienced and aspiring Java programmers. He has also published articles on Java Programming in Java Pro magazine.
Richard holds an MSEE degree from Southern Methodist University and has many years of experience in the application of computer technology to real-world problems.