A top-level window (that is, a window that is not contained inside another window) is called a frame in Java. The AWT library has a class, called Frame, for this top level. The Swing version of this class is called JFrame and extends the Frame class. The JFrame is one of the few Swing components that is not painted on a canvas. Thus, the decorations (buttons, title bar, icons, and so on) are drawn by the user’s windowing system, not by Swing.
1. Creating a Frame
In this section, we will go over the most common methods for working with a Swing JFrame. Listing 10.1 lists a simple program that displays an empty frame on the screen, as illustrated in Figure 10.1.
Let’s work through this program, line by line.
The Swing classes are placed in the javax.swing package. The package name javax indicates a Java extension package, not a core package. For historical reasons, Swing is considered an extension. However, it is present in every Java implementation since version 1.2.
By default, a frame has a rather useless size of 0 x 0 pixels. We define a subclass SimpleFrame whose constructor sets the size to 300 x 200 pixels. This is the only difference between a SimpleFrame and a JFrame.
In the main method of the SimpleFrameTest class, we construct a SimpleFrame object and make it visible.
There are two technical issues that we need to address in every Swing program.
First, all Swing components must be configured from the event dispatch thread, the thread of control that passes events such as mouse clicks and keystrokes to the user interface components. The following code fragment is used to execute statements in the event dispatch thread:
Next, we define what should happen when the user closes the application’s frame. For this particular program, we want the program to exit. To select this behavior, we use the statement
In other programs with multiple frames, you would not want the program to exit just because the user closed one of the frames. By default, a frame is hidden when the user closes it, but the program does not terminate. (It might have been nice if the program terminated once the last frame becomes invisible, but that is not how Swing works.)
Simply constructing a frame does not automatically display it. Frames start their life invisible. That gives the programmer the chance to add components into the frame before showing it for the first time. To show the frame, the main method calls the setVisibte method of the frame.
After scheduling the initialization statements, the main method exits. Note that exiting main does not terminate the program—just the main thread. The event dispatch thread keeps the program alive until it is terminated, either by closing the frame or by calling the System.exit method.
The running program is shown in Figure 10.1—it is a truly boring top-level window. As you can see in the figure, the title bar and the surrounding decorations, such as resize corners, are drawn by the operating system and not the Swing library. The Swing library draws everything inside the frame. In this program, it just fills the frame with a default background color.
2. Frame Properties
The JFrame class itself has only a few methods for changing how frames look. Of course, through the magic of inheritance, most of the methods for working with the size and position of a frame come from the various superclasses of JFrame. Here are some of the most important methods:
- The setLocation and setBounds methods for setting the position of the frame
- The setIconImage method, which tells the windowing system which icon to display in the title bar, task switcher window, and so on
- The setTitte method for changing the text in the title bar
- The setResizabte method, which takes a boolean to determine if a frame will be resizeable by the user
Figure 10.2 illustrates the inheritance hierarchy for the JFrame class.
As the API notes indicate, the Component class (which is the ancestor of all GUI objects) and the Window class (which is the superclass of the Frame class) are where you need to look for the methods to resize and reshape frames. For example, the setLocation method in the Component class is one way to reposition a component. If you make the call
the top left corner is located x pixels across and y pixels down, where (0, 0) is the top left corner of the screen. Similarly, the setBounds method in Component lets you resize and relocate a component (in particular, a JFrame) in one step, as
setBounds(x, y, width, height)
Many methods of component classes come in getter/setter pairs, such as the following methods of the Frame class:
public String getTitte()
public void setTitle(String title)
Such a getter/setter pair is called a property. A property has a name and a type. The name is obtained by changing the first letter after the get or set to lowercase. For example, the Frame class has a property with name title and type String.
Conceptually, title is a property of the frame. When we set the property, we expect the title to change on the user’s screen. When we get the property, we expect to get back the value that we have set.
There is one exception to the get/set convention: For properties of type boolean, the getter starts with is. For example, the following two methods define the resizable property:
public boolean isResizable()
public void setResizable(boolean resizable)
To determine an appropriate size for a frame, first find out the screen size. Call the static getDefaultToolkit method of the Toolkit class to get the Toolkit object. (The Toolkit class is a dumping ground for a variety of methods interfacing with the native windowing system.) Then call the getScreenSize method, which returns the screen size as a Dimension object. A Dimension object simultaneously stores a width and a height, in public (!) instance variables width and height. Then you can use a suitable percentage of the screen size to size the frame. Here is the code:
Toolkit kit = Toolkit.getDefaultToolkit();
Dimension screenSize = kit.getScreenSize();
int screenWidth = screenSize.width;
int screenHeight = screenSize.height;
setSize(screenWidth / 2, screenHeight / 2);
You can also supply frame icon:
Image img = new ImageIcon(“icon.gif”).getImage();
Source: Horstmann Cay S. (2019), Core Java. Volume I – Fundamentals, Pearson; 11th edition.