Section 7.11 Handling Text in a Graphics Context (Optional)
In order to create attractive GUIs, it is often necessary to be able to select and control the font that is used. Even a simple drawing task, such as being able to center a message in a panel, requires that we know the font's dimensions and be able to manipulate them. In this section, we learn how to work with Java's fonts and font control methods.
Each graphics context has an associated
FontMetrics object, and the
Graphics class (Figure 7.11.1) provides several methods to access them. A
FontMetrics is an object that encapsulates important data about a font, such as its height and width.
Java assigns a default font to each
Graphics object. The particular font used is system dependent, but to override the default one can simply invoke the
g.setFont(new Font("TimesRoman", Font.ITALIC, 12));
In this case, the
Font() constructor is used to specify a 12-point, italicized, TimesRoman font. Once the font is set, it will be used in all subsequent drawings.
Subsection 7.11.1 The
Font class (Figure 7.11.2) provides a platform-independent representation of an individual font. A font is distinguished by its name, size, and style, and the
Font class includes
protected instance variables for these properties, as well as a constructor method that allows these three characteristics to be specified.
In order to understand how fonts work, it is necessary to distinguish between a character, which is a symbol that represents a certain letter or digit, and a glyph, which is a shape used to display the character. When you display a string, such as “Hello”, Java maps each individual character into a corresponding shape, as defined by the particular font that is selected.
Java distinguishes between physical and logical fonts. A physical font is an actual font library that contains the data and tables needed to associate the correct glyph with a given character. Generally speaking, a given platform (host computer plus operating system) will have a collection of such fonts available.
A logical font is one of five font families that are supported by the Java runtime environment. These include Serif, SansSerif, Monospaced, Dialog, and DialogInput. Java also supports the following font styles:
Whereas the physical fonts are platform dependent, the logical fonts and styles are platform independent. When used in a program, they are mapped to real fonts available on the host system. If the host system does not have an exact match for the specified font, it will supply a substitute. For example, if you specify a 48-point, italic, Monospaced font,
Font myFont = new Font("Monospaced", Font.ITALIC, 48);
the system may map this to a 24-point, italic Courier font, if that is the largest fixed-spaced font available.
Font() constructor is designed to work with any set of arguments. Thus, if you supply the name of a font that is not available, the system will supply a default font as a substitute. For example, on my system, specifying a nonexistent font named Random,
g.setFont(new Font("Random", Font.ITALIC, 12) ); g.drawString("Hello, World!! (random, italic, 12)", 30, 45);
produces the same font used as the mapping for a font named Dialog.
Principle 7.11.3. EFFECTIVE DESIGN: Font Portability.
The fact that
Font() will produce a font for virtually any set of arguments is important in ensuring that a Java program will run on any platform. This is another example of how Java has been designed for portability.
Component.setFont() method can be used to assign a specific font to a button or window or other graphics component. All AWT and JFC components have an associated font, which can be accessed using the
Component.getFont() methods. For example, the following code could be used to override a
Button b = new Button("Label"); b.setFont(new Font("Times", Font.ITALIC, 14));
If 14-point, italic, Times font is not available on the host system, a substitute will be supplied.
Subsection 7.11.2 Font Metrics
To illustrate how to use the
FontMetrics class, let's write a “Hello, World!” application that centers its message both horizontally and vertically in its window. The message should be centered regardless of the size of the application window. Thus, we will have to position the text relative to the window size.
The message should also be centered no matter what font is used. This will require us to know certain characteristics of the font itself, such as the height and width of its characters, whether the characters have a fixed or variable width, and so on. In order to get access to these properties, we will use the
Figure 7.11.4 illustrates the various properties that are associated with a font. The baseline of a font refers to the line on which the bottom of most characters occurs. When drawing a string, the x- and y-coordinates determine the baseline of the string's first character. Thus, in
g.drawString("Hello, World!", 10, 40);
the bottom left of the H in “Hello, World!” would be located at (10, 40).
All characters ascend some distance above the baseline. This is known as the character's ascent. Some characters, such as y, may extend below the baseline, into what's known as the descent. Each font has a maximum descent. Similarly, some characters, such as accent characters, may extend above the maximum ascent into a space known as the leading.
The height of a font is defined as the sum (in pixels) of the ascent, descent, and leading values. The height is a property of the font itself rather than of any individual character. Except for fixed-width fonts, in which the width of all characters is the same, the characters that make up a font have varying widths. The width of an individual character is known as its advance.
FontMetrics class (Figure 7.11.5) provides methods for accessing a font's properties. These can be useful to control the layout of text on a GUI. For example, when drawing multiple lines of text, the
getHeight() method is useful for determining how much space should be left between lines. When drawing character by character, the
charWidth() method can be used to determine how much space must be left between characters. Alternatively, the
stringWidth() method can be used to determine the number of pixels required to draw the entire string.
Subsection 7.11.3 Example: Centering a Line of Text
Given this background, let's take on the task of centering a message in an application window. In order for this application to work for any font, we must take care not to base its design on characteristics of the particular font that we happen to be using. To underscore this point, let's design it to work for a font named Random, which, as we noted earlier, will be mapped to some font by the system on which the application is run. In other words, we will let the system pick a font for this application's message. An interesting experiment would be to run the application on different platforms to see what fonts are chosen.
The only method we need for this application is the
paint() method. Let's begin by setting the font used by the graphics context to a random font. To get the characteristics of this font, we create a
FontMetrics object and get the font metrics for the font we just created:
g.setFont(new Font("Random", Font.BOLD, 24)); FontMetrics metrics = g.getFontMetrics();
The next step is to determine the
JFrame's dimensions using the
getSize() method. This method returns an object of type
java.awt.Dimension class (Figure 7.11.6) represents the size (width and height) of a GUI component. A
Dimension makes it possible to manipulate an object's width and height as a single entity.
Note that the
width variables are defined as
public, which is an exception from the usual convention of defining instances variables as
protected. The justification for this exception is probably to simplify the syntax of referring to an object's width and height. For example, the following syntax can be used to refer to a component's dimensions:
Dimension d = new Dimension(100, 50); System.out.println("width = " + d.width + " height = " + d.height);
Note the redundancy built into the
Dimension class. For example, in addition to being able to set a
Dimension's instance variables directly, public access methods are provided. Also, by defining more than one version of some access methods, the class achieves a higher level of flexibility. The same can be said for providing several different constructors, including a copy constructor. Finally, note how it overrides the
toString() methods. These are all examples of good object-oriented design.
Principle 7.11.7. EFFECTIVE DESIGN: Redundancy.
Redundancy is often a desirable characteristic of object design. It makes the object easier to use and more widely applicable.
Dimension object is used to calculate the x- and y-coordinates for the string. In order to center the string horizontally, we need to know its width, which is supplied by the
metrics object. If the
d.width pixels wide, then the following expression subtracts the width of the string from the width of the
JFrame and then divides the leftover space in half:
// Calculate coordinates int x = (d.width - metrics.stringWidth(str)) / 2;
Similarly, the following expression adds the height of the string to the height of the
JFrame and divides the leftover space in half:
int y = (d.height + metrics.getHeight()) / 2;
Taken together, these calculations give the coordinates for the lower left pixel of the first character in “Hello, World!!” The only remaining task is to draw the string (Listing 7.11.8). Because the
paint() method is called automatically whenever the
JFrame is resized, this application, whose output is shown in Figure 7.11.10, will re-center its message whenever it is resized by the user.
Principle 7.11.9. PROGRAMMING TIP: Generality.
By using a component's size and font as the determining factors, you can center text on virtually any component. These values are available via the component's