Java Swing处理图形、颜色和字体

Published on 2017 - 02 - 19

Working With 2D Shapes

To draw shapes in the Java 2D library, you need to obtain an object of the Graphics2D class. This class is a subclass of the Graphics class. Ever since Java SE 2, methods such as paintComponent automatically receive an object of the Graphics2D class. Simply use a cast, as follows:

public void paintComponent(Graphics g)
{
   Graphics2D g2 = (Graphics2D) g;
   . . .
}

The Java 2D library organizes geometric shapes in an object-oriented fashion. In particular, there are classes to represent lines, rectangles, and ellipses:

Line2D
Rectangle2D
Ellipse2D

These classes all implement the Shape interface.

To draw a shape, you first create an object of a class that implements the Shape interface and then call the draw method of the Graphics2D class. For example:

Rectangle2D rect = . . .;
g2.draw(rect);

Using the Java 2D shape classes introduces some complexity. Unlike the 1.0 draw methods, which used integer pixel coordinates, Java 2D shapes use floating-point coordinates. In many cases, that is a great convenience because it allows you to specify your shapes in coordinates that are meaningful to you (such as millimeters or inches) and then translate them to pixels. The Java 2D library uses single-precision float quantities for many of its internal floating-point calculations. Single precision is sufficient—after all, the ultimate purpose of the geometric computations is to set pixels on the screen or printer. As long as any roundoff errors stay within one pixel, the visual outcome is not affected. Furthermore, float computations are faster on some platforms, and float values require half the storage of double values.

However, manipulating float values is sometimes inconvenient for the programmer because Java is adamant about requiring casts when converting double values into float values. For example, consider the following statement:

float f = 1.2; // Error

This statement does not compile because the constant 1.2 has type double, and the compiler is nervous about loss of precision. The remedy is to add an F suffix to the floating-point constant:

float f = 1.2F; // Ok

Now consider this statement:

Rectangle2D r = . . .
float f = r.getWidth(); // Error

This statement does not compile either, for the same reason. The getWidth method returns a double. This time, the remedy is to provide a cast:

float f = (float) r.getWidth(); // Ok

These suffixes and casts are a bit of a pain, so the designers of the 2D library decided to supply two versions of each shape class: one with float coordinates for frugal programmers, and one with double coordinates for the lazy ones. (In this book, we fall into the second camp and use double coordinates whenever we can.)

The library designers chose a curious, and initially confusing, method for packaging these choices. Consider the Rectangle2D class. This is an abstract class with two concrete subclasses, which are also static inner classes:

Rectangle2D.Float
Rectangle2D.Double

Actually, since both Rectangle2D.Float and Rectangle2D.Double extend the common Rectangle2D class and the methods in the subclasses simply override those in the Rectangle2D superclass, there is no benefit in remembering the exact shape type. You can simply use Rectangle2D variables to hold the rectangle references.

Rectangle2D floatRect = new Rectangle2D.Float(10.0F, 25.0F, 22.5F, 20.0F);
Rectangle2D doubleRect = new Rectangle2D.Double(10.0, 25.0, 22.5, 20.0);

That is, you only need to use the pesky inner classes when you construct the shape objects.

The construction parameters denote the top left corner, width, and height of the rectangle.

The Rectangle2D methods use double parameters and return values. For example, the getWidth method returns a double value, even if the width is stored as a float in a Rectangle2D.Float object.

What we just discussed for the Rectangle2D classes holds for the other shape classes as well. Furthermore, there is a Point2D class with subclasses Point2D.Float and Point2D.Double. Here is how to make a point object.

Point2D p = new Point2D.Double(10, 20);

The classes Rectangle2D and Ellipse2D both inherit from the common superclass RectangularShape. Admittedly, ellipses are not rectangular, but they have a bounding rectangle.

The RectangularShape class defines over 20 methods that are common to these shapes, among them such useful methods as getWidth, getHeight, getCenterX, and getCenterY (but, sadly, at the time of this writing, not a getCenter method that would return the center as a Point2D object).

Finally, a couple of legacy classes from Java 1.0 have been fitted into the shape class hierarchy. The Rectangle and Point classes, which store a rectangle and a point with integer coordinates, extend the Rectangle2D and Point2D classes.

Figure 7.11 shows the relationships between the shape classes. However, the Double and Float subclasses are omitted. Legacy classes are marked with a gray fill.

Rectangle2D and Ellipse2D objects are simple to construct. You need to specify

  • The x and y coordinates of the top left corner; and
  • The width and height.

For ellipses, these refer to the bounding rectangle. For example,

Ellipse2D e = new Ellipse2D.Double(150, 200, 100, 50);

constructs an ellipse that is bounded by a rectangle with the top left corner at (150, 200), width of 100, and height of 50.

However, sometimes you don’t have the top left corner readily available. It is quite common to have two diagonal corner points of a rectangle, but perhaps they aren’t the top left and bottom right corners. You can’t simply construct a rectangle as

Rectangle2D rect = new Rectangle2D.Double(px, py, qx - px, qy - py); // Error

If p isn’t the top left corner, one or both of the coordinate differences will be negative and the rectangle will come out empty. In that case, first create a blank rectangle and use the setFrameFromDiagonal method, as follows:

Rectangle2D rect = new Rectangle2D.Double();
rect.setFrameFromDiagonal(px, py, qx, qy);

Or, even better, if you have the corner points as Point2D objects p and q, use

rect.setFrameFromDiagonal(p, q);

When constructing an ellipse, you usually know the center, width, and height, but not the corner points of the bounding rectangle (which don’t even lie on the ellipse). The setFrameFromCenter method uses the center point, but it still requires one of the four corner points. Thus, you will usually end up constructing an ellipse as follows:

Ellipse2D ellipse = new Ellipse2D.Double(centerX - width / 2, centerY - height / 2, width, height);

To construct a line, you supply the start and end points, either as Point2D objects or as pairs of numbers:

Line2D line = new Line2D.Double(start, end);
//or
Line2D line = new Line2D.Double(startX, startY, endX, endY);

Using Color

The setPaint method of the Graphics2D class lets you select a color that is used for all subsequent drawing operations on the graphics context. For example:

g2.setPaint(Color.RED);
g2.drawString("Warning!", 100, 100);

You can fill the interiors of closed shapes (such as rectangles or ellipses) with a color. Simply call fill instead of draw:

Rectangle2D rect = . . .;
g2.setPaint(Color.RED);
g2.fill(rect); // fills rect with red color

To draw in multiple colors, select a color, draw or fill, then select another color, and draw or fill again.

Define colors with the Color class. The java.awt.Color class offers predefined constants for the following 13 standard colors:

BLACK, BLUE, CYAN, DARK_GRAY, GRAY, GREEN, LIGHT_GRAY, MAGENTA, ORANGE, PINK, RED, WHITE, YELLOW

You can specify a custom color by creating a Color object by its red, green, and blue components. Using a scale of 0–255 (that is, one byte) for the redness, blueness, and greenness, call the Color constructor like this:

Color(int redness, int greenness, int blueness)

Here is an example of setting a custom color:

g2.setPaint(new Color(0, 128, 128)); // a dull blue-green
g2.drawString("Welcome!", 75, 125);

To set the background color, use the setBackground method of the Component class, an ancestor of JComponent.

MyComponent p = new MyComponent();
p.setBackground(Color.PINK);

There is also a setForeground method. It specifies the default color that is used for drawing on the component.

Java gives you predefined names for many more colors in its SystemColor class. The constants in this class encapsulate the colors used for various elements of the user’s system. For example,

p.setBackground(SystemColor.window)

sets the background color of the component to the default used by all windows on the user’s desktop. (The background is filled in whenever the window is repainted.) Using the colors in the SystemColor class is particularly useful when you want to draw user interface elements so that the colors match those already found on the user’s desktop. Table 7.1 lists the system color names and their meanings.

Using Special Fonts for Text

To find out which fonts are available on a particular computer, call the getAvailableFontFamilyNames method of the GraphicsEnvironment class. The method returns an array of strings containing the names of all available fonts. To obtain an instance of the GraphicsEnvironment class that describes the graphics environment of the user’s system, use the static getLocalGraphicsEnvironment method. The following program prints the names of all fonts on your system:

import java.awt.*;

public class ListFonts
{
   public static void main(String[] args)
    {
      String[] fontNames = GraphicsEnvironment
         .getLocalGraphicsEnvironment()
         .getAvailableFontFamilyNames();
      for (String fontName : fontNames)
         System.out.println(fontName);
    }
 }

On one system, the list starts out like this:

Abadi MT Condensed Light
Arial
Arial Black
Arial Narrow
Arioso
Baskerville
Binner Gothic
 . . .

To establish a common baseline, the AWT defines five logical font names:

SansSerif
Serif
Monospaced
Dialog
DialogInput

These names are always mapped to some fonts that actually exist on the client machine. For example, on a Windows system, SansSerif is mapped to Arial.

In addition, the Oracle JDK always includes three font families named “Lucida Sans,” “Lucida Bright,” and “Lucida Sans Typewriter.”

To draw characters in a font, you must first create an object of the class Font. Specify the font face name, the font style, and the point size. Here is an example of how you construct a Font object:

Font sansbold14 = new Font("SansSerif", Font.BOLD, 14);

The third argument is the point size. Points are commonly used in typography to indicate the size of a font. There are 72 points per inch.

You can use a logical font name in place of the font face name in the Font constructor. Specify the style (plain, bold, italic, or bold italic) by setting the second Font constructor argument to one of the following values:

Font.PLAIN
Font.BOLD
Font.ITALIC
Font.BOLD + Font.ITALIC

You can read font files in TrueType, OpenType, or PostScript Type 1 formats. You need an input stream for the font—typically from a file or URL.Then, call the static Font.createFont method:

URL url = new URL("http://www.fonts.com/Wingbats.ttf");
InputStream in = url.openStream();
Font f1 = Font.createFont(Font.TRUETYPE_FONT, in);

The font is plain with a font size of 1 point. Use the deriveFont method to get a font of the desired size:

Font f = f1.deriveFont(14.0F);

Here’s the code that displays the string “Hello, World!” in the standard sans serif font on your system, using 14-point bold type:

Font sansbold14 = new Font("SansSerif", Font.BOLD, 14);
g2.setFont(sansbold14);
String message = "Hello, World!";
g2.drawString(message, 75, 100);

Next, let’s center the string in its component instead of drawing it at an arbitrary position. We need to know the width and height of the string in pixels. These dimensions depend on three factors:

  • The font used (in our case, sans serif, bold, 14 point);
  • The string (in our case, “Hello, World!”); and
  • The device on which the font is drawn (in our case, the user’s screen).

To obtain an object that represents the font characteristics of the screen device, call the getFontRenderContext method of the Graphics2D class. It returns an object of the FontRenderContext class. Simply pass that object to the getStringBounds method of the Font class:

FontRenderContext context = g2.getFontRenderContext();
Rectangle2D bounds = f.getStringBounds(message, context);

The getStringBounds method returns a rectangle that encloses the string.

To interpret the dimensions of that rectangle, you should know some basic typesetting terms (see Figure 7.13). The baseline is the imaginary line where, for example, the bottom of a character like “e” rests. The ascent is the distance from the baseline to the top of an ascender, which is the upper part of a letter like “b” or “k,” or an uppercase character. The descent is the distance from the baseline to a descender, which is the lower portion of a letter like “p” or “g."

Leading is the space between the descent of one line and the ascent of the next line. (The term has its origin from the strips of lead that typesetters used to separate lines.) The height of a font is the distance between successive baselines, which is the same as descent + leading + ascent.

The width of the rectangle that the getStringBounds method returns is the horizontal extent of the string. The height of the rectangle is the sum of ascent, descent, and leading. **The rectangle has its origin at the baseline of the string. The top y coordinate of the rectangle is negative. **Thus, you can obtain string width, height, and ascent as follows:

double stringWidth = bounds.getWidth();
double stringHeight = bounds.getHeight();
double ascent = -bounds.getY();

If you need to know the descent or leading, use the getLineMetrics method of the Font class. That method returns an object of the LineMetrics class, which has methods to obtain the descent and leading:

LineMetrics metrics = f.getLineMetrics(message, context);
float descent = metrics.getDescent();
float leading = metrics.getLeading();

The following code uses all this information to center a string in its surrounding component:

FontRenderContext context = g2.getFontRenderContext();
Rectangle2D bounds = f.getStringBounds(message, context);
// (x,y) = top left corner of text
double x = (getWidth() - bounds.getWidth()) / 2;
double y = (getHeight() - bounds.getHeight()) / 2;
// add ascent to y to reach the baseline
double ascent = -bounds.getY();
double baseY = y + ascent;
g2.drawString(message, (int) x, (int) baseY);

To understand the centering, consider that getWidth() returns the width of the component. A portion of that width, namely, bounds.getWidth(), is occupied by the message string. The remainder should be equally distributed on both sides. Therefore, the blank space on each side is half the difference. The same reasoning applies to the height.

Displaying Images

Once images are stored in local files or someplace on the Internet, you can read them into a Java application and display them on Graphics objects. There are many ways of reading images. Here, we use the ImageIcon class that you already saw:

Image image = new ImageIcon(filename).getImage();

Now the variable image contains a reference to an object that encapsulates the image data. You can display the image with the drawImage method of the Graphics class.

public void paintComponent(Graphics g)
{
   . . .
   g.drawImage(image, x, y, null);
}

Listing 7.6 takes this a little bit further and tiles the window with the graphics image. The result looks like the screen shown in Figure 7.15. We do the tiling in the paintComponent method. We first draw one copy of the image in the top left corner and then use the copyArea call to copy it into the entire window:

for (int i = 0; i * imageWidth <= getWidth(); i++)
  for (int j = 0; j * imageHeight <= getHeight(); j++)
      if (i + j > 0)
         g.copyArea(0, 0, imageWidth, imageHeight, i * imageWidth, j * imageHeight);

Reference