March 6, 2000
Java Programming, Lecture Notes # 312
by Richard G. Baldwin
In an earlier lesson, I explained that the Graphics2D class extends the Graphics class to provide more sophisticated control over geometry, coordinate transformations, color management, and text layout. Beginning with JDK 1.2, Graphics2D is the fundamental class for rendering two-dimensional shapes, text and images.
I also explained that without understanding the behavior of other classes and interfaces such as Shape, AffineTransform, GraphicsConfiguration, PathIterator, and Stroke, it is not possible to fully understand the inner workings of the Graphics2D class.
Earlier lessons have explained a number of Java 2D concepts, including Shape, AffineTransform, and PathIterator. I am saving GraphicsConfiguration until later because it is fairly complicated. Before, I can explain the Stroke class, I need to explain how to fill a Shape. The purpose of this lesson, and the next couple of lessons is to help you to understand how the fill process works in Java 2D.
As you can see, each of the lessons in this series on Java 2D builds on the knowledge that you have gained by studying previous lessons. For that reason, I hope that you are taking the time to study the lessons in sequence.
The Graphics2D class has dozens of methods. In this lesson, I will be primarily concerned with the following three methods:
I have used the draw() method in numerous previous lessons, so it isn’t new to this lesson and won’t merit much discussion. However, setPaint() and fill() are new to this lesson. I will discuss them both in detail
Here is part of what Sun has to say about the setPaint() method:
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Sets the Paint attribute for the Graphics2D context. Calling this method with a null Paint object does not have any effect on the current Paint attribute of this Graphics2D.
Parameters: paint - the Paint object to be used to generate color during the rendering process, or null |
The terminology here can be a little confusing, especially with respect to the use of the word paint.
First, the class named Graphics2D has a property named paint. The method setPaint() is a typical setter method used to set the paint property.
The parameter that is passed to this method must be a reference to an object of a class that implements the interface named Paint.
Here is what Sun has to say about the Paint interface.
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“This Paint interface defines how color patterns can be generated for Graphics2D operations. A class implementing the Paint interface is added to the Graphics2D context in order to define the color pattern used by the draw and fill methods.” |
This interface declares a single method named createContext() that returns a reference to an object of a class that implements the PaintContext interface. (I tell my Java students at least four or five times during each semester that if they don’t understand the Java interface, they really don’t understand Java.)
Here is what Sun has to say about the PaintContext interface.
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The PaintContext interface defines the encapsulated and optimized environment to generate color patterns in device space for fill or stroke operations on a Graphics2D. The PaintContext provides the necessary colors for Graphics2D operations in the form of a Raster associated with a ColorModel. The PaintContext maintains state for a particular paint operation. In a multi-threaded environment, several contexts can exist simultaneously for a single Paint object. |
Obviously, I could continue tracking the path through the hierarchy, but that won’t be necessary.
The bad news is that this can all be very complicated. The good news is that unless you intend to define a class that implements the Paint interface, you don’t need to be concerned about many of these details.
(See a later reference to Knudsen’s book for an example of how to define your own class to implement the Paint interface.)
All you really need to know is the following:
To set the paint property of a Graphics2D object, all that you must do is invoke the setPaint() method on that object, passing a reference to an object instantiated from a class that implements the Paint interface as a parameter.
Fortunately, there are several useful classes in the 2D API that implement the Paint interface so you may not need to define your own class that implements Paint.
Why would you want to set the paint property of the Graphics2D object in the first place?
Because, when you invoke the fill() method, passing a reference to a Shape object to that method, the Shape object will be filled using the Paint object that was previously established by invoking the setPaint() method.
The bottom line is, if you want to fill a Shape object before you draw it, you accomplish this with the following two steps:
As I mentioned in an earlier lesson, in his book, Java Foundation Classes in a Nutshell, David Flanagan tells us that the Java 2D definition of a Shape does not require the shape to enclose an area. In other words, a Shape object can represent an open curve. According to Flanagan, if an open curve is passed to a method that requires a closed curve (such as fill()), the curve is automatically closed by connecting its end points with a straight line.
The Java2D API provides at least three classes that implement the Paint interface:
(As of JDK 1.2.2, this is apparently all of the classes in the API that implement Paint, but you can always define your own.)
The Color class can be used to fill a Shape object with a solid color. This will be the topic of the remainder of this lesson.
The GradientPaint class can be used to fill a Shape with a color gradient. The gradient progresses from one specified color at one point to a different specified color at a different point. The two colors can be stabilized beyond the two points (acyclic) or the gradient can be caused to repeat in a cyclic fashion beyond the two points (cyclic).
In his book entitled Java 2D Graphics, Jonathan Knudsen provides a sample program that produces a radial color gradient. This is a good example program to take a look at if you need to define your own class that implements the Paint interface.
Use of the GradientPaint class will be the topic of a subsequent lesson.
The TexturePaint class can be used to fill a Shape with a tiled version of a BufferedImage object. This will also be the topic of a subsequent lesson.
The name of this program is PaintColor01. It illustrates the use of a Paint object to fill a Shape with a solid color. In this case, the Paint object is an instance of the Color class, which implements the interface named Paint.
The program draws a four-inch by four-inch Frame on the screen. It translates the origin to the center of the Frame. Then it draws a pair of X and Y-axes centered on the new origin. So far, this is very similar to the sample programs that I have explained in previous lessons.
The program then draws one two-inch diameter circle in each quadrant. It fills the upper left circle with solid red, the upper right circle with solid green, the lower left circle with solid blue, and the lower right circle with solid yellow
The program was tested using JDK 1.2.2 under WinNT Workstation 4.0
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This discussion of dimensions in inches on the screen depends on the method named getScreenResolution() returning the correct value. However, the getScreenResolution() method always seems to return 120 on my computer regardless of the actual screen resolution settings. |
As is often the case, I will discuss this program in fragments. The controlling class and the constructor for the GUI class are essentially the same as you have seen in several previous lessons, so, I won’t bore you by repeating that discussion here. You can view that material in the complete listing of the program at the end of the lesson.
All of the interesting action takes place in the overridden paint() method, so I will begin the discussion there.
The beginning portions of the overridden paint() method should be familiar to you by now as well. So, I am simply going to let the comments in Figure 1 speak for themselves.
That brings us to the interesting part, which, if you understand the previous discussion, you will find to be very straightforward.
The code in the next four fragments draws a circle in the upper left quadrant of the Frame and fills the circle with the color red.
The code in Figure 2 instantiates an object of the Ellipse2D.Double class. This is one of several classes in the API that can be used to produce geometric objects that implement the Shape interface.
Recall that the parameters required for the constructor of this class specify a bounding rectangle for the ellipse. If that bounding rectangle describes a square, the ellipse turns into a circle.
Assuming that the variable ds contains the actual screen resolution, the parameters used in this fragment describe a bounding rectangle (square) that is two inches one each side, and whose upper left-hand corner is positioned two inches above and two inches to the left of the origin. This will produce a circle with a two-inch diameter, located in the upper left-hand quadrant of the Frame.
Figure 3 instantiates a new object of the class Color, initialized to the color red, and passes it to the setPaint() method of the Graphics2D object. (Recall that the Color class implements the Paint interface, so this satisfies the type requirements of the parameter to the setPaint() method.)
Figure 4 invokes the fill() method on the Graphics2D object, passing the circle in the upper left-hand quadrant as a parameter. This causes the circle to be filled using the Color object discussed in the previous fragment. This, in turn, causes the circle to be filled with the color red
At this point, the red circle has not yet been rendered onto the screen. That happens in the Figure 5, which invokes the draw() method on the Graphics2D object, passing a reference to the red circle as a parameter.
When the circle is rendered, those attributes that have previously been established in the Graphics2D object (such as scaling, translation, rotation, etc.) will be used to render the circle.
Just for the record, recall that the draw() method requires a parameter that is a reference to an object of a class that implements the Shape interface. The Ellipse2D.Double class, of which this circle is an instance, is one of the geometric classes in the API that implements the Shape interface.
Once you understand all of the above, the remainder of the program, shown in Figure 6, is completely straightforward.
This remaining code simply creates, fills, and draws three more circles in the remaining three quadrants of the Frame. Except for variable names and parameter values, this code is the same as that shown in the previous several fragments, so I will let the comments speak for themselves.
You can view a complete listing of the program at the end of the lesson.
I have explained the basics of how fill is handled in Java 2D, and have shown you how to accomplish solid fill.
I will show you how to fill a Shape object with a color gradient, or with a BufferedImage object in subsequent lessons.
A complete listing of the program is provided in Figure 7.
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.
About the author
Richard Baldwin is a college professor and private consultant whose primary focus is a combination of Java and XML. In addition to the many platform-independent benefits of Java applications, he believes that a combination of Java and XML will become the primary driving force in the delivery of structured information on the Web.
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.
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