Printing in Java, Part 3

Jean-Pierre Dubé introduces the print framework that works on top of the Java Print API

In this third part of a five-part series on printing in Java, I will explain the design of the print framework. This framework will work on top of the API to ease the burden of creating printed output. With it, you will be able to create pages with running headers/footers, and insert paragraphs, images, and tables. The coding phase will begin in Part 4 and continue into Part 5. We have a lot of ground to cover, so let's start.

Goals of the print framework

In Parts 1 and 2, you had the opportunity to examine in detail the Java Print API. With this knowledge, you are now able to evaluate its strengths and weaknesses and build a framework on top of it. That framework will compensate for the lack of high-level features in the Java Print API.

The print framework will:

  1. Be easy to use.
  2. Provide the high-level functionality required to efficiently render pages.
  3. Loosely couple with the Print API. Because the framework has no direct ties to the Java Print API, it will allow greater support for other output formats, such as PDF or HTML.
  4. Provide a structure that clearly defines each component involved in creating documents (document, page, paragraph, and so on).
  5. Use an abstract measurement system. Developers will thus be able work with the measurement system of their choice or even create their own. The framework will provide three default units of measurements: inches, centimeters, and points.
  6. Provide a print-preview facility. The Java Print API has no support for previewing the output before you print.
  7. Support a standard and portable page-setup dialog. As mentioned in Part 2, even though Java has been categorized as a WORA (Write Once, Run Anywhere) language, its functionality varies from platform to platform. The print framework will provide one page-setup dialog that will be uniform across all platforms.
  8. Offer export features. Although the framework will support several output formats (PDF, HTML, Postscript), their implementation is beyond the scope of this series. I leave the implementation up to you.
  9. Offer text handling, which is of primary importance. The framework will provide all the functionality needed to effectively render text, including right justification, left justification, full justification, and support for the AttributedString class.
  10. Support graphics primitives such as rectangles, circles, and lines.
  11. Support GIF and JPEG image types.
  12. Support headers and footers. These can be set at the document or the page level, and the first page can feature a different header/footer.
  13. Support sticky position, which easily sticks a component to a specific location. As an example, let's assume that you want to center an object in a page. Instead of doing all the math yourself, you can set the vertical and horizontal sticky value to a CENTER value; the framework centers the object within its parent container -- in this case, the page. Since all print objects are containers, the sticky values would be applied within the boundaries of the parent object.
  14. Support sticky dimensions, with which you can set a component's width and/or height to the parent container's width and/or height.
  15. Support tables, though their implementation will be limited.

In addition, all print objects are containers in the print framework.

A detailed look at the framework

To better understand the design of the framework, select UML Diagram 1 below:

UML Diagram 1

The diagram shows a hierarchical structure. Starting from the top of the hierarchy, you will find a PFDocument object that contains PFPage objects that contain any objects that are instances of PFPrintObject, where each PFPrintObject can contain other PFPrintObjects. If you got lost in the above explanation, take a look at Figure 1, which presents a better view of the print framework's structure. A closer look at the print objects reveals that the design is based on the composite pattern. I present more on the composite pattern later.

Figure 1. Document structure Click on thumbnail to view full-size image (44 KB)

The following sections will explain in detail the print framework's design; I begin with the universal measurement system.

Universal measurement system

An important feature of the print framework is that it supports conventional measurement units. Most of us prefer working with inches or centimeters, rather than with points or pixels. I created an abstract class called PFUnit to define the basic functionality required to convert from a given measurement system to points.

I also created three classes that implement the PFUnit class: PFInchUnit, PFCmUnit, and PFPointUnit. With these classes, you can work with your preferred unit of measurement. You can also create your own measurement system by subclassing PFUnit. When using these classes, you will use two methods: setUnit(), to set a value in the measurement system, and getPoints(), to get the value converted into points.

Using classes to represent numbers, instead of the standard double, float, or integer types, can cause problems when performing calculations. Another drawback to using classes is the lack of operator overloading in the Java language. To alleviate this problem, I added all the basic math operations to the PFUnit class. In this way, we preserve the encapsulation of data while providing basic math operations. As an example, let's add 2 inches to a PFInchUnit of 3 inches. You could complete that task in the non-object-oriented way:

PFInchUnit inch = new PFInchUnit (3.0);
inch.setUnit (new PFInchUnit (2.0).getUnit () + inch.getUnit ());

Or, by using the built-in math methods, you could do it like so:

PFInchUnit inch = new PFInchUnit (3.0);
inch = inch.add (2.0);

I created three other classes to complement the measurement system:

  1. PFPoint, which represents an X,Y point
  2. PFSize, which represents the size of an object
  3. PFRectangle, which represents a rectangle area

All of the above classes use the PFUnit class.

The PFDocument class

The PFDocument class is located at the top of the hierarchy in the print framework library. The class acts as a page container and provides the necessary functionality to handle the print and export processes. This class also offers some methods related to document-handling.

With the PFDocument class, you can set headers and footers for a whole document using the setHeader()/setFooter() methods. You can set a different header/footer for the first page by using the setFirstPageHeader() and the setFirstPageFooter() methods.

PFDocument also includes a print-preview window. By calling the printPreview() method, a print-preview window will display. I will discuss the preview window in further detail later.

One of the goals of the print framework is to provide functionality for exporting documents to formats other than paper. Since the print framework model is totally independent of the output, it's possible to export to any format. To export your document to such formats as HTML or PDF, use either the printPDF() or the printHTML() methods. Note that these methods appear in the design but have not been implemented yet. I leave their implementation to you.

PFDocument offers several other methods to set document properties. For example, with setDocumentName() you can set a document's name, which will identify your document in the host operating system's print queue. The print-preview window also uses the document name to identify your document in the window's title bar. Finally the showPrintDialog() and showPageDialog() methods will show the appropriate dialog. Note that the page dialog called by showPageDialog() does not originate in the Java Print API. I decided to implement my own page dialog after I tried printing on different platforms and discovered that page dialogs vary from platform to platform.

Now that you know about the functionality implemented by the PFDocument class, let's dive into the print method's details. Take a look at UML Diagram 2 below.

UML Diagram 2

As shown in this diagram, the messages flow between each component when the PFDocument print() method is called. The PFDocument will instantiate the print job. It will first create a Book object and will enter in a loop to add all of the document pages to the Book. After PFDocument completes the loop, PrinterJob print() is called, and the Java Print API handles the rest of the printing process.

The following sections will elaborate on each object's role in the print framework. I begin these explanations with the PFPage class.

PFPage class

Next in the hierarchy, the PFPage class defines a page and acts as a container for objects that will be rendered on it. Each page in the print framework can have its own format. There is no limit to the number of objects that you can add to a page.

Each page can either have its own header/footer or use the one defined in the PFDocument class. How does the print framework decide which header/footer to use? First, the page checks if its own header/footer is null. If so, it will try to obtain the header/footer from the PFDocument object. If the header/footer has a non-null value, the page will use the document header/footer. Note that the document has two methods, getHeader() and getFooter(), that handle the logistics of supplying the appropriate header/footer. The header/footer value is based on the value of the first page header/footer and whether or not the page under consideration is the first one.

The PFPage class also contains two important methods, getPrintableAreaOrigin() and getPrintableAreaSize(). getPrintableAreaOrigin() provides the location of the printable area's top left corner, which is determined by the margins, the gutter, and the height of the page header. getPrintableAreaSize(), on the other hand, returns the printable area's size. Figure 2 illustrates the printable area origin and the printable area size.

Figure 2. Page layout Click on thumbnail to view full-size image (13 KB)

To print, PFPage must implement the Printable interface, which Part 1 explains.

You are now ready to see how the framework handles the print objects.

Understand the PFPrintObject

The design of the print object architecture is based on the composite pattern. I decided to use that pattern after considering the following: Suppose you wish to draw a circle in the middle of a rectangle with a drawing program. You decide to group these two objects together to form one object so that you can move, resize, copy, and delete the objects as a whole. But how does a good object-oriented program handle such a feature?

You could create container object and another object that is a graphics primitive. Your program would have to differentiate between these two distinct objects and treat them accordingly. I've tried this approach before, and the results were buggy and hard to maintain.

Instead, I suggest using a composite pattern. It ensures uniform access to all graphics objects. Because each graphics object also acts as a container, no difference exists between a container object and a graphics primitive object. The same objects handle all functionality. The composite structure will produce a hierarchy of objects.

Figure 3 illustrates how the composite pattern works.

Figure 3. Composite structure Click on thumbnail to view full-size image (15 KB)

Figure 3 shows that you can create a complex object composition with only one type of object. You can also group objects together with the composite pattern, and even group together several groups of objects. By using the pattern, you simplify the framework's design considerably.

The composite pattern is also easy to expand; you can create your own print objects by extending the PFPrintObject. Since all the basic functionality is located in one place, expanding the PFPrintObject gives your extended objects all the container functionality as well as sticky positioning, sticky fill, and the rendering of all child objects.

To see how this pattern would help solve a real-world problem, consider the following example. Let's say you want to build a page header as shown in Figure 4.

Figure 4. Page header structure

To build the header, you could draw everything by hand and work out the math necessary to position each component. Or you could take advantage of the PFPrintObject's container feature.

By examining the structure of the page header, you see that the master object contains everything. Two paragraph objects are added to the master object. The first paragraph object will print the string "Printing in Java". That object will have the following properties set: verticalSticky = CENTER, horizontalSticky = LEFT. The second paragraph object will contain the string "Page nn", with the properties verticalSticky = CENTER and the horizontalSticky = RIGHT. Finally, we add a line object with the following properties: verticalSticky = BOTTOM, horizontalSticky = CENTER, and horizontalFill = TRUE. Don't forget to set a margin around the main component to ensure that everything centers within the header area. Refer to Figure 5 to see where everything will fit.

Figure 5. Page header structure Click on thumbnail to view full-size image (9 KB)

The use of the sticky functionality saves us many headaches. The code that we created in that example is 100 percent reusable and will automatically adapt to any situation. If the user decides to change the orientation of the page, it will adjust itself automatically; no additional code is required.

So, what is sticky functionality, anyway? The following section should answer your questions.

Sticky positioning

Many of you may be wondering why I've been using the term sticky. I devised this term while working as a graphics editor. I had to provide functionality with which a user could glue a component to another component using a relative position such as top, bottom, center, right, or left. The component had to stick to the assigned position even if the user moved or resized the position. I came up with the term sticky component, and it has stuck with me ever since.

A sticky component facilitates the positioning of print components on a page or inside another component. With these sticky methods, you can place a component within a parent component at the following positions: on the vertical axis, top, center, bottom and on the horizontal axis left, center, and right. Figure 6 shows all the possibilities along with their properties.

Figure 6. Sticky positioning Click on thumbnail to view full-size image (6 KB)

Two methods allow you to set the sticky position values: setVerticalSticky() and setHorizontalSticky().

Sticky sizing

Another useful bit of functionality is sticky sizing. With sticky sizing you can set a component's width and/or height to the size of its parent component. The parent component can either be the page to which the component was added or another component.

Two methods set sticky sizing: setHorizontalFill (boolean) and setVerticalFill (boolean).

Margins

If pages can have margins, then PFPrintObject can have margins too. Having the ability to set a margin around a component can be useful and will enhance your printed output. You can set margins independently using the current measurement system.

How do you implement all of the margins' properties? The computePositionAndSize() method handles everything. You probably noted upon examining UML Diagram 1 that this method is private; only the PFPrintPrintObject class has access to it. computePositionAndSize() computes the sticky positioning, the sticky filling, and the margins. When finished computing, the method returns the top left corner of the area where you can draw, and the width and height of the drawing area for the object. Everything that falls outside this area will be clipped.

Let's take a look inside computePositionAndSize().

First, the parent object returns the actual drawing origin and size. The parent object can be either the page to which the object -- PFPage -- was added, or another object -- PFPrintObject. Next, set the margins and then calculate the sticky sizing. Finally, determine the sticky positioning properties. By completing the calculations in this order, you solve all possible conflicts within each property without relying on any if code. To prevent any ill-behaved object from drawing outside of the drawing area, Graphics2D is clipped accordingly. The drawingOrigin and drawingSize objects store the result of that computation. To access these values, use the following two methods: getDrawingOrigin() and getDrawingSize().

To produce different effects, each component can be rotated. The method rotate (double) in PFPrintObject adds this functionality. All angles are in degrees.

Paragraph

Efficient text management is of primary importance in this framework, and PFParagraph manages this task. The object will accept two types of input: a standard String object and an AttributedString object. AttributedString, explained in Part 2, is currently the only way to add reformatted text. Some of you may ask, "What about HTML, or RTF, or XML?" I'm leaving their implementation to you. You could easily add support for the XML format, whereas the other two formats would require extra work. Even so, the framework features enough functionality to satisfy your immediate needs.

The PFParagraph supports left, center, right, and full justification. You can set a default text color, but it will be overridden if AttributedString sets the text color.

Four private methods handle the text justification. I took the implementation code from the text examples in Part 2.

Drawing primitives

The framework would not be complete without the basic graphics primitives: rectangles, circles, and lines. All of these extend the PFPrintObject object. Each object uses a corresponding Graphics2D draw method to render its graphics. Several methods are available to set different properties such as line width, line color, and fill color.

Images

The PFImage object supports JPEG and GIF files. You could easily add other formats. I kept PFImage as simple as possible, while providing enough flexibility for advanced image processing. Instead of trying to provide all the functionality of the Java 2D API inside a single object, I opted to implement the functionality directly related to the print framework, and left the image processing to you. I included a method for obtaining a copy of the BufferedImage object. Once you have access to a BufferedImage, complete all the image processing that you want. Load an image using the setURL (String) method. The string parameter must be a valid URL.

Tables

Our framework would also not be complete if it didn't sustain tables. When creating complex layouts, tables often come to the rescue. A PFPrintObject will represent each cell in the table, so right from the start you have all that class's functionality. Included in the framework are methods that apply properties to a complete row, column, or table.

However, the framework's table implementation features a few limitations. First, the table class does not support column spanning. A cell cannot occupy the space of more than one cell at a time. Second, the print framework cannot import tabular data. You will need to build each row one at time by completing some additional coding.

Printing visual components

Sometimes it may be useful to render visual components on paper. The PFVisualObject has only one method, setComponent(). Pass the visual component that you want to print to that method. The print method will size and position the visual component to the size and position of the PFVisualComponent, and then call the visual component paint method with a Graphics2D object.

You must remember one important factor when printing visual components. As explained in Part 1, the location at which you create your visual object affects the resolution of the Graphics object that is assigned to it. When a Graphics object is created, an AffineTransform is automatically assigned to it. AffineTransform maps the user space to the device space. For example, when you create your components inside a Frame, it sets the AffineTransform to map the device space -- in this case, a screen that has a lower resolution. Thus, your component will print at the same resolution as the screen. Always make sure that you create your components outside a graphic context if you want the maximum resolution.

Print preview

The print preview implemented here will only show a single page at a time. In order to illustrate a page on screen, you must resize it to fit the view window. Preserving the aspect ratio of the page is important. For example, the aspect ratio for a page that measures 8.5 by 11 inches is 0.77 (8.5 divided by 11). The size of the view window for such a page must therefore have a width that is 0.77 of the height. Once you size the view window accordingly, you can resize the page to fit within this view window.

Since the print framework renders a Graphics2D object, we can use that same object to draw on the screen. You must scale the drawing to fit the page in the preview window. To achieve that task, apply an AffineTransform to Graphics2D.

Using the preview window, the user can navigate to the first page, last page, next page, and previous page, and can zoom in and out. In addition, the window includes a button to print the document.

The preview window is structured as two objects, the first one being PFPrintPreviewToolbar. That object creates the toolbar and will send the user actions to PFPrintPreview, the second object; it renders a given page on screen. To see how these two objects collaborate, select UML Diagram 3 below.

UML Diagram 3

Print job dialog

To make sure that users of all platforms have access to the same page setup functionality, I devised a standard dialog for setting such printing parameters as the margins and paper orientation. Because adding the number of copies and page range selection would double the parameters found in print dialog, I omitted that functionality. UML Diagram 1 provides a good overview of the PFPageSetup class.

Conclusion

This concludes our discussion on the design of the print framework. Now that you understand the goals of the framework, we will move to the coding phase. In Part 4, you will start coding all the basic objects, such as the PFDocument, PFPage, and the PFPrintObject. Until then, happy printing.

Jean-Pierre Dubé is an independent Java consultant. He founded Infocom in 1988. Since then, Infocom has developed custom applications in fields including manufacturing, document management, and large-scale electrical power line management. Jean-Pierre has extensive programming experience in C, Visual Basic, and Java; the latter is now the primary language for all new projects. He dedicates this series to his mother, who passed away while he was writing this article.

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