打印多页文档

You have already learned how to use the Printable interface to print a single page document. However, documents are usually more than one physical page in length. Pagination is the process of identifying the location in a document where page breaks and printing accordingly.

In case of printing several graphics images, one per page, use the page index to iterate through these pages and print one on each page. For example, if several images are represented in the following array:

BufferedImage[] images = new BufferedImage[10];

then use the print() method as shown in the following code fragment:

public int print(Graphics graphics,
           PageFormat pageFormat, int pageIndex)
           throws PrinterException {

    if (pageIndex < images.length) {
        graphics.drawImage(images[pageIndex], 100, 100, null);
        return PAGE_EXISTS;
    } else {
        return NO_SUCH_PAGE:
    }
}

If the document is continuous, the application must calculate how much content can fit on each page, and break the page at that point. If text document consists of many lines, then an application must calculate how many of these lines can fit entirely on a page. The Point class creates a point representing a location in (x,y)

To calculate the height of a single line of text, use the FontMetrics class.

Font font = new Font("Serif", Font.PLAIN, 10);
FontMetrics metrics = graphics.getFontMetrics(font);
int lineHeight = metrics.getHeight();

The PageFormat parameter describes the printable area of the page. In particular, to find the vertical span of the page use the following code fragment:

double pageHeight = pageFormat.getImageableHeight();

Use the following code fragment to calculate the number of lines that fit on a page and the number of page breaks:

int linesPerPage = ((int)pageHeight)/lineHeight);
int numBreaks = (textLines.length-1)/linesPerPage;
int[] pageBreaks = new int[numBreaks];
for (int b=0; b < numBreaks; b++) {
    pageBreaks[b] = (b+1)*linesPerPage; 
}

Use the print() method to calculate the printable area for the following reasons:

• Text measurement depends on the FontRenderContext and this is implicit in the FontMetrics object returned by the printer graphics which is not available except inside the print() method.
• The page format may not be disclosured until printing occurs. Since if the user selected a landscape mode in the print dialog, then this setting needs to be accounted for. The PageFormat object passed into the print() method provides this information.

The page break positions are used as represented in the following code fragment:

/* Draw each line that is on this page.
 * Increment 'y' position by lineHeight
 * for each line.
 */
int y = 0; 
int start = (pageIndex == 0) ? 0 : pageBreaks[pageIndex-1];
int end   = (pageIndex == pageBreaks.length)
                 ? textLines.length : pageBreaks[pageIndex];
for (int line=start; line<end; line++) {
    y += lineHeight;
    g.drawString(textLines[line], 0, y);
}

If a document contains 100 lines and only 48 lines fit on a page, then an application prints 3 pages with page breaks after 48 and 96 lines of text. The remaining 4 lines are printed on the last page. The complete code for this example is in PaginationExample.java.

The following simplifying factors are used in the PaginationExample code:

• Each page has the same height.
• The same font is used.