Home > Articles > Programming > Java

  • Print
  • + Share This
Like this article? We recommend

The API

My first take on writing a syntax highlighter centered around creating a View for the Document that would display parentheses differently from other characters, and depending on the current position of the caret. That turns out to involve a fair amount of effort, because the View classes are responsible for a lot of things: breaking the text into paragraphs, and then into lines. However, it doesn't break lines into words (at least not for the plain views I was interested in); and getting words to display in order, and properly communicating their position afterwards, would be a lot of work.

It also involved creating an EditorKit, which doesn't quite fit cleanly into the whole model/view/controller paradigm. It's kind of a way of moderating the interaction between the view and the controller allowing customization. Clearly, we're making a very complicated task out of what I'd hoped would be a simple one.

It turns out that the JTextComponent designers anticipated my needs, although I've never seen their trick used anywhere. JTextComponent is the base class of the commonly used JEditorPane, JTextArea, and JTextField classes. It's an odd bit of package design: The JEditorPane, JTextArea, and JTextField classes are in the javax.swing package, where they're highly visible, while JTextComponent is "hidden" in the sub-package javax.swing.text. Ordinarily, base classes are considered more general, and therefore belong in higher-level packages. The implication is that JTextComponent bears common abstract and low-level functions, but is less interesting than the particular implementations that provide more complete functionality. You can use JTextField, for example, without ever using any of the JTextComponent methods.

JTextComponent provides a number of features common to all text components, but they remain somewhat obscure because they're in a base class with dozens of methods. I didn't know that JTextComponent has a NavigationFilter designed to prevent you from clicking in certain places, and until I began this project I didn't know that they'd anticipated the need for a highlighter.

Highlighting is represented by a Highlighter object, obtained by calling getHighlighter on the JTextComponent. A default highlighter is provided for you; you don't have to call setHighlighter first. Which is good, because it saves us having to figure out what kind of Highlighter we want. This same highlighter is also responsible for highlighting text that has been selected.

The most immediately relevant method of Highlighter is shown here:

public Object addHighlight(int p0,
              int p1,
              Highlighter.HighlightPainter p)
          throws BadLocationException

This adds a highlight from p0 to p1, using p to paint the highlighting background.

An Aside: API Design and Strong Typing

The object that's returned is an arbitrary, featureless tag. That's kind of odd, and the corresponding code to remove a highlight is as follows:

public void removeHighlight(Object tag)

This is prone to all sorts of misuse; it's easy to pass in the wrong object. You could pass in any object, even a String, although that would be meaningless. Usually, an API designer would use a typed object to prevent that kind of mistake, perhaps called Highlight. There would still be ways to get it wrong (for example, using a Highlight from a different Highlighter), but it would prevent some mistakes.

Even odder, there's already a Highlighter.Highlight class that could have been used, and if you poke around in the code for the DefaultHighlighter you'll find that addHighlight does in fact return a Highlighter.Highlight object (by way of LayeredHighlightInfo, an internal class).

I'm not the first person to discover this oddity; it was dealt with more than two years ago in Java's Bug Parade as bug ID 4200254. (Note: Requires login.) They provided the rather weak excuse that it was to allow flexibility in the implementations (an interface would have allowed strong typing without limiting flexibility). They also make the much better excuse that changing it at this point would break binary compatibility. It may not be the best design, but it works, and functioning code is usually to be preferred to reengineered code that breaks your existing code base.

If you don't keep an eye on the Bug Parade, you should. If you think that something is wrong that can be blamed on the Java APIs, or even if you just think that a design is odd, the odds are good that somebody has already complained. The Bug Parade may be able to give you a workaround you hadn't considered. It may reveal nothing more than a case like this, where Sun has looked at the problem and chosen not to fix it, but at least you'll know you aren't crazy. It may reveal a known bug that's going to be fixed someday, but not now. It won't solve your problem, but it'll allow you to pick a different route to achieving your goal rather than beating your head against a wall. Or it may just be the proof you need to tell your boss to end an ill-considered project entirely.

The addHighlight method takes three arguments. The first two identify the characters you want lit. They form a range, where 0 is the first character. The second argument is the first character that you don't want lit. So, to highlight just the first character, the arguments should be (0, 1).

The numbers might make more sense if you look at them the way Document wants you to, where the numbers refer to the positions between characters rather than the characters themselves, as in Figure 2 (from the Document documentation):

Figure 2Figure 2 Offsets in Documents.

The addHighlight method also takes a Highlighter.HighlightPainter. Highlighter.HighlightPainter is an interface, containing only a single method:

/* Renders the highlight. */
public void paint(Graphics g,
         int p0,
         int p1,
         Shape bounds,
         JTextComponent c)

Fortunately, we don't have to implement it. Java has provided a default implementation: DefaultHighlighter.DefaultHighlightPainter. It takes a Color, such as Color.green. It highlights the characters by painting their background to the color you provide. We're going to want two different painters: one for when the parentheses match, and another for when they don't:

Highlighter.HighlightPainter goodPainter =
  new DefaultHighlighter.DefaultHighlightPainter(Color.cyan);
Highlighter.HighlightPainter badPainter =
  new DefaultHighlighter.DefaultHighlightPainter(Color.magenta);
  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020