Comments and embedded documentation

Thursday Mar 1st 2001
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There are two types of comments in Java. The first is the traditional C-style comment that was inherited by C++. These comments begin with a /* and continue, possibly across many lines, until a */ . Note that many programmers will begin each line of a continued comment with a * , so you’ll often see:

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There are two types of comments in Java. The first is the traditional C-style comment that was inherited by C++. These comments begin with a /* and continue, possibly across many lines, until a */. Note that many programmers will begin each line of a continued comment with a *, so you’ll often see:

/* This is

* A comment that continues

* Across lines

*/

Remember, however, that everything inside the /* and */ is ignored so it’s no different to say:

/* This is a comment that

continues across lines */

The second form of comment comes from C++. It is the single-line comment, which starts at a // and continues until the end of the line. This type of comment is convenient and commonly used because it’s easy. You don’t need to hunt on the keyboard to find / and then * (you just press the same key twice), and you don’t need to close the comment. So you will often see:

// this is a one-line comment

Comment documentation

One of the thoughtful parts of the Java language is that the designers didn’t consider writing code to be the only important activity – they also thought about documenting it. Possibly the biggest problem with documenting code has been maintaining that documentation. If the documentation and the code are separate, it becomes a hassle to change the documentation every time you change the code. The solution seems simple: link the code to the documentation. The easiest way to do this is to put everything in the same file. To complete the picture, however, you need a special comment syntax to mark special documentation and a tool to extract those comments and put them in a useful form. This is what Java has done.

The tool to extract the comments is called javadoc. It uses some of the technology from the Java compiler to look for special comment tags you put in your programs. It not only extracts the information marked by these tags, but it also pulls out the class name or method name that adjoins the comment. This way you can get away with the minimal amount of work to generate decent program documentation.

The output of javadoc is an HTML file that you can view with your Web browser. This tool allows you to create and maintain a single source file and automatically generate useful documentation. Because of javadoc we have a standard for creating documentation, and it’s easy enough that we can expect or even demand documentation with all Java libraries.

Syntax

All of the javadoc commands occur only within /** comments. The comments end with */ as usual. There are two primary ways to use javadoc: embed HTML, or use “doc tags.” Doc tags are commands that start with a ‘ @’ and are placed at the beginning of a comment line. (A leading ‘ *’, however, is ignored.)

There are three “types” of comment documentation, which correspond to the element the comment precedes: class, variable, or method. That is, a class comment appears right before the definition of a class; a variable comment appears right in front of the definition of a variable and a method comment appears right in front of the definition of a method. As a simple example:

/** A class comment */
public class docTest {
  /** A variable comment */
  public int i;
  /** A method comment */
  public void f() {}
}

Note that javadoc will process comment documentation for only public and protected members. Comments for private and “friendly” (see Chapter 5) members are ignored and you’ll see no output. (You can use the -private flag to include private members as well.) This makes sense, since only public and protected members are available outside the file, which is the client programmer’s perspective. However, all class comments are included in the output.

The output for the above code is an HTML file that has the same standard format as all the rest of the Java documentation, so users will be comfortable with the format and can easily navigate your classes. It’s worth entering the above code, sending it through javadoc and viewing the resulting HTML file to see the results.

Embedded HTML

Javadoc passes HTML commands through to the generated HTML document. This allows you full use of HTML; however, the primary motive is to let you format code, such as:

/**

* <pre>

* System.out.println(new Date());

* </pre>

*/

You can also use HTML just as you would in any other Web document to format the regular text in your descriptions:

/**

* You can <em>even</em> insert a list:

* <ol>

* <li> Item one

* <li> Item two

* <li> Item three

* </ol>

*/

Note that within the documentation comment, asterisks at the beginning of a line are thrown away by javadoc, along with leading spaces. Javadoc reformats everything so that it conforms to the standard documentation appearance. Don’t use headings such as <h1> or <hr> as embedded HTML because javadoc inserts its own headings and yours will interfere with them.

All types of comment documentation – class, variable, and method – can support embedded HTML.

@see: referring to other classes

All three types of comment documentation can contain @see tags, which allow you to refer to the documentation in other classes. Javadoc will generate HTML with the @see tags hyperlinked to the other documentation. The forms are:

@see classname
@see fully-qualified-classname
@see fully-qualified-classname#method-name

Each one adds a hyperlinked “See Also” entry to the generated documentation. Javadoc will not check the hyperlinks you give it to make sure they are valid.

Class documentation tags

Along with embedded HTML and @see references, class documentation can include tags for version information and the author’s name. Class documentation can also be used for interfaces (described later in the book).

@version
This is of the form:

@version version-information

in which version-information is any significant information you see fit to include. When the -version flag is placed on the javadoc command line, the version information will be called out specially in the generated HTML documentation.

@author
This is of the form:

@author author-information

in which author-information is, presumably, your name, but it could also include your email address or any other appropriate information. When the -author flag is placed on the javadoc command line, the author information will be called out specially in the generated HTML documentation.

You can have multiple author tags for a list of authors, but they must be placed consecutively. All the author information will be lumped together into a single paragraph in the generated HTML.

Variable documentation tags

Variable documentation can include only embedded HTML and @see references.

Method documentation tags

As well as embedded documentation and @see references, methods allow documentation tags for parameters, return values, and exceptions.

@param
This is of the form:

@param parameter-name description

in which parameter-name is the identifier in the parameter list, and description is text that can continue on subsequent lines. The description is considered finished when a new documentation tag is encountered. You can have any number of these, presumably one for each parameter.

@return
This is of the form:

@return description

in which description gives you the meaning of the return value. It can continue on subsequent lines.

@exception
Exceptions will be described in Chapter 9, but briefly they are objects that can be “thrown” out of a method if that method fails. Although only one exception object can emerge when you call a method, a particular method might produce any number of different types of exceptions, all of which need descriptions. So the form for the exception tag is:

@exception fully-qualified-class-name description

in which fully-qualified-class-name gives an unambiguous name of an exception class that’s defined somewhere, and description (which can continue on subsequent lines) tells you why this particular type of exception can emerge from the method call.

@deprecated
This is new in Java 1.1. It is used to tag features that were superseded by an improved feature. The deprecated tag is a suggestion that you no longer use this particular feature, since sometime in the future it is likely to be removed. Methods that are marked @deprecated cause the compiler to issue warnings if it is used.

Documentation example

Here is the first Java program again, this time with documentation comments added:

//: Property.java
import java.util.*;
 
/** The first Thinking in Java example program.
 * Lists system information on current machine.
 * @author Bruce Eckel
 * @author http://www.BruceEckel.com
 * @version 1.0 
*/
public class Property {
  /** Sole entry point to class &amp; application
   * @param args array of string arguments
   * @return No return value
   * @exception exceptions No exceptions thrown
  */
  public static void main(String[] args) {
    System.out.println(new Date());
    Properties p = System.getProperties();
    p.list(System.out);
    System.out.println("--- Memory Usage:");
    Runtime rt = Runtime.getRuntime();
    System.out.println("Total Memory = "
                       + rt.totalMemory()
                       + " Free Memory = "
                       + rt.freeMemory());
  }
} ///:~ 

The first line:

//: Property.java

uses my own technique of putting a ‘ :’ as a special marker for the comment line containing the source file name. The last line also finishes with a comment, and this one indicates the end of the source code listing, which allows it to be automatically extracted from the text of the book and checked with a compiler. This is described in detail in Chapter 17.

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