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How to Write Doc Comments for the Javadoc Tool

Javadoc Home Page

This document describes the style guide, tag and image conventions we use in documentation comments for Java programs written at Java Software, Oracle. It does not rehash related material covered elsewhere:

Terminology

Source files, format of a doc comment, doc comment checking tool, descriptions, a style guide.

Documenting Default Constructors

Documenting exceptions with @throws tag, package-level comments, documenting anonymous inner classes, including images, examples of doc comments, troubleshooting curly quotes (microsoft word), introduction.

At Java Software, we have several guidelines that might make our documentation comments different than those of third party developers. Our documentation comments define the official Java Platform API Specification . To this end, our target audience is those who write Java compatibility tests, or conform or re-implement the Java platform, in addition to developers. We spend time and effort focused on specifying boundary conditions, argument ranges and corner cases rather than defining common programming terms, writing conceptual overviews, and including examples for developers.

Thus, there are commonly two different ways to write doc comments -- as API specifications, or as programming guide documentation. These two targets are described in the following sections. A staff with generous resources can afford to blend both into the same documentation (properly "chunked"); however, our priorities dictate that we give prime focus to writing API specifications in doc comments. This is why developers often need to turn to other documents, such as Java SE Technical Documentation and The Java Tutorials for programming guides.

Writing API Specifications

Ideally, the Java API Specification comprises all assertions required to do a clean-room implementation of the Java Platform for "write once, run anywhere" -- such that any Java applet or application will run the same on any implementation. This may include assertions in the doc comments plus those in any architectural and functional specifications (usually written in FrameMaker) or in any other document. This definition is a lofty goal and there is some practical limitation to how fully we can specify the API. The following are guiding principles we try to follow:

The Java Platform API Specification is defined by the documentation comments in the source code and any documents marked as specifications reachable from those comments.

Notice that the specification does not need to be entirely contained in doc comments. In particular, specifications that are lengthy are sometimes best formatted in a separate file and linked to from a doc comment.

The Java Platform API Specification is a contract between callers and implementations.

The Specification describes all aspects of the behavior of each method on which a caller can rely. It does not describe implementation details, such as whether the method is native or synchronized. The specification should describe (textually) the thread-safety guarantees provided by a given object. In the absence of explicit indication to the contrary, all objects are assumed to be "thread-safe" (i.e., it is permissible for multiple threads to access them concurrently). It is recognized that current specifications don't always live up to this ideal.

Unless otherwise noted, the Java API Specification assertions need to be implementation-independent. Exceptions must be set apart and prominently marked as such.

We have guidelines for how to prominently document implementation differences .

The Java API Specification should contain assertions sufficient to enable Software Quality Assurance to write complete Java Compatibility Kit (JCK) tests.

This means that the doc comments must satisfy the needs of the conformance testing by SQA. The comments should not document bugs or how an implementation that is currently out of spec happens to work.

Writing Programming Guide Documentation

What separates API specifications from a programming guide are examples, definitions of common programming terms, certain conceptual overviews (such as metaphors), and descriptions of implementation bugs and workarounds. There is no dispute that these contribute to a developer's understanding and help a developer write reliable applications more quickly. However, because these do not contain API "assertions", they are not necessary in an API specification. You can include any or all of this information in documentation comments (and can include custom tags , handled by a custom doclet, to facilitate it). At Java Software, we consciously do not include this level of documentation in doc comments, and instead include either links to this information (links to the Java Tutorial and list of changes) or include this information in the same documentation download bundle as the API spec -- the JDK documentation bundle includes the API specs as well as demos, examples, and programming guides.

It's useful to go into further detail about how to document bugs and workarounds. There is sometimes a discrepancy between how code should work and how it actually works. This can take two different forms: API spec bugs and code bugs. It's useful to decide up front whether you want to document these in the doc comments. At Java Software we have decided to document both of these outside of doc comments, though we do make exceptions.

API spec bugs are bugs that are present in the method declaration or in the doc comment that affects the syntax or semantics. An example of such a spec bug is a method that is specified to throw a NullPointerException when null is passed in, but null is actually a useful parameter that should be accepted (and was even implemented that way). If a decision is made to correct the API specification, it would be useful to state that either in the API specification itself, or in a list of changes to the spec, or both. Documenting an API difference like this in a doc comment, along with its workaround, alerts a developer to the change where they are most likely to see it. Note that an API specification with this correction would still maintain its implementation-independence.

Code bugs are bugs in the implementation rather than in the API specification. Code bugs and their workarounds are often likewise distributed separately in a bug report. However, if the Javadoc tool is being used to generate documentation for a particular implementation, it would be quite useful to include this information in the doc comments, suitably separated as a note or by a custom tag (say @bug ).

Who Owns and Edits the Doc Comments

The doc comments for the Java platform API specification is owned programmers. However, they are edited by both programmers and writers. It is a basic premise that writers and programmers honor each other's capabilities and both contribute to the best doc comments possible. Often it is a matter of negotiation to determine who writes which parts of the documentation, based on knowledge, time, resources, interest, API complexity, and on the state of the implementation itself. But the final comments must be approved by the responsible engineer.

Ideally, the person designing the API would write the API specification in skeleton source files, with only declarations and doc comments, filling in the implementation only to satisfy the written API contract. The purpose of an API writer is to relieve the designer from some of this work. In this case, the API designer would write the initial doc comments using sparse language, and then the writer would review the comments, refine the content, and add tags.

If the doc comments are an API specification for re-implementors, and not simply a guide for developers, they should be written either by the programmer who designed and implemented the API, or by a API writer who is or has become a subject matter expert. If the implementation is written to spec but the doc comments are unfinished, a writer can complete the doc comments by inspecting the source code or writing programs that test the API. A writer might inspect or test for exceptions thrown, parameter boundary conditions, and for acceptance of null arguments. However, a much more difficult situation arises if the implementation is not written to spec. Then a writer can proceed to write an API specification only if they either know the intent of the designer (either through design meetings or through a separately-written design specification) or have ready access to the designer with their questions. Thus, it may be more difficult for a writer to write the documentation for interfaces and abstract classes that have no implementors.

With that in mind, these guidelines are intended to describe the finished documentation comments. They are intended as suggestions rather than requirements to be slavishly followed if they seem overly burdensome, or if creative alternatives can be found. When a complex system such as Java (which contains about 60 packages) is being developed, often a group of engineers contributing to a particular set of packages, such as javax.swing may develop guidelines that are different from other groups. This may be due to the differing requirements of those packages, or because of resource constraints.

API documentation (API docs) or API specifications (API specs)

On-line or hardcopy descriptions of the API, intended primarily for programmers writing in Java. These can be generated using the Javadoc tool or created some other way. An API specification is a particular kind of API document, as described above . An example of an API specification is the on-line Java Platform, Standard Edition 7 API Specification .

Documentation comments (doc comments)

The special comments in the Java source code that are delimited by the /** ... */ delimiters. These comments are processed by the Javadoc tool to generate the API docs.

The JDK tool that generates API documentation from documentation comments.

The Javadoc tool can generate output originating from four different types of "source" files:

For more details, see: Source Files .

Writing Doc Comments

A doc comment is written in HTML and must precede a class, field, constructor or method declaration. It is made up of two parts -- a description followed by block tags. In this example, the block tags are @param , @return , and @see .

For more examples, see Simple Examples .

So lines won't wrap, limit any doc-comment lines to 80 characters.

Here is what the previous example would look like after running the Javadoc tool:

Returns an Image object that can then be painted on the screen. The url argument must specify an absolute URL. The name argument is a specifier that is relative to the url argument.

This method always returns immediately, whether or not the image exists. When this applet attempts to draw the image on the screen, the data will be loaded. The graphics primitives that draw the image will incrementally paint on the screen.

Parameters:

url - an absolute URL giving the base location of the image.

name - the location of the image, relative to the url argument.

the image at the specified URL.

Also see Troubleshooting Curly Quotes (Microsoft Word) at the end of this document.

At Oracle, we have developed a tool for checking doc comments, called the Oracle Doc Check Doclet, or DocCheck. You run it on source code and it generates a report describing what style and tag errors the comments have, and recommends changes. We have tried to make its rules conform to the rules in this document.

DocCheck is a Javadoc doclet, or "plug-in", and so requires that the Javadoc tool be installed (as part of the Java 2 Standard Edition SDK).

First Sentence

The first sentence of each doc comment should be a summary sentence, containing a concise but complete description of the API item. This means the first sentence of each member, class, interface or package description. The Javadoc tool copies this first sentence to the appropriate member, class/interface or package summary. This makes it important to write crisp and informative initial sentences that can stand on their own.

This sentence ends at the first period that is followed by a blank, tab, or line terminator, or at the first tag (as defined below). For example, this first sentence ends at "Prof.":

However, you can work around this by typing an HTML meta-character such as "&" or "<" immediately after the period, such as:

In particular, write summary sentences that distinguish overloaded methods from each other. For example:

Implementation-Independence

Write the description to be implementation-independent, but specifying such dependencies where necessary. This helps engineers write code to be "write once, run anywhere."

On Windows systems, the path search behavior of the loadLibrary method is identical to that of the Windows API's LoadLibrary procedure.

The use of "On Windows" at the beginning of the sentence makes it clear up front that this is an implementation note.

Automatic re-use of method comments

You can avoid re-typing doc comments by being aware of how the Javadoc tool duplicates (inherits) comments for methods that override or implement other methods. This occurs in three cases:

In the first two cases, if a method m() overrides another method, The Javadoc tool will generate a subheading "Overrides" in the documentation for m() , with a link to the method it is overriding.

In the third case, if a method m() in a given class implements a method in an interface, the Javadoc tool will generate a subheading "Specified by" in the documentation for m() , with a link to the method it is implementing.

In all three of these cases, if the method m() contains no doc comments or tags, the Javadoc tool will also copy the text of the method it is overriding or implementing to the generated documentation for m() . So if the documentation of the overridden or implemented method is sufficient, you do not need to add documentation for m() . If you add any documentation comment or tag to m() , the "Overrides" or "Specified by" subheading and link will still appear, but no text will be copied.

The following are useful tips and conventions for writing descriptions in doc comments.

Keywords and names are offset by <code>...</code> when mentioned in a description. This includes:

You are encouraged to add links for API names (listed immediately above) using the {@link} tag. It is not necessary to add links for all API names in a doc comment. Because links call attention to themselves (by their color and underline in HTML, and by their length in source code doc comments), it can make the comments more difficult to read if used profusely. We therefore recommend adding a link to an API name if:

Our audience is advanced (not novice) programmers, so it is generally not necessary to link to API in the java.lang package (such as String), or other API you feel would be well-known.

When referring to a method or constructor that has multiple forms, and you mean to refer to a specific form, use parentheses and argument types. For example, ArrayList has two add methods: add(Object) and add(int, Object):

The add(int, Object) method adds an item at a specified position in this arraylist.

However, if referring to both forms of the method, omit the parentheses altogether. It is misleading to include empty parentheses, because that would imply a particular form of the method. The intent here is to distinguish the general method from any of its particular forms. Include the word "method" to distinguish it as a method and not a field.

The add method enables you to insert items. (preferred)

The add() method enables you to insert items. (avoid when you mean "all forms" of the add method)

OK to use phrases instead of complete sentences, in the interests of brevity. This holds especially in the initial summary and in @param tag descriptions.

The description is in 3rd person declarative rather than 2nd person imperative.

Gets the label. (preferred)

Get the label. (avoid)

A method implements an operation, so it usually starts with a verb phrase:

Gets the label of this button. (preferred)

This method gets the label of this button.

These API often describe things rather than actions or behaviors:

A button label. (preferred)

This field is a button label. (avoid)

For example, the description of the getToolkit method should read as follows:

Gets the toolkit for this component. (preferred)

Gets the toolkit for the component. (avoid)

The best API names are "self-documenting", meaning they tell you basically what the API does. If the doc comment merely repeats the API name in sentence form, it is not providing more information. For example, if method description uses only the words that appear in the method name, then it is adding nothing at all to what you could infer. The ideal comment goes beyond those words and should always reward you with some bit of information that was not immediately obvious from the API name.

Avoid - The description below says nothing beyond what you know from reading the method name. The words "set", "tool", "tip", and "text" are simply repeated in a sentence.

Preferred - This description more completely defines what a tool tip is, in the larger context of registering and being displayed in response to the cursor.

Be aware that the word "field" has two meanings:

use "also known as" instead of "aka", use "that is" or "to be specific" instead of "i.e.", use "for example" instead of "e.g.", and use "in other words" or "namely" instead of "viz."

Tag Conventions

This section covers:

Order of Tags

Ordering Multiple Tags

Required Tags

Most of the following tags are specified in the Java Language Specification, First Edition . Also see the Javadoc reference page .

Include tags in the following order:

We employ the following conventions when a tag appears more than once in a documentation comment. If desired, groups of tags, such as multiple @see tags, can be separated from the other tags by a blank line with a single asterisk.

Multiple @author tags should be listed in chronological order, with the creator of the class listed at the top.

Multiple @param tags should be listed in argument-declaration order. This makes it easier to visually match the list to the declaration.

Multiple @throws tags (also known as @exception) should be listed alphabetically by the exception names.  

Multiple @see tags should be ordered as follows, which is roughly the same order as their arguments are searched for by javadoc , basically from nearest to farthest access, from least-qualified to fully-qualified, The following list shows this progression. Notice the methods and constructors are in "telescoping" order, which means the "no arg" form first, then the "1 arg" form, then the "2 arg" form, and so forth. Where a second sorting key is needed, they could be listed either alphabetically or grouped logically.

An @param tag is "required" (by convention) for every parameter, even when the description is obvious. The @return tag is required for every method that returns something other than void , even if it is redundant with the method description. (Whenever possible, find something non-redundant (ideally, more specific) to use for the tag comment.)

These principles expedite automated searches and automated processing. Frequently, too, the effort to avoid redundancy pays off in extra clarity.

Tag Comments

As a reminder, the fundamental use of these tags is described on the Javadoc Reference page . Java Software generally uses the following additional guidelines to create comments for each tag:

@author (reference page)

You can provide one @author tag, multiple @author tags, or no @author tags. In these days of the community process when development of new APIs is an open, joint effort, the JSR can be consider the author for new packages at the package level. For example, the new package java.nio has "@author JSR-51 Expert Group" at the package level. Then individual programmers can be assigned to @author at the class level. As this tag can only be applied at the overview, package and class level, the tag applies only to those who make significant contributions to the design or implementation, and so would not ordinarily include technical writers.

The @author tag is not critical, because it is not included when generating the API specification, and so it is seen only by those viewing the source code. (Version history can also be used for determining contributors for internal purposes.)

If someone felt strongly they need to add @author at the member level, they could do so by running javadoc using the new 1.4 -tag option:

-tag author:a:"Author:"

If the author is unknown, use "unascribed" as the argument to @author. Also see order of multiple @author tags .

@version (reference page)

The Java Software convention for the argument to the @version tag is the SCCS string "%I%, %G%", which converts to something like " 1.39, 02/28/97 " (mm/dd/yy) when the file is checked out of SCCS.

@param (reference page)

The @param tag is followed by the name (not data type) of the parameter, followed by a description of the parameter. By convention, the first noun in the description is the data type of the parameter. (Articles like "a", "an", and "the" can precede the noun.) An exception is made for the primitive int , where the data type is usually omitted. Additional spaces can be inserted between the name and description so that the descriptions line up in a block. Dashes or other punctuation should not be inserted before the description, as the Javadoc tool inserts one dash.

Parameter names are lowercase by convention. The data type starts with a lowercase letter to indicate an object rather than a class. The description begins with a lowercase letter if it is a phrase (contains no verb), or an uppercase letter if it is a sentence. End the phrase with a period only if another phrase or sentence follows it.

Do not bracket the name of the parameter after the @param tag with <code>...</code> since Javadoc 1.2 and later automatically do this. (Beginning with 1.4, the name cannot contain any HTML, as Javadoc compares the @param name to the name that appears in the signature and emits a warning if there is any difference.)

When writing the comments themselves, in general, start with a phrase and follow it with sentences if they are needed.

@param x  the x-coordinate, measured in pixels

@param x  the x-coordinate. Measured in pixels.

  @param x  Specifies the x-coordinate, measured in pixels.

  @param x  Specifies the x-coordinate. Measured in pixels.

Also see order of multiple @param tags .

@return (reference page)

Omit @return for methods that return void and for constructors; include it for all other methods, even if its content is entirely redundant with the method description. Having an explicit @return tag makes it easier for someone to find the return value quickly. Whenever possible, supply return values for special cases (such as specifying the value returned when an out-of-bounds argument is supplied).

Use the same capitalization and punctuation as you used in @param.

@deprecated (reference page)

The @deprecated description in the first sentence should at least tell the user when the API was deprecated and what to use as a replacement. Only the first sentence will appear in the summary section and index. Subsequent sentences can also explain why it has been deprecated. When generating the description for a deprecated API, the Javadoc tool moves the @deprecated text ahead of the description, placing it in italics and preceding it with a bold warning: "Deprecated". An @see tag (for Javadoc 1.1) or {@link} tag (for Javadoc 1.2 or later) should be included that points to the replacement method:

If the member has no replacement, the argument to @deprecated should be "No replacement".

Do not add @deprecated tags without first checking with the appropriate engineer. Substantive modifications should likewise be checked first.

@since (reference page)

Specify the product version when the Java name was added to the API specification (if different from the implementation). For example, if a package, class, interface or member was added to the Java 2 Platform, Standard Edition, API Specification at version 1.2, use:

The Javadoc standard doclet displays a "Since" subheading with the string argument as its text. This subheading appears in the generated text only in the place corresponding to where the @since tag appears in the source doc comments (The Javadoc tool does not proliferate it down the hierarchy).

(The convention once was " @since JDK1.2 " but because this is a specification of the Java Platform, not particular to the Oracle JDK or SDK, we have dropped "JDK".)

When a package is introduced, specify an @since tag in its package description and each of its classes. (Adding @since tags to each class is technically not needed, but is our convention, as enables greater visibility in the source code.) In the absence of overriding tags, the value of the @since tag applies to each of the package's classes and members.

When a class (or interface) is introduced, specify one @since tag in its class description and no @since tags in the members. Add an @since tag only to members added in a later version than the class. This minimizes the number of @since tags.

If a member changes from protected to public in a later release, the @since tag would not change, even though it is now usable by any caller, not just subclassers.

@throws ( @exception was the original tag) (reference page)

A @throws tag should be included for any checked exceptions (declared in the throws clause), as illustrated below, and also for any unchecked exceptions that the caller might reasonably want to catch, with the exception of NullPointerException . Errors should not be documented as they are unpredictable. For more details, please see Documenting Exceptions with the @throws Tag .

See the Exceptions chapter of the Java Language Specification, Second Edition for more on exceptions. Also see order of multiple @throws tags .

@see (reference page)

Also see order of multiple @see tags .

@serialField

@serialData  

(All added in Javadoc 1.2) (reference page)

For information about how to use these tags, along with an example, see "Documenting Serializable Fields and Data for a Class," Section 1.6 of the Java Object Serialization Specification . Also see Oracle's criteria for including classes in the serialized form specification.

{@link} (Added in Javadoc 1.2) (reference page)

For conventions, see Use In-Line Links Economically .

Custom Tags and Annotations

If annotations are new to you, when you need to markup your source code, it might not be immediately clear whether to use an annotation or a Javadoc custom tag. Here is a quick comparison of the two.

In general, if the markup is intended to affect or produce documentation, it should probably be a javadoc tag; otherwise, it should be an annotation.

If you need to affect both program semantics and documentation, you probably need both an annotation and a tag. For example, our guidelines now recommend using the @Deprecated annotation for alerting the compiler warning and the @deprecated tag for the comment text.

Section 8.8.7 of the Java Language Specification, Second Edition describes a default constructor: If a class contains no constructor declarations, then a default constructor that takes no parameters is automatically provided. It invokes the superclass constructor with no arguments. The constructor has the same access as its class.

The Javadoc tool generates documentation for default constructors. When it documents such a constructor, Javadoc leaves its description blank, because a default constructor can have no doc comment. The question then arises: How do you add a doc comment for a default constructor? The simple answer is that it is not possible -- and, conveniently, our programming convention is to avoid default constructors. (We considered but rejected the idea that the Javadoc tool should generate a default comment for default constructors.)

Good programming practice dictates that code should never make use of default constructors in public APIs: All constructors should be explicit. That is, all default constructors in public and protected classes should be turned into explicit constructor declarations with the appropriate access modifier. This explicit declaration also gives you a place to write documentation comments.

The reason this is good programming practice is that an explicit declaration helps prevents a class from inadvertently being made instantiable, as the design engineer has to actually make a decision about the constructor's access. We have had several cases where we did not want a public class to be instantiable, but the programmer overlooked the fact that its default constructor was public. If a class is inadvertently allowed to be instantiable in a released version of a product, upward compatibility dictates that the unintentional constructor be retained in future versions. Under these unfortunate circumstances, the constructor should be made explicit and deprecated (using @deprecated ).

Note that when creating an explicit constructor, it must match precisely the declaration of the automatically generated constructor; even if the constructor should logically be protected, it must be made public to match the declaration of the automatically generated constructor, for compatibility. An appropriate doc comment should then be provided. Often, the comment should be something as simple as:

NOTE - The tags @throws and @exception are synonyms.

Documenting Exceptions in API Specs

The API specification for methods is a contract between a caller and an implementor. Javadoc-generated API documentation contains two ways of specifying this contract for exceptions -- the "throws" clause in the declaration, and the @throws Javadoc tag. These guidelines describe how to document exceptions with the @throws tag.

The purpose of the @throws tag is to indicate which exceptions the programmer must catch (for checked exceptions) or might want to catch (for unchecked exceptions).

Guidelines - Which Exceptions to Document

Document the following exceptions with the @throws tag:

Documenting Unchecked Exceptions

It is generally desirable to document the unchecked exceptions that a method can throw: this allows (but does not require) the caller to handle these exceptions. For example, it allows the caller to "translate" an implementation-dependent unchecked exception to some other exception that is more appropriate to the caller's exported abstraction.

Since there is no way to guarantee that a call has documented all of the unchecked exceptions that it may throw, the programmer must not depend on the presumption that a method cannot throw any unchecked exceptions other than those that it is documented to throw. In other words, you should always assume that a method can throw unchecked exceptions that are undocumented.

Note that it is always inappropriate to document that a method throws an unchecked exception that is tied to the current implementation of that method. In other words, document exceptions that are independent of the underlying implementation. For example, a method that takes an index and uses an array internally should not be documented to throw an ArrayIndexOutOfBoundsException , as another implementation could use a data structure other than an array internally. It is, however, generally appropriate to document that such a method throws an IndexOutOfBoundsException .

Keep in mind that if you do not document an unchecked exception, other implementations are free to not throw that exception. Documenting exceptions properly is an important part of write-once, run-anywhere.

Background on Checked and Unchecked Exceptions

The idea behind checking an exception is that the compiler checks at compile-time that the exception is properly being caught in a try-catch block.

You can identify checked and unchecked exceptions as follows.

Note that whether an exception is checked or unchecked is not defined by whether it is included in a throws clause.

Background on the Throws Clause

Checked exceptions must be included in a throws clause of the method. This is necessary for the compiler to know which exceptions to check. For example (in java.lang.Class):

By convention, unchecked exceptions should not be included in a throws clause. (Including them is considered to be poor programming practice. The compiler treats them as comments, and does no checking on them.) The following is poor code -- since the exception is a RuntimeException, it should be documented in the @throws tag instead.

java.lang.Byte source code:

Note that the Java Language Specification also shows unchecked exceptions in throws clauses (as follows). Using the throws clause for unchecked exceptions in the spec is merely a device meant to indicate this exception is part of the contract between the caller and implementor. The following is an example of this (where "final" and "synchronization" are removed to make the comparison simpler).

java.util.Vector source code:

public Object elementAt(int index)

java.util.Vector spec in the Java Language Specification, 1st Ed. (p. 656):

With Javadoc 1.2, package-level doc comments are available. Each package can have its own package-level doc comment source file that The Javadoc tool will merge into the documentation that it produces. This file is named package.html (and is same name for all packages). This file is kept in the source directory along with all the *.java files. (Do not put the packages.html file in the new doc-files source directory, because those files are only copied to the destination and are not processed.)

The file package.html is an example of a package-level source file for java.text code> and package-summary.html is the file that the Javadoc tool generates.

The Javadoc tool processes package.html by doing three things:

Template for package.html source file

At Oracle, we use the following template, Empty Template for Package-Level Doc Comment File, when creating a new package doc comment file. This contains a copyright statement. Obviously, if you are from a different company, you would supply your own copyright statement. An engineer would copy this whole file, rename it to package.html , and delete the lines set off with hash marks: ##### . One such file should go into each package directory of the source tree.

Contents of package.html source file

The package doc comment should provide (directly or via links) everything necessary to allow programmers to use the package. It is a very important piece of documentation: for many facilities (those that reside in a single package but not in a single class) it is the first place where programmers will go for documentation. It should contain a short, readable description of the facilities provided by the package (in the introduction, below) followed by pointers to detailed documentation, or the detailed documentation itself, whichever is appropriate. Which is appropriate will depend on the package: a pointer is appropriate if it's part of a larger system (such as, one of the 37 packages in Corba), or if a Framemaker document already exists for the package; the detailed documentation should be contained in the package doc comment file itself if the package is self-contained and doesn't require extensive documentation (such as java.math ).

To sum up, the primary purpose of the package doc comment is to describe the purpose of the package, the conceptual framework necessary to understand and to use it, and the relationships among the classes that comprise it. For large, complex packages (and those that are part of large, complex APIs) a pointer to an external architecture document is warranted.

The following are the sections and headings you should use when writing a package-level comment file. There should be no heading before the first sentence, because the Javadoc tool picks up the first text as the summary statement.

An assertion is a statement a conforming implementor would have to know in order to implement the Java platform.

On that basis, at Oracle, references in this section are critical to the Java Compatibility Kit (JCK). The Java Compatibility Kit includes a test to verify each assertion, to determine what passes as Java Compatible. The statement "Returns an int" is an assertion. An example is not an assertion.

Some "specifications" that engineers have written contain no assertions not already stated in the API specs (javadoc) -- they just elaborate on the API specs. In this respect, such a document should not be referred to in this section, but rather should be referred to in the next section.

Anonymous inner classes are defined in Java Language Specification, Second Edition, at Anonymous Class Declaration . The Javadoc tool does not directly document anonymous classes -- that is, their declarations and doc comments are ignored. If you want to document an anonymous class, the proper way to do so is in a doc comment of its outer class, or another closely associated class.

For example, if you had an anonymous TreeSelectionListener inner class in a method makeTree that returns a JTree object that users of this class might want to override, you could document in the method comment that the returned JTree has a TreeSelectionListener attached to it:

This section covers images used in the doc comments, not images directly used by the source code.

NOTE: The bullet and heading images required with Javadoc version 1.0 and 1.1 are located in the images directory of the JDK download bundle: jdk1.1/docs/api/images/ . Those images are no longer needed starting with 1.2.

Prior to Javadoc 1.2, the Javadoc tool would not copy images to the destination directory -- you had to do it in a separate operation, either manually or with a script. Javadoc 1.2 looks for and copies to the destination directory a directory named "doc-files" in the source tree (one for each package) and its contents. (It does a shallow copy for 1.2 and 1.3, and a deep copy for 1.4 and later.) Thus, you can put into this directory any images (GIF, JPEG, etc) or other files not otherwise processed by the Javadoc tool.

The following are the Java Software proposals for conventions for including images in doc comments. The master images would be located in the source tree; when the Javadoc tool is run with the standard doclet, it would copy those files to the destination HTML directory.

Images in Source Tree

Button-1.gif

java/awt/Button.java (source file)

java/awt/doc-files/Button-1.gif (image file)

Images in HTML Destination

api/java/awt/doc-files/Button-1.gif

With flat file output, such as Javadoc 1.1, directories would be located in the package directory and named "images-<package>". For example:

Problem - A problem occurs if you are working in an editor that defaults to curly (rather than straight) single and double quotes, such as Microsoft Word on a PC -- the quotes disappear when displayed in some browsers (such as Unix Netscape). So a phrase like "the display's characteristics" becomes "the displays characteristics."

The illegal characters are the following:

What should be used instead is:

Preventive Solution - The reason the "illegal" quotes occurred was that a default Word option is "Change 'Straight Quotes' to 'Smart Quotes'". If you turn this off, you get the appropriate straight quotes when you type.

Fixing the Curly Quotes - Microsoft Word has several save options -- use "Save As Text Only" to change the quotes back to straight quotes. Be sure to use the correct option:

[1] At Java Software, we use @version for the SCCS version. See "man sccs-get" for details. The consensus seems to be the following:

%I% gets incremented each time you edit and delget a file

%G% is the date mm/dd/yy

When you create a file, %I% is set to 1.1. When you edit and delget it, it increments to 1.2.

Some developers omit the date %G% (and have been doing so) if they find it too confusing -- for example, 3/4/96, which %G% would produce for March 4th, would be interpreted by those outside the United States to mean the 3rd of April. Some developers include the time %U% only if they want finer resolution (due to multiple check-ins in a day).

The clearest numeric date format would be to have the date formatted with the year first, something like yyyy-mm-dd, as proposed in ISO 8601 and elsewhere (such as http://www.cl.cam.ac.uk/~mgk25/iso-time.html), but that enhancement would need to come from SCCS.

method write javadoc

Java Tutorial

Java - Documentation Comments

The Java language supports three types of comments −

This chapter is all about explaining Javadoc. We will see how we can make use of Javadoc to generate useful documentation for Java code.

What is Javadoc?

Javadoc is a tool which comes with JDK and it is used for generating Java code documentation in HTML format from Java source code, which requires documentation in a predefined format.

Following is a simple example where the lines inside /*….*/ are Java multi-line comments. Similarly, the line which preceeds // is Java single-line comment.

You can include required HTML tags inside the description part. For instance, the following example makes use of <h1>....</h1> for heading and <p> has been used for creating paragraph break −

The javadoc Tags

The javadoc tool recognizes the following tags −

Following program uses few of the important tags available for documentation comments. You can make use of other tags based on your requirements.

The documentation about the AddNum class will be produced in HTML file AddNum.html but at the same time a master file with a name index.html will also be created.

Now, process the above AddNum.java file using javadoc utility as follows −

You can check all the generated documentation here − AddNum . If you are using JDK 1.7 then javadoc does not generate a great stylesheet.css , so we suggest to download and use standard stylesheet from https://docs.oracle.com/javase/7/docs/api/stylesheet.css

Javadoc Tutorial

Introduction.

Javadoc Comments

Javadoc Tags

Javadoc Compilation

Javadoc In Eclipse 3.0

Javadoc References

Chapter 10. Documentation with Javadoc

Table of Contents

Documenting your code is crucial to help others understand it, and even to remind yourself how your own older programs work. Unfortunately, it is easy for most external documentation to become out of date as a program changes. For this reason, it is useful to write documentation as comments in the code itself, where they can be easily updated with other changes. Javadoc is a documentation tool which defines a standard format for such comments, and which can generate HTML files to view the documentation from a web broswer. (As an example, see Oracle's Javadoc documentation for the Java libraries at http://download.oracle.com/javase/6/docs/api/ .)

You can easily run Javadoc over your programs from within DrJava, using the "Javadoc All Documents" and "Preview Javadoc for Current Document" commands in the Tools menu. These commands will generate Javadoc HTML files from the comments you have written and display them in a browser. This chapter gives a brief overview of these commands and how to write Javadoc comments. More detailed information on writing Javadoc comments can be found online at http://www.oracle.com/technetwork/java/javase/documentation/index-137868.html .

Writing Javadoc Comments

In general, Javadoc comments are any multi-line comments (" /** ... */ ") that are placed before class, field, or method declarations. They must begin with a slash and two stars, and they can include special tags to describe characteristics like method parameters or return values. The HTML files generated by Javadoc will describe each field and method of a class, using the Javadoc comments in the source code itself. Examples of different Javadoc comments are listed below.

Simple Comments.  Normal Javadoc comments can be placed before any class, field, or method declaration to describe its intent or characteristics. For example, the following simple Student class has several Javadoc comments.

Using Tags.  Tags can be used at the end of each Javadoc comment to provide more structured information about the code being described. For example, most Javadoc comments for methods include " @param " and " @return " tags when applicable, to describe the method's parameters and return value. The " @param " tag should be followed by the parameter's name, and then a description of that parameter. The " @return " tag is followed simply by a description of the return value. Examples of these tags are given below.

Other common tags include " @throws e " (to describe some Exception "e" which is thrown by a method) and " @see #foo " (to provide a link to a field or method named "foo").

Javadoc Tutorial

Introduction.

Javadoc Comments

Javadoc Tags

import java.io.*;

/** * <h1>Add Two Numbers!</h1> * The AddNum program implements an application that * simply adds two given integer numbers and Prints * the output on the screen. * <p> * <b>Note:</b> Giving proper comments in your program makes it more * user friendly and it is assumed as a high quality code. * * @author Zara Ali * @version 1.0 * @since 2014-03-31 */ public class AddNum { /** * This method is used to add two integers. This is * a the simplest form of a class method, just to * show the usage of various javadoc Tags. * @param numA This is the first paramter to addNum method * @param numB This is the second parameter to addNum method * @return This returns sum of numA and numB. */ public int addNum(int numA, int numB) { return numA + numB; }

/** * This is the main method which makes use of addNum method. * @param args Unused. * @exception IOException On input error. * @see IOException */ public static void main(String args[]) throws IOException {

AddNum obj = new AddNum(); int sum = obj.addNum(10, 20);

System.out.println("Sum of 10 and 20 is :" + sum); } }

Javadoc References

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good example of Javadoc [closed]

is there a good example of a source file containing Javadoc ?

I can find lots of good examples of Javadoc on the internet, I would just like to find out the particular syntax used to create them, and assume I can pore through the source of some library somewhere but that seems like a lot of work.

Jonny Henly's user avatar

11 Answers 11

How about the JDK source code, but accessed through a 3rd party like docjar? For example, the Collections source .

That way, there's no big download.

GaryF's user avatar

The page How to Write Doc Coments for the Javadoc Tool contains a good number of good examples. One section is called Examples of Doc Comments and contains quite a few usages.

Also, the Javadoc FAQ contains some more examples to illustrate the answers.

rluba's user avatar

How about the JDK source code?

Abhijeet's user avatar

I use a small set of documentation patterns:

dfa's user avatar

Have a look at Spring framework source, it has excellent javadocs

Gaël Marziou's user avatar

ANT for example - source code browsable online: http://svn.apache.org/viewvc/ant/core/trunk/src/main/org/apache/tools/ant/DefaultLogger.java?view=co

To choose other files start from: http://svn.apache.org/viewvc/ant/core/trunk/src/main/org/apache/tools/ant/?pathrev=761528

hasanghaforian's user avatar

The documentation of Google Guava's EventBus package and classes is a good example of Javadoc. Especially the package documentation with the quick start is well written.

Marvin Frommhold's user avatar

Download the sources of Lucene and see how they do it. They have good JavaDocs.

RichieHindle's user avatar

If you are using Eclipse , then you can setup your JDK (not JRE ) in Installed JREs , and then use Open Type ( Ctrl + Shift + T ), give something like java.util.Collections

KrishPrabakar's user avatar

If all your looking for is the syntax, then this may help:

How to Write Doc Comments for the Javadoc Tool

Jesse's user avatar

If you install a JDK and choose to install sources too, the src.zip contains the source of ALL the public Java classes. Most of these have pretty good javadoc.

Thorbjørn Ravn Andersen's user avatar

Not the answer you're looking for? Browse other questions tagged java javadoc or ask your own question .

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It is important always to document your code clearly. For Java programmers, some general commenting conventions have been established by a tool called javadoc , the Java API documentation generator. This tool can automatically extract documentation comments from source code and generate HTML class descriptions, like those in the ">cs1705 Package API . This tool is so popular and so commonly used that it has set the standard for how people document the externally visible features in their Java code.

The javadoc tool expects comments to be written in a particular way--other comments are ignored. JavaDoc comments (also called just "doc comments") always start with " /** " and end with " */ ". Any other comments are ignored when generating documentation for your code. Further, a JavaDoc comment describing something always appears immediately before the thing it documents.

Within a JavaDoc comment, special tags can be embedded to signal particular kinds of information. These doc tags enable complete, well-formatted API documentation to be automatically generated from your source code. All JavaDoc tags start with an at-sign ( @ ).

If you already know some HTML, you can even embed simple HTML markup inside your JavaDoc comments, and it will appear in the generated documentation for your classes. This is handy if you want to add bullet lists in a class description, or wish to make part of your comment stand out in boldface, and so on.

You should place a descriptive JavaDoc comment just before the start of each class you write:

Class descriptions typically use two tags: @author indicates who wrote the file, and @version indicates the "version" of this file or project. You can use your full name, or just PID, in an @author tag. In this course, it is fine to use the date when the file was written as the version information in the @version tag.

In lab, list all lab partners on the @author line. Also, if you started off with a comment generated by BlueJ (like the example above), don't forget to replace the text inside the comment with your own. The javadoc tool will use the very first sentence of your comment as a one-sentence summary of your class, and will use the entire text of your comment as the full class description.

You should place a descriptive JavaDoc comment just before each method or constructor you write:

As with other JavaDoc comments, make sure this appears just before the method it describes. For methods that have parameters, you should also include a brief description of what each parameter means. For example, we might have a Harvester robot class that provides its own constructor:

Here, a @param tag has been used to give a description of the meaning and use of each of the parameters. Be sure to start these tags at the beginning of a comment line, and group all of the tags with the same name together (i.e., all @param tags should be next to each other).

Again, javadoc will take the first sentence in your comment as a one-sentence summary of what the method does. The remainder of the comment will be used in generating a full description of the method.

Some methods have return values--that is, they give back information to their caller. For example, a wallOnRight() predicate might return a boolean true or false result to indicate whether or not there is a wall to the Robot's immediate right. You can document what information is returned using a @return tag :

Within a BlueJ project, you can use the Tools->Project Documentation command to generate full documentation for your own project straight from your source code. It may take a minute, but once complete, a new browser window will open showing all of the generated documentation for your classes. It will be similar to the ">cs1705 Package API , but for your own code.

Also, when editing a single file, you will notice a drop-down list on the upper right of the edit window. This list gives you two choices: "Implementation", which shows the code you normally edit, and "Interface", which will instead show the generated documentation view for the current class.

Using these two approaches within BlueJ, you can check out how your comments look in the generated documentation.

JavaDoc comments are "public" documentation of the externally accessible features of your classes. Often, you may also wish to include "internal" (that is, private) documentation that is only useful to someone reading the source code directly. Any comment that does not begin with /** is treated as private, purely for someone with access to the source code. You are free to use such comments where ever you like to improve the readability of your code, but ...

Choose all names carefully so that a naïve reader's first interpretation will always be right. Do not choose names that might mislead someone about what a method is supposed to do, or what information a variable holds. Choosing poor names or convoluted logic structure and then trying to explain it in lengthy comments does little to improve readability. This is doubly true for methods, because half the time a reader will see your method name where it is called, not when they are reading your method itself. If it is not immediately clear what the method should do, that affects the readability of all the code calling this method, no matter how many comments you put in the method itself.

Strive to write code that is clear and understandable on its own, simply by virtue of the names you have chosen and the structure you use. If you feel you have to add an internal comment to explain something, ask yourself what needs explaining. If you need to explain what a name refers to or how you intend to use it, consider choosing a better name. If you have to explain a complex series of if statements or some other convoluted structure, ask yourself (or a TA) if there is a better way. Only after considering these alternatives should you add descriptive comments.

Consider these comments:

These are examples of useless comments. Many students add comments to their code just to "make sure everything is documented", or because they believe copious comments are what the instructor is looking for. Comments like this just get in the way of reading the code, however. You should only add comments when they express something that isnt already evident from the code itself . Comments are more code that the poor reader has to wade through, so you need to carefully balance their benefits against the cost of having to read them.

First, you must begin a javadoc comment block with /** (see line 1 above).  Both asterisks are required.  The block must be terminated with an  */ (see line 18 above).

Notice the use of the phases brief description and detailed description in the text on lines 3 and 4 above.  The first sentence of your description will be the brief description and the detailed description will be the entire description. Take a look at some documentaion in the Java Standard Library to get a feel for what these phrases mean and examples of how they are typically written.

Next you document any and all parameters to the method using the @param tag (lines 7 & 10), one tag for each parameter in the order they appear in the method's parameter list.  You must state the name of the parameter exactly as it appears in the actual method or an error will occur when you generate the html documentation.  If the method has no parameter you do not write any @param tags.

Now you document the return, if any, for the method using the @return tag (line 13). If the method has a void return-type you do not write the @return tag.

Finally, you document the exceptions, if any, that the method is designed to throw during execution using the @throw tag (lines 16), one tag for each exception the method might throw.  If the method does not, by design, throw any exceptions you do not write any @throws tags.

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COMMENTS

  1. How to Write Doc Comments for the Javadoc Tool

    A doc comment is written in HTML and must precede a class, field, constructor or method declaration. It is made up of two parts -- a description followed by

  2. Introduction to JavaDoc

    2.4. Javadoc at Method Level · @param provides any useful description about a method's parameter or input it should expect · @return provides a

  3. Java

    Javadoc is a tool which comes with JDK and it is used for generating Java code documentation in HTML format from Java source code, which requires documentation

  4. Javadoc Tutorial

    Javadoc allows you to attach descriptions to classes, constructors, fields, interfaces and methods in the generated html documentation by placing Javadoc

  5. Chapter 10. Documentation with Javadoc

    Javadoc is a documentation tool which defines a standard format for such comments, and which can generate HTML files to view the documentation from a web

  6. Writing Javadoc Comments and creating an API with the ...

    Javadoc comments are those that start with a /** and end with a */. They are typically found above every class and method definition.

  7. Javadoc Comments

    Javadoc allows you to attach descriptions to classes, constructors, fields, interfaces and methods in the generated html documentation by placing Javadoc

  8. good example of Javadoc [closed]

    The documentation of Google Guava's EventBus package and classes is a good example of Javadoc. Especially the package documentation with the

  9. Documenting Your Classes and Methods

    You should place a descriptive JavaDoc comment just before the start of each class you write: /** * Write a one-sentence summary of your robot task class here.

  10. How to write Java Documentation

    The javadoc-style of comments allows a utility that comes with the Java development environment (called javadoc) to read your source code and generate html