Docs [D]

These are rules about the public docs (JavaDocs) for APIs.

All public APIs must be documented

All public APIs must have sufficient javadocs to explaining how a developer would use the API. Assume the developer found it via auto-complete or while browsing through API reference docs and has a minimal amount of context from the adjacent API surface (ex. on the same class).


Method parameters and return values must be documented using @param and @return docs annotations, respectively. The javadoc body should be formatted as though it is preceded by “This method...”.

In cases where a method takes no parameters, has no special considerations, and simply returns what the method name says it does, the @return may be omitted and docs may be written similar to:

 * Returns the priority of the thread.
@IntRange(from = 1, to = 10)
public int getPriority() { ... }

Always use links in JavaDocs

Docs should link to other docs for related constants, methods, etc. Use Javadoc tags (e.g., @see and {@link foo}), not just plain-text words.


public static final int FOO = 0;
public static final int BAR = 1;


 * Sets value to one of FOO or <code>BAR</code>.
 * @param value the value being set, one of FOO or BAR
public void setValue(int value) { ... }
 * Sets value to one of {@link ##FOO} or {@link ##BAR}.
 * @param value the value being set
public void setValue(@ValueType int value) { ... }

Note that using an IntDef annotation such as @ValueType on a parameter will automatically generate documentation specifying the allowed types. See the guidance on annotations for more information on IntDef.

Run update-api or docs target when adding JavaDocs

This rule is particularly important when adding @link or @see tags, and make sure the output looks as expected. It is common to see ERROR output in JavaDocs from bad links. Either the update-api or docs Make target will perform this check, but the docs target might be quicker if you are simply changing javadocs and do not otherwise need to run the update-api target.

Use {@code foo} to distinguish Java values

Java values like true, false, and null should be wrapped with {@code ...} to distinguish them from documentation text.

@param and @return summaries should be a single sentence fragment

Parameter and return value summaries should start with a lowercase character and contain only a single sentence fragment. If you have additional information that extends beyond a single sentence, move it to the method javadoc body.

 * @param e The element to be appended to the list. This must not be
 *       null. If the list contains no entries, this element will
 *       be added at the beginning.
 * @return This method returns true on success.
 * @param e element to be appended to this list, must be non-{@code null}
 * @return {@code true} on success, {@code false} otherwise

Docs annotations need explanations

Annotations @hide and @removed should include documentation as to why they are hidden from public API. Use of @deprecated annotation must include instructions on how to replace usages of the deprecated API.

Use @throws to document exceptions

If a method throws a checked exception, for example IOException, the exception must be documented with @throws. For Kotlin-sourced APIs intended for use by Java clients, annotate functions with @Throws.

If a method throws an unchecked exception indicating a preventable error, for example IllegalArgumentException or IllegalStateException, the exception must be documented with an explanation of why the exception is thrown. The thrown exception should also indicate why it was thrown.

 * ...
 * @throws IOException If it cannot find the schema for {@code toVersion}
 * @throws IllegalStateException If the schema validation fails
public SupportSQLiteDatabase runMigrationsAndValidate(String name, int version,
    boolean validateDroppedTables, Migration... migrations) throws IOException {
  // ...
  if (!dbPath.exists()) {
    throw new IllegalStateException("Cannot find the database file for " + name
        + ". Before calling runMigrations, you must first create the database "
        + "via createDatabase.");
  // ...
 * ...
 * @throws IOException If something goes wrong reading the file, such as a bad
 *                     database header or missing permissions
fun readVersion(databaseFile: File): Int {
  // ...
  val read =
    if (read != 4) {
      throw IOException("Bad database header, unable to read 4 bytes at " +
          "offset 60")
  // ...

If the method invokes asynchronous code that may throw exceptions, please consider how the developer will find out about and respond to such exceptions. Typically this involves forwarding the exception to a callback and documenting the exceptions thrown on the method that receives them. Asynchronous exceptions should not be documented with @throws unless they are actually re-thrown from the annotated method.

End the first sentence of docs with a period

The doclava tool parses docs simplistically, ending the synopsis doc (the first sentence, used in the quick description at the top of the class docs) as soon as it sees a period (.) followed by a space. There are two problems that this causes:

  • If a short doc is not ended with a period, and if that member has inherited docs that are picked up by the tool, then the synopsis also picks up those inherited docs. See, for example, actionBarTabStyle in the R.attr docs, which has the description of the dimension added into the synopsis.
  • Avoid “e.g.” in the first sentence for the same reason, because doclava will end the synopsis docs after “g.”. See, for example, TEXT_ALIGNMENT_CENTER in Note that Metalava will automatically correct this error by inserting a non-breaking space after the period; however, please don’t make this mistake in the first place.

Format docs to be rendered in HTML

Javadocs will be rendered in HTML, so format them accordingly:

  • Line breaks should use an explicit <p> tag. Do not add a closing </p> tag.

  • Do not use ASCII to render lists or tables.

  • Lists should use <ul> or <ol> for unordered and ordered, respectively. Each item should begin with a <li> tag, but does not need a closing </li> tag. A closing </ul> or </ol> tag is required after the last item.

  • Tables should use <table>, <tr> for rows, <th> for headers, and <td> for cells. All table tags require matching closing tags. You may use class="deprecated" on any tag to denote deprecation.

  • To create inline code font, use {@code foo}.

  • To create code blocks, use <pre>.

  • All text inside a <pre> block is parsed by the browser, so be careful with brackets <>. You can escape them with &lt; and &gt; HTML entities.

  • Alternatively, you can leave raw brackets <> in your code snippet if you wrap the offending sections in {@code foo}. For example:

    <pre>{@code <manifest>}</pre>

Follow the API reference style guide

To ensure consistency in the style for class summaries, method descriptions, parameter descriptions, etc., follow the recommendations in the official Java language guidelines at How to Write Doc Comments for the Javadoc Tool.