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## au.id.jericho.lib.html
Class Tag

    java.lang.Object
      Segment
          Tag

All Implemented Interfaces:
  java.lang.CharSequence, java.lang.Comparable, HTMLElementName

Direct Known Subclasses:
  EndTag, StartTag

public abstract class Tag
extends Segment
implements HTMLElementName

Represents either a `StartTag` or `EndTag` in a specific source document.

Take the following HTML segment as an example:

`<p>This is a sample paragraph.</p>`

The "`<p>`" is represented by a `StartTag` object, and the "`</p>`" is represented by an `EndTag` object, both of which are subclasses of the `Tag` class. The whole segment, including the start tag, its corresponding end tag and all of the content in between, is represented by an `Element` object.

### Tag Parsing Process

The following process describes how each tag is identified by the parser:

1. Every '`<`' character found in the source document is considered to be the start of a tag. The characters following it are compared with the start delimiters of all the registered tag types, and a list of matching tag types is determined.
2. A more detailed analysis of the source is performed according to the features of each matching tag type from the first step, in order of precedence, until a valid tag is able to be constructed.

   The analysis performed in relation to each candidate tag type is a two-stage process:

   1. The position of the tag is checked to determine whether it is valid. In theory, a server tag is valid in any position, but a non-server tag is not valid inside another non-server tag.

      The `TagType.isValidPosition(Source, int pos, int[] fullSequentialParseData)` method is responsible for this check and has a common default implementation for all tag types (although custom tag types can override it if necessary). Its behaviour differs depending on whether or not a full sequential parse is peformed. See the documentation of the `isValidPosition` method for full details.

   2. A final analysis is performed by the `TagType.constructTagAt(Source, int pos)` method of the candidate tag type. This method returns a valid `Tag` object if all conditions of the candidate tag type are met, otherwise it returns `null` and the process continues with the next candidate tag type.
3. If the source does not match the start delimiter or syntax of any registered tag type, the segment spanning it and the next '`>`' character is taken to be an unregistered tag. Some tag search methods ignore unregistered tags. See the `isUnregistered()` method for more information.

See the documentation of the `TagType` class for more details on how tags are recognised.

### Tag Search Methods

Methods that find tags in a source document are collectively referred to as Tag Search Methods. They are found mostly in the `Source` and `Segment` classes, and can be generally categorised as follows:

Open Search:
  These methods search for tags of any name and type.
  - `findNextTag()`
  - `findPreviousTag()`
  - `Segment.findAllElements()`
  - `Segment.findAllTags()`
  - `Source.getTagAt(int pos)`
  - `Source.findPreviousTag(int pos)`
  - `Source.findNextTag(int pos)`
  - `Source.findEnclosingTag(int pos)`
  - `Segment.findAllStartTags()`
  - `Source.findPreviousStartTag(int pos)`
  - `Source.findNextStartTag(int pos)`
  - `Source.findPreviousEndTag(int pos)`
  - `Source.findNextEndTag(int pos)`
Named Search:
  These methods usually include a parameter called `name` which is used to specify the name of the tag to search for. In some cases named search methods do not require this parameter because the context or name of the method implies the name to search for. In tag search methods specifically looking for start tags, specifying a name that ends in a colon (`:`) searches for all start tags in the specified XML namespace.
  - `Segment.findAllElements(String name)`
  - `Segment.findAllStartTags(String name)`
  - `Source.findPreviousStartTag(int pos, String name)`
  - `Source.findNextStartTag(int pos, String name)`
  - `Source.findPreviousEndTag(int pos, String name)`
  - `Source.findNextEndTag(int pos, String name)`
  - `Source.findNextEndTag(int pos, String name, EndTagType)`
Tag Type Search:
  These methods usually include a parameter called `tagType` which is used to specify the type of the tag to search for. In some methods the search parameter is restricted to the `StartTagType` subclass of `TagType`.
  - `Segment.findAllElements(StartTagType)`
  - `Segment.findAllTags(TagType)`
  - `Source.findPreviousTag(int pos, TagType)`
  - `Source.findNextTag(int pos, TagType)`
  - `Source.findEnclosingTag(int pos, TagType)`
  - `Source.findNextEndTag(int pos, String name, EndTagType)`
Other Search:
  A small number of methods do not fall into any of the above categories, such as the methods that search on attribute values.
  - `Segment.findAllStartTags(String attributeName, String value, boolean valueCaseSensitive)`
  - `Source.findNextStartTag(int pos, String attributeName, String value, boolean valueCaseSensitive)`

Field Summary

Fields inherited from interface HTMLElementName
`A, ABBR, ACRONYM, ADDRESS, APPLET, AREA, B, BASE, BASEFONT, BDO, BIG, BLOCKQUOTE, BODY, BR, BUTTON, CAPTION, CENTER, CITE, CODE, COL, COLGROUP, DD, DEL, DFN, DIR, DIV, DL, DT, EM, FIELDSET, FONT, FORM, FRAME, FRAMESET, H1, H2, H3, H4, H5, H6, HEAD, HR, HTML, I, IFRAME, IMG, INPUT, INS, ISINDEX, KBD, LABEL, LEGEND, LI, LINK, MAP, MENU, META, NOFRAMES, NOSCRIPT, OBJECT, OL, OPTGROUP, OPTION, P, PARAM, PRE, Q, S, SAMP, SCRIPT, SELECT, SMALL, SPAN, STRIKE, STRONG, STYLE, SUB, SUP, TABLE, TBODY, TD, TEXTAREA, TFOOT, TH, THEAD, TITLE, TR, TT, U, UL, VAR`

Method Summary
`Tag` | `findNextTag()` Returns the next tag in the source document.
`Tag` | `findPreviousTag()` Returns the previous tag in the source document.
`abstract Element` | `getElement()` Returns the element that is started or ended by this tag.
`java.lang.String` | `getName()` Returns the name of this tag, always in lower case.
`Segment` | `getNameSegment()` Returns the segment spanning the name of this tag.
`abstract TagType` | `getTagType()` Returns the type of this tag.
`java.lang.Object` | `getUserData()` Returns the general purpose user data object that has previously been associated with this tag via the `setUserData(Object)` method.
`abstract boolean` | `isUnregistered()` Indicates whether this tag has a syntax that does not match any of the registered tag types.
`static boolean` | `isXMLName(java.lang.CharSequence text)` Indicates whether the specified text is a valid XML Name.
`static boolean` | `isXMLNameChar(char ch)` Indicates whether the specified character is valid anywhere in an XML Name.
`static boolean` | `isXMLNameStartChar(char ch)` Indicates whether the specified character is valid at the start of an XML Name.
`void` | `setUserData(java.lang.Object userData)` Associates the specified general purpose user data object with this tag.
`abstract java.lang.String` | `tidy()` Returns an XML representation of this tag.

Methods inherited from class Segment
`charAt, compareTo, encloses, encloses, equals, extractText, extractText, findAllCharacterReferences, findAllElements, findAllElements, findAllElements, findAllElements, findAllStartTags, findAllStartTags, findAllStartTags, findAllTags, findAllTags, findFormControls, findFormFields, getBegin, getChildElements, getDebugInfo, getEnd, getRenderer, getTextExtractor, hashCode, ignoreWhenParsing, isWhiteSpace, isWhiteSpace, length, parseAttributes, subSequence, toString`

Methods inherited from class java.lang.Object
`clone, finalize, getClass, notify, notifyAll, wait, wait, wait`

Method Detail

### getElement

    public abstract Element getElement()

  Returns the element that is started or ended by this tag.

  `StartTag.getElement()` is guaranteed not `null`.

  `EndTag.getElement()` can return `null` if the end tag is not properly matched to a start tag.

  Returns:
    the element that is started or ended by this tag.

### getName

    public final java.lang.String getName()

  Returns the name of this tag, always in lower case.

  The name always starts with the name prefix defined in this tag's type. For some tag types, the name consists only of this prefix, while in others it must be followed by a valid XML name (see `StartTagType.isNameAfterPrefixRequired()`).

  If the name is equal to one of the constants defined in the `HTMLElementName` interface, this method is guaranteed to return the constant itself. This allows comparisons to be performed using the `==` operator instead of the less efficient `String.equals(Object)` method.

  For example, the following expression can be used to test whether a `StartTag` is from a `SELECT` element:
  `startTag.getName()==HTMLElementName.SELECT`

  To get the name of this tag in its original case, use `getNameSegment()``.toString()`.

  Returns:
    the name of this tag, always in lower case.

### getNameSegment

    public Segment getNameSegment()

  Returns the segment spanning the name of this tag.

  The code `getNameSegment().toString()` can be used to retrieve the name of this tag in its original case.

  Every call to this method constructs a new `Segment` object.

  Returns:
    the segment spanning the name of this tag.
  See Also:
    `getName()`

### getTagType

    public abstract TagType getTagType()

  Returns the type of this tag.

  Returns:
    the type of this tag.

### getUserData

    public java.lang.Object getUserData()

  Returns the general purpose user data object that has previously been associated with this tag via the `setUserData(Object)` method.

  If `setUserData(Object)` has not been called, this method returns `null`.

  Returns:
    the generic data object that has previously been associated with this tag via the `setUserData(Object)` method.

### setUserData

    public void setUserData(java.lang.Object userData)

  Associates the specified general purpose user data object with this tag.

  This property can be useful for applications that need to associate extra information with tags. The object can be retrieved later via the `getUserData()` method.

  Parameters:
    `userData` - general purpose user data of any type.

### findNextTag

    public Tag findNextTag()

  Returns the next tag in the source document.

  If a full sequential parse has been performed, this method is very efficient.

  If not, it is equivalent to `source.``findNextTag``(``Segment.getBegin()``+1)`.

  See the `Tag` class documentation for more details about the behaviour of this method.

  Returns:
    the next tag in the source document, or `null` if this is the last tag.

### findPreviousTag

    public Tag findPreviousTag()

  Returns the previous tag in the source document.

  If a full sequential parse has been performed, this method is very efficient.

  If not, it is equivalent to `source.``findPreviousTag``(``Segment.getBegin()``-1)`.

  See the `Tag` class documentation for more details about the behaviour of this method.

  Returns:
    the previous tag in the source document, or `null` if this is the first tag.

### isUnregistered

    public abstract boolean isUnregistered()

  Indicates whether this tag has a syntax that does not match any of the registered tag types.

  The only requirement of an unregistered tag type is that it starts with '`<`' and there is a closing '`>`' character at some position after it in the source document.

  The absence or presence of a '`/`' character after the initial '`<`' determines whether an unregistered tag is respectively a `StartTag` with a type of `StartTagType.UNREGISTERED` or an `EndTag` with a type of `EndTagType.UNREGISTERED`.

  There are no restrictions on the characters that might appear between these delimiters, including other '`<`' characters. This may result in a '`>`' character that is identified as the closing delimiter of two separate tags, one an unregistered tag, and the other a tag of any type that begins in the middle of the unregistered tag. As explained below, unregistered tags are usually only found when specifically looking for them, so it is up to the user to detect and deal with any such nonsensical results.

  Unregistered tags are only returned by the `Source.getTagAt(int pos)` method, named search methods, where the specified `name` matches the first characters inside the tag, and by tag type search methods, where the specified `tagType` is either `StartTagType.UNREGISTERED` or `EndTagType.UNREGISTERED`.

  Open tag searches and other searches always ignore unregistered tags, although every discovery of an unregistered tag is logged by the parser.

  The logic behind this design is that unregistered tag types are usually the result of a '`<`' character in the text that was mistakenly left unencoded, or a less-than operator inside a script, or some other occurrence which is of no interest to the user. By returning unregistered tags in named and tag type search methods, the library allows the user to specifically search for tags with a certain syntax that does not match any existing `TagType`. This expediency feature avoids the need for the user to create a custom tag type to define the syntax before searching for these tags. By not returning unregistered tags in the less specific search methods, it is providing only the information that most users are interested in.

  Returns:
    `true` if this tag has a syntax that does not match any of the registered tag types, otherwise `false`.

### tidy

    public abstract java.lang.String tidy()

  Returns an XML representation of this tag.

  This is an abstract method which is implemented in the `StartTag` and `EndTag` subclasses. See the documentation of the `StartTag.tidy()` and `EndTag.tidy()` methods for details.

  Returns:
    an XML representation of this tag.

### isXMLName

    public static final boolean isXMLName(java.lang.CharSequence text)

  Indicates whether the specified text is a valid XML Name.

  This implementation first checks that the first character of the specified text is a valid XML Name start character as defined by the `isXMLNameStartChar(char)` method, and then checks that the rest of the characters are valid XML Name characters as defined by the `isXMLNameChar(char)` method.

  Note that this implementation does not exactly adhere to the formal definition of an XML Name, but the differences are unlikely to be significant in real-world XML or HTML documents.

  Parameters:
    `text` - the text to test.
  Returns:
    `true` if the specified text is a valid XML Name, otherwise `false`.
  See Also:
    `Source.findNameEnd(int pos)`

### isXMLNameStartChar

    public static final boolean isXMLNameStartChar(char ch)

  Indicates whether the specified character is valid at the start of an XML Name.

  The XML 1.0 specification section 2.3 defines a `Name` as starting with one of the characters
  `(Letter | '_' | ':')`.

  This method uses the expression
  `Character.isLetter(ch) || ch=='_' || ch==':'`.

  Note that there are many differences between the `Character.isLetter()` definition of a Letter and the XML definition of a Letter, but these differences are unlikely to be significant in real-world XML or HTML documents.

  Parameters:
    `ch` - the character to test.
  Returns:
    `true` if the specified character is valid at the start of an XML Name, otherwise `false`.
  See Also:
    `Source.findNameEnd(int pos)`

### isXMLNameChar

    public static final boolean isXMLNameChar(char ch)

  Indicates whether the specified character is valid anywhere in an XML Name.

  The XML 1.0 specification section 2.3 uses the entity `NameChar` to represent this set of characters, which is defined as
  `(Letter | Digit | '.' | '-' | '_' | ':' | CombiningChar | Extender)`.

  This method uses the expression
  `Character.isLetterOrDigit(ch) || ch=='.' || ch=='-' || ch=='_' || ch==':'`.

  Note that there are many differences between these definitions, but these differences are unlikely to be significant in real-world XML or HTML documents.

  Parameters:
    `ch` - the character to test.
  Returns:
    `true` if the specified character is valid anywhere in an XML Name, otherwise `false`.
  See Also:
    `Source.findNameEnd(int pos)`

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