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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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package javax.swing.text;

import javax.swing.event.DocumentListener;
import javax.swing.event.UndoableEditListener;

/**
 * <p>
 * The <code>Document</code> is a container for text that serves
 * as the model for swing text components.  The goal for this
 * interface is to scale from very simple needs (a plain text textfield)
 * to complex needs (an HTML or XML document, for example).
 *
 * <p><b>Content</b>
 * <p>
 * At the simplest level, text can be
 * modeled as a linear sequence of characters. To support
 * internationalization, the Swing text model uses
 * <a href="http://www.unicode.org/">unicode</a> characters.
 * The sequence of characters displayed in a text component is
 * generally referred to as the component's <em>content</em>.
 * <p>
 * To refer to locations within the sequence, the coordinates
 * used are the location between two characters.  As the diagram
 * below shows, a location in a text document can be referred to
 * as a position, or an offset. This position is zero-based.
 * <p style="text-align:center"><img src="doc-files/Document-coord.gif"
 * alt="The following text describes this graphic.">
 * <p>
 * In the example, if the content of a document is the
 * sequence "The quick brown fox," as shown in the preceding diagram,
 * the location just before the word "The" is 0, and the location after
 * the word "The" and before the whitespace that follows it is 3.
 * The entire sequence of characters in the sequence "The" is called a
 * <em>range</em>.
 * <p>The following methods give access to the character data
 * that makes up the content.
 * <ul>
 * <li>{@link #getLength()}
 * <li>{@link #getText(int, int)}
 * <li>{@link #getText(int, int, javax.swing.text.Segment)}
 * </ul>
 * <p><b>Structure</b>
 * <p>
 * Text is rarely represented simply as featureless content. Rather,
 * text typically has some sort of structure associated with it.
 * Exactly what structure is modeled is up to a particular Document
 * implementation.  It might be as simple as no structure (i.e. a
 * simple text field), or it might be something like diagram below.
 * <p style="text-align:center"><img src="doc-files/Document-structure.gif"
 * alt="Diagram shows Book->Chapter->Paragraph">
 * <p>
 * The unit of structure (i.e. a node of the tree) is referred to
 * by the <a href="Element.html">Element</a> interface.  Each Element
 * can be tagged with a set of attributes.  These attributes
 * (name/value pairs) are defined by the
 * <a href="AttributeSet.html">AttributeSet</a> interface.
 * <p>The following methods give access to the document structure.
 * <ul>
 * <li>{@link #getDefaultRootElement()}
 * <li>{@link #getRootElements()}
 * </ul>
 *
 * <p><b>Mutations</b>
 * <p>
 * All documents need to be able to add and remove simple text.
 * Typically, text is inserted and removed via gestures from
 * a keyboard or a mouse.  What effect the insertion or removal
 * has upon the document structure is entirely up to the
 * implementation of the document.
 * <p>The following methods are related to mutation of the
 * document content:
 * <ul>
 * <li>{@link #insertString(int, java.lang.String, javax.swing.text.AttributeSet)}
 * <li>{@link #remove(int, int)}
 * <li>{@link #createPosition(int)}
 * </ul>
 *
 * <p><b>Notification</b>
 * <p>
 * Mutations to the <code>Document</code> must be communicated to
 * interested observers.  The notification of change follows the event model
 * guidelines that are specified for JavaBeans.  In the JavaBeans
 * event model, once an event notification is dispatched, all listeners
 * must be notified before any further mutations occur to the source
 * of the event.  Further, order of delivery is not guaranteed.
 * <p>
 * Notification is provided as two separate events,
 * <a href="../event/DocumentEvent.html">DocumentEvent</a>, and
 * <a href="../event/UndoableEditEvent.html">UndoableEditEvent</a>.
 * If a mutation is made to a <code>Document</code> through its api,
 * a <code>DocumentEvent</code> will be sent to all of the registered
 * <code>DocumentListeners</code>.  If the <code>Document</code>
 * implementation supports undo/redo capabilities, an
 * <code>UndoableEditEvent</code> will be sent
 * to all of the registered <code>UndoableEditListener</code>s.
 * If an undoable edit is undone, a <code>DocumentEvent</code> should be
 * fired from the Document to indicate it has changed again.
 * In this case however, there should be no <code>UndoableEditEvent</code>
 * generated since that edit is actually the source of the change
 * rather than a mutation to the <code>Document</code> made through its
 * api.
 * <p style="text-align:center"><img src="doc-files/Document-notification.gif"
 * alt="The preceding text describes this graphic.">
 * <p>
 * Referring to the above diagram, suppose that the component shown
 * on the left mutates the document object represented by the blue
 * rectangle. The document responds by dispatching a DocumentEvent to
 * both component views and sends an UndoableEditEvent to the listening
 * logic, which maintains a history buffer.
 * <p>
 * Now suppose that the component shown on the right mutates the same
 * document.  Again, the document dispatches a DocumentEvent to both
 * component views and sends an UndoableEditEvent to the listening logic
 * that is maintaining the history buffer.
 * <p>
 * If the history buffer is then rolled back (i.e. the last UndoableEdit
 * undone), a DocumentEvent is sent to both views, causing both of them to
 * reflect the undone mutation to the document (that is, the
 * removal of the right component's mutation). If the history buffer again
 * rolls back another change, another DocumentEvent is sent to both views,
 * causing them to reflect the undone mutation to the document -- that is,
 * the removal of the left component's mutation.
 * <p>
 * The methods related to observing mutations to the document are:
 * <ul>
 *   <li>{@link #addDocumentListener(DocumentListener)}
 *   <li>{@link #removeDocumentListener(DocumentListener)}
 *   <li>{@link #addUndoableEditListener(UndoableEditListener)}
 *   <li>{@link #removeUndoableEditListener(UndoableEditListener)}
 * </ul>
 *
 * <p><b>Properties</b>
 * <p>
 * Document implementations will generally have some set of properties
 * associated with them at runtime.  Two well known properties are the
 * <a href="#StreamDescriptionProperty">StreamDescriptionProperty</a>,
 * which can be used to describe where the <code>Document</code> came from,
 * and the <a href="#TitleProperty">TitleProperty</a>, which can be used to
 * name the <code>Document</code>.  The methods related to the properties are:
 * <ul>
 * <li>{@link #getProperty(java.lang.Object)}
 * <li>{@link #putProperty(java.lang.Object, java.lang.Object)}
 * </ul>
 *
 * <p><b>Overview and Programming Tips</b>
 * <p><u>{@link javax.swing.text.Element}</u> is an important interface used in constructing a Document.
 * It has the power to describe various structural parts of a document,
 * such as paragraphs, lines of text, or even (in HTML documents) items in lists.
 * Conceptually, the Element interface captures some of the spirit of an SGML document.
 * So if you know SGML, you may already have some understanding of Swing's Element interface.
 * <p>In the Swing text API's document model, the interface Element defines a structural piece of a Document,
 * like a paragraph, a line of text, or a list item in an HTML document.
 * <p>Every Element is either a <i>branch</i> or a <i>leaf</i>. If an element is a branch,
 * the <code>isLeaf()</code> method returns false. If an element is a leaf, <code>isLeaf()</code> returns true.
 * <p>Branches can have any number of children. Leaves do not have children.
 * To determine how many children a branch has, you can call <code>getElementCount()</code>.
 * To determine the parent of an Element, you can call <code>getParentElement()</code>.
 * Root elements don't have parents, so calling <code>getParentElement()</code> on a root returns null.
 * <p>An Element represents a specific region in a Document that begins with startOffset
 * and ends just before endOffset.
 * The start offset of a branch Element is usually the start offset of its first child.
 * Similarly, the end offset of a branch Element is usually the end offset of its last child.
 * <p>Every Element is associated with an AttributeSet that you can access by calling <code>getAttributes()</code>.
 * In an Element, and AttributeSet is essentially a set of key/value pairs.
 * These pairs are generally used for markup -- such as determining the Element's
 * foreground color, font size, and so on. But it is up to the model, and the developer,
 * to determine what is stored in the AttributeSet.
 * <p>You can obtain the root Element (or Elements) of a Document by calling the
 * methods <code>getDefaultRootElement()</code> and <code>getRootElements()</code>, which are defined in the Document interface.
 * <p>The Document interface is responsible for translating a linear view of the
 * characters into Element operations. It is up to each Document implementation
 * to define what the Element structure is.
 *
 * <p><b>The PlainDocument class</b>
 * <p>The <u>{@link javax.swing.text.PlainDocument}</u> class defines an Element
 * structure in which the root node has a child node for each line of text in the model.
 * <u>Figure 1</u> shows how two lines of text would be modeled by a PlainDocument
 * <p style="text-align:center"><img src="doc-files/plain1.gif"
 * alt="The preceding text describes this graphic.">
 * <p><u>Figure 2</u> shows how how those same two lines of text might map to actual content:
 * <p style="text-align:center"><img src="doc-files/plain2.gif"
 * alt="The preceding text describes this graphic.">
 *
 * <p><b>Inserting text into a PlainDocument</b>
 * <p>As just mentioned, a PlainDocument contains a root Element, which in turn
 * contains an Element for each line of text.
 * When text is inserted into a PlainDocument, it creates the Elements that
 * are needed for an Element to exist for each newline.
 * To illustrate, let's say you wanted to insert a newline at offset 2 in <u>Figure 2</u>, above.
 * To accomplish this objective, you could use the Document method <code>insertString()</code>,
 * using this syntax:
 * <pre><code>document.insertString(2, "\n", null);</code></pre>
 * <p>After invoking the <code>insertString()</code> method, the Element structure would look
 * like the one shown in <u>Figure 3</u>.
 * <p style="text-align:center"><img src="doc-files/plain3.gif"
 * alt="The preceding text describes this graphic.">
 * <p>As another example, let's say you wanted to insert the pattern "new\ntext\n"
 * at offset 2 as shown previously in <u>Figure 2</u>. This operation would have the
 * result shown in <u>Figure 4</u>.
 * <p style="text-align:center"><img src="doc-files/plain4.gif"
 * alt="The preceding text describes this graphic.">
 * <p>In the preceding illustrations, the name of the line Elements is changed
 * after the insertion to match the line numbers.
 * But notice that when this is done, the AttributeSets remain the same.
 * For example, in <u>Figure 2</u>, the AttributeSet of Line 2 matches that of the
 * AttributeSet of Line 4 in <u>Figure 4</u>.
 *
 * <p><b>Removing text from a PlainDocument</b>
 * <p>Removal of text results in a structure change if the deletion spans more than one line.
 * Consider a deletion of seven characters starting at Offset 1 shown previously in <u>Figure 3</u>.
 * In this case, the Element representing Line 2 is completely removed, as the
 * region it represents is contained in the deleted region.
 * The Elements representing Lines 1 and 3 are joined, as they are partially
 * contained in the deleted region. Thus, we have the result:
 * <p style="text-align:center"><img src="doc-files/plain5.gif"
 * alt="The preceding text describes this graphic.">
 *
 * <p><b>The Default StyledDocument Class</b>
 * <p>The <u>{@link javax.swing.text.DefaultStyledDocument}</u> class, used for styled text,
 * contains another level of Elements.
 * This extra level is needed so that each paragraph can contain different styles of text.
 * In the two paragraphs shown in <u>Figure 6</u>, the first paragraph contains
 * two styles and the second paragraph contains three styles.
 * <p style="text-align:center"><img src="doc-files/plain6.gif"
 * alt="The preceding text describes this graphic.">
 * <p><u>Figure 7</u> shows how those same Elements might map to content.
 * <p style="text-align:center"><img src="doc-files/plain7.gif"
 * alt="The preceding text describes this graphic.">
 *
 * <p><b>Inserting text into a DefaultStyledDocument</b>
 * <p>As previously mentioned, DefaultStyledDocument maintains an Element structure
 * such that the root Element
 * contains a child Element for each paragraph. In turn, each of these
 * paragraph Elements contains an Element for each style of text in the paragraph.
 * As an example, let's say you had a document containing one paragraph,
 * and that this paragraph contained two styles, as shown in <u>Figure 8</u>.
 * <p style="text-align:center"><img src="doc-files/plain8.gif"
 * alt="The preceding text describes this graphic.">
 * <p>If you then wanted to insert a newline at offset 2, you would again use the
 * method <code>insertString()</code>, as follows:
 *
 *  <pre><code> styledDocument.insertString(2, "\n",
                styledDocument.getCharacterElement(0).getAttributes());</code></pre>

 * <p>This operation would have the result shown in <u>Figure 9</u>.
 * <p style="text-align:center"><img src="doc-files/plain9.gif"
 * alt="The preceding text describes this graphic.">
 * <p>It's important to note that the AttributeSet passed to <code>insertString()</code> matches
 * that of the attributes of Style 1. If the AttributeSet passed to <code>insertString()</code>
 *  did not match, the result would be the situation shown in <u>Figure 10</u>.
 * <p style="text-align:center"><img src="doc-files/plain10.gif"
 * alt="The preceding text describes this graphic.">
 * <p><b>Removing text from a DefaultStyledDocument</b>
 * <p>Removing text from a DefaultStyledDocument is similar to removing text from
 * a PlainDocument. The only difference is the extra level of Elements.
 * Consider what would happen if you deleted two characters at Offset 1
 * from Figure 10, above. Since the second Element of Paragraph 1 is
 * completely contained in the deleted region, it would be removed.
 * Assuming the attributes of Paragraph 1's first child matched those of
 * Paragraph2's first child, the results would be those shown in <u>Figure 11</u>.
 * <p style="text-align:center"><img src="doc-files/plain11.gif"
 * alt="The preceding text describes this graphic.">
 * <p>If the attributes did not match, we would get the results shown in <u>Figure 12</u>.
 * <p style="text-align:center"><img src="doc-files/plain12.gif"
 * alt="The preceding text describes this graphic.">
 *
 * <p><b>The StyledDocument Class</b>
 * <p>The <u>{@link javax.swing.text.StyledDocument}</u> class provides a method
 * named <code>setCharacterAttributes()</code>, which allows you to set the attributes
 * on the character Elements in a given range:

 *   <pre><code> public void setCharacterAttributes
 *          (int offset, int length, AttributeSet s, boolean replace);</code></pre>
 *
 * <p>Recall that in the diagrams shown in the previous section, all leaf Elements
 * shown in the drawings were also character Elements.
 * That means that the <code>setCharacterAttributes()</code> method could be used to set their attributes.
 * <p>The <code>setCharacterAttributes()</code> method takes four arguments .
 * The first and second arguments identify a region in the Document that is
 * to be changed. The third argument specifies the new attributes
 * (as an AttributeSet), and the fourth argument determines if the new attributes
 * should be added to the existing attributes (a value of false) or
 * if the character Element should replace its existing attributes
 * with the new attributes (a value of true).
 * <p>As an example, let's say you wanted to change the attributes of the
 * first three characters in <u>Figure 9</u>, shown previously.
 * The first two arguments passed to <code>setCharacterAttributes()</code> would be 0 and 3.
 * The third argument would be the AttributeSet containing the new attributes.
 * In the example we are considering, it doesn't matter what the fourth argument is.
 * <p>As the start and end offsets of the changed region (0 and 3) fall on
 * character Element boundaries, no structure change is needed.
 * That is, only the attributes of the character Element style 1 will change.
 * <p>Now let's look at an example that requires a structure change.
 * Instead of changing the first three characters shown in <u>Figure 9</u>,
 * let's change the first two characters.
 * Because the end change offset (2) does not fall on a character Element boundary,
 * the Element at offset 2 must be split in such a way
 * that offset 2 is the boundary of two Elements.
 * Invoking <code>setCharacterAttributes()</code> with a start offset of 0
 * and length of 2 has the result shown earlier in <u>Figure 10</u>.
 * <p><b>Changing Paragraph Attributes in a StyledDocument</b>
 * <p>The StyledDocument class provides a method named <code>setParagraphAttributes()</code>,
 * which can be used to change the attributes of a paragraph Element:

 *   <pre><code> public void setParagraphAttributes
 *         (int offset, int length, AttributeSet s, boolean replace);</code></pre>
 *
 *  <p>This method is similar to <code>setCharacterAttributes()</code>,
 *  but it allows you to change the attributes of paragraph Elements.
 *  It is up to the implementation of a StyledDocument to define which Elements
 *  are paragraphs. DefaultStyledDocument interprets paragraph Elements
 *  to be the parent Element of the character Element.
 *  Invoking this method does not result in a structure change;
 *  only the attributes of the paragraph Element change.
 *
 * <p>It is recommended to look into {@link javax.swing.text.EditorKit} and
 * {@link javax.swing.text.View}.
 * View is responsible for rendering a particular Element, and
 * EditorKit is responsible for a ViewFactory that is able to decide what
 * View should be created based on an Element.
 *
 * @author  Timothy Prinzing
 *
 * @see javax.swing.event.DocumentEvent
 * @see javax.swing.event.DocumentListener
 * @see javax.swing.event.UndoableEditEvent
 * @see javax.swing.event.UndoableEditListener
 * @see Element
 * @see Position
 * @see AttributeSet
 */
public interface Document {

    /**
     * Returns number of characters of content currently
     * in the document.
     *
     * @return number of characters &gt;= 0
     */
    public int getLength();

    /**
     * Registers the given observer to begin receiving notifications
     * when changes are made to the document.
     *
     * @param listener the observer to register
     * @see Document#removeDocumentListener
     */
    public void addDocumentListener(DocumentListener listener);

    /**
     * Unregisters the given observer from the notification list
     * so it will no longer receive change updates.
     *
     * @param listener the observer to register
     * @see Document#addDocumentListener
     */
    public void removeDocumentListener(DocumentListener listener);

    /**
     * Registers the given observer to begin receiving notifications
     * when undoable edits are made to the document.
     *
     * @param listener the observer to register
     * @see javax.swing.event.UndoableEditEvent
     */
    public void addUndoableEditListener(UndoableEditListener listener);

    /**
     * Unregisters the given observer from the notification list
     * so it will no longer receive updates.
     *
     * @param listener the observer to register
     * @see javax.swing.event.UndoableEditEvent
     */
    public void removeUndoableEditListener(UndoableEditListener listener);

    /**
     * Gets the properties associated with the document.
     *
     * @param key a non-<code>null</code> property key
     * @return the properties
     * @see #putProperty(Object, Object)
     */
    public Object getProperty(Object key);

    /**
     * Associates a property with the document.  Two standard
     * property keys provided are: <a href="#StreamDescriptionProperty">
     * <code>StreamDescriptionProperty</code></a> and
     * <a href="#TitleProperty"><code>TitleProperty</code></a>.
     * Other properties, such as author, may also be defined.
     *
     * @param key the non-<code>null</code> property key
     * @param value the property value
     * @see #getProperty(Object)
     */
    public void putProperty(Object key, Object value);

    /**
     * Removes a portion of the content of the document.
     * This will cause a DocumentEvent of type
     * DocumentEvent.EventType.REMOVE to be sent to the
     * registered DocumentListeners, unless an exception
     * is thrown.  The notification will be sent to the
     * listeners by calling the removeUpdate method on the
     * DocumentListeners.
     * <p>
     * To ensure reasonable behavior in the face
     * of concurrency, the event is dispatched after the
     * mutation has occurred. This means that by the time a
     * notification of removal is dispatched, the document
     * has already been updated and any marks created by
     * <code>createPosition</code> have already changed.
     * For a removal, the end of the removal range is collapsed
     * down to the start of the range, and any marks in the removal
     * range are collapsed down to the start of the range.
     * <p style="text-align:center"><img src="doc-files/Document-remove.gif"
     *  alt="Diagram shows removal of 'quick' from 'The quick brown fox.'">
     * <p>
     * If the Document structure changed as result of the removal,
     * the details of what Elements were inserted and removed in
     * response to the change will also be contained in the generated
     * DocumentEvent. It is up to the implementation of a Document
     * to decide how the structure should change in response to a
     * remove.
     * <p>
     * If the Document supports undo/redo, an UndoableEditEvent will
     * also be generated.
     *
     * @param offs  the offset from the beginning &gt;= 0
     * @param len   the number of characters to remove &gt;= 0
     * @throws BadLocationException  some portion of the removal range
     *   was not a valid part of the document.  The location in the exception
     *   is the first bad position encountered.
     * @see javax.swing.event.DocumentEvent
     * @see javax.swing.event.DocumentListener
     * @see javax.swing.event.UndoableEditEvent
     * @see javax.swing.event.UndoableEditListener
     */
    public void remove(int offs, int len) throws BadLocationException;

    /**
     * Inserts a string of content.  This will cause a DocumentEvent
     * of type DocumentEvent.EventType.INSERT to be sent to the
     * registered DocumentListers, unless an exception is thrown.
     * The DocumentEvent will be delivered by calling the
     * insertUpdate method on the DocumentListener.
     * The offset and length of the generated DocumentEvent
     * will indicate what change was actually made to the Document.
     * <p style="text-align:center"><img src="doc-files/Document-insert.gif"
     *  alt="Diagram shows insertion of 'quick' in 'The quick brown fox'">
     * <p>
     * If the Document structure changed as result of the insertion,
     * the details of what Elements were inserted and removed in
     * response to the change will also be contained in the generated
     * DocumentEvent.  It is up to the implementation of a Document
     * to decide how the structure should change in response to an
     * insertion.
     * <p>
     * If the Document supports undo/redo, an UndoableEditEvent will
     * also be generated.
     *
     * @param offset  the offset into the document to insert the content &gt;= 0.
     *    All positions that track change at or after the given location
     *    will move.
     * @param str    the string to insert
     * @param a      the attributes to associate with the inserted
     *   content.  This may be null if there are no attributes.
     * @throws BadLocationException  the given insert position is not a valid
     * position within the document
     * @see javax.swing.event.DocumentEvent
     * @see javax.swing.event.DocumentListener
     * @see javax.swing.event.UndoableEditEvent
     * @see javax.swing.event.UndoableEditListener
     */
    public void insertString(int offset, String str, AttributeSet a) throws BadLocationException;

    /**
     * Fetches the text contained within the given portion
     * of the document.
     *
     * @param offset  the offset into the document representing the desired
     *   start of the text &gt;= 0
     * @param length  the length of the desired string &gt;= 0
     * @return the text, in a String of length &gt;= 0
     * @throws BadLocationException  some portion of the given range
     *   was not a valid part of the document.  The location in the exception
     *   is the first bad position encountered.
     */
    public String getText(int offset, int length) throws BadLocationException;

    /**
     * Fetches the text contained within the given portion
     * of the document.
     * <p>
     * If the partialReturn property on the txt parameter is false, the
     * data returned in the Segment will be the entire length requested and
     * may or may not be a copy depending upon how the data was stored.
     * If the partialReturn property is true, only the amount of text that
     * can be returned without creating a copy is returned.  Using partial
     * returns will give better performance for situations where large
     * parts of the document are being scanned.  The following is an example
     * of using the partial return to access the entire document:
     *
     * <pre><code>
     *
     * &nbsp; int nleft = doc.getDocumentLength();
     * &nbsp; Segment text = new Segment();
     * &nbsp; int offs = 0;
     * &nbsp; text.setPartialReturn(true);
     * &nbsp; while (nleft &gt; 0) {
     * &nbsp;     doc.getText(offs, nleft, text);
     * &nbsp;     // do something with text
     * &nbsp;     nleft -= text.count;
     * &nbsp;     offs += text.count;
     * &nbsp; }
     *
     * </code></pre>
     *
     * @param offset  the offset into the document representing the desired
     *   start of the text &gt;= 0
     * @param length  the length of the desired string &gt;= 0
     * @param txt the Segment object to return the text in
     *
     * @throws BadLocationException  Some portion of the given range
     *   was not a valid part of the document.  The location in the exception
     *   is the first bad position encountered.
     */
    public void getText(int offset, int length, Segment txt) throws BadLocationException;

    /**
     * Returns a position that represents the start of the document.  The
     * position returned can be counted on to track change and stay
     * located at the beginning of the document.
     *
     * @return the position
     */
    public Position getStartPosition();

    /**
     * Returns a position that represents the end of the document.  The
     * position returned can be counted on to track change and stay
     * located at the end of the document.
     *
     * @return the position
     */
    public Position getEndPosition();

    /**
     * This method allows an application to mark a place in
     * a sequence of character content. This mark can then be
     * used to tracks change as insertions and removals are made
     * in the content. The policy is that insertions always
     * occur prior to the current position (the most common case)
     * unless the insertion location is zero, in which case the
     * insertion is forced to a position that follows the
     * original position.
     *
     * @param offs  the offset from the start of the document &gt;= 0
     * @return the position
     * @throws BadLocationException  if the given position does not
     *   represent a valid location in the associated document
     */
    public Position createPosition(int offs) throws BadLocationException;

    /**
     * Returns all of the root elements that are defined.
     * <p>
     * Typically there will be only one document structure, but the interface
     * supports building an arbitrary number of structural projections over the
     * text data. The document can have multiple root elements to support
     * multiple document structures.  Some examples might be:
     * </p>
     * <ul>
     * <li>Text direction.
     * <li>Lexical token streams.
     * <li>Parse trees.
     * <li>Conversions to formats other than the native format.
     * <li>Modification specifications.
     * <li>Annotations.
     * </ul>
     *
     * @return the root element
     */
    public Element[] getRootElements();

    /**
     * Returns the root element that views should be based upon,
     * unless some other mechanism for assigning views to element
     * structures is provided.
     *
     * @return the root element
     */
    public Element getDefaultRootElement();

    /**
     * Allows the model to be safely rendered in the presence
     * of concurrency, if the model supports being updated asynchronously.
     * The given runnable will be executed in a way that allows it
     * to safely read the model with no changes while the runnable
     * is being executed.  The runnable itself may <em>not</em>
     * make any mutations.
     *
     * @param r a <code>Runnable</code> used to render the model
     */
    public void render(Runnable r);

    /**
     * The property name for the description of the stream
     * used to initialize the document.  This should be used
     * if the document was initialized from a stream and
     * anything is known about the stream.
     */
    public static final String StreamDescriptionProperty = "stream";

    /**
     * The property name for the title of the document, if
     * there is one.
     */
    public static final String TitleProperty = "title";


}
