"Show me your flowchart and conceal your tables, and I shall continue to be mystified. Show me your tables, and I won't usually need your flowchart; it'll be obvious." — Fred Brooks, The Mythical Man Month

Looking at the diagram below, the class Multivalent, at top center, is the central point for shared state. It holds Browsers, which correspond to top-level windows. Each Browser holds a document tree, with exactly one Root. The document tree includes in one unified tree both graphical user interface (GUI) elements such as the menubar and toolbar (shown in the bottom half of the diagram in the leftmost of the three rough columns), and tree nodes that correspond to document content (middle column). Regardless of whether the concrete document being displayed is PDF, HTML, DVI, manual pages, scanned paper, or something else, the document tree looks the same at this level. All of the functionality in the system is implemented by groups or Layers of Behaviors (rightmost column), which can apply to the entire Browser if they hook on to the Root, or document "genres" such as "HTML" and "PDF", or specific documents by hooking onto Documents.

The diagram depicts the high level data structures; in dynamic operation, control flow passes through the tree and through behaviors in protocols: restore, format, paint, events, semantic events, and so on; all activity pertaining to a particular function should be done in the corresponding protocol, which is carefully sequenced under system control so as to avoid conflicts among competing classes.

We now examine the diagram in more detail. The class Multivalent starts up the system when invoked by the Java Virtual Machine. It holds shared state such as preferences (including arbitrary persisten variables, and mappings from MIME type or file suffix to behavior that reads that document format), hooks to the network file cacheing, and methods that can create a new Browser or fetch a previously created one by name. Any class anywhere can obtain the singleton instance of this class with the static call Multivalent.getMultivalent().

A Browser corresponds to an independent window that can be moved about the screen, iconified, and so on. It holds state shared within a Browser such as the selection and the focus subwindow. It interfaces between the operating system and the Multivalent world: paint requests and mouse and keyboard events initiated by the user come through the OS to this class, which then routes them to the appropriate protocol for passing through the tree or behaviors.

All visual content is drawn via the document tree with a single Root. Tree nodes have the tree navigation facilities common to all tree packages, such as getting a child (getChild()), getting one's parent (getParentNode()), getting one's name (getName()), and so on. The Root is special in that it is a gateway between the document tree and the containing Browser. As a convenience, the method getRoot() on any Node climbs up the tree and returns the Root switch to a browser that can handle PNG (Diagram also available in .pdf for printing.)

The document tree contains both GUI nodes and nodes corresponding to concrete document formats. In both cases, the "structure" is represented in internal nodes, and "words" (text, image, button) in nodes at or near the leaves. In most other GUI toolkits, GUI elements are typically organized in a tree with layout widgets containing more specialized widgets as leaves. In Multivalent, which focuses on human readable documents, GUI widgets and text/image nodes are the same: a button is simply a word with a specialized function when you click on it, and widget layout is done with the same layout functions used for, say, laying out an HTML table. This has benefits in both directions: when documents are faced with, say, a HTML form, the tree is uniform, and there is no need to bridge two different systems, tree nodes and system GUI widgets. In the other direction, GUI widgets benefit from sophisticated display (buttons can display a HTML table and be controlled by stylesheets).

In the GUI subtree specifically (left column), VMenuButton, which displays a label like "File" or "Edit" in the menubar and when clicked displays its associated menu, is an internal node whose appearance is given by its subtree, which can be of arbitrary complexity, anything from a simple ASCII text to a full HTML table to a page of PDF. VMenuButtons can have explicit menus in the tree, but often they generate them on demand by creating an empty menu with a label and passing it to all the Behavior in the system to add entries.

The subtree corresponding to the content of the concrete document format (middle column) represents the structure of the document in internal nodes and the words and images and shapes in leaves. For SGML and XML the structure is identical to the parse tree, for HTML it is the corrected parse tree with appearance tags like I, B, TT, and EMPH separated out as Spans (see below), and for PDF and scanned paper the only available structure is often just text regions.

The Document at the head of the subtree represents an independent document in that it has a URI, a stylesheet, and, as available, attributes for author, title, and so on. Document has a link to the Behaviors associated with it, and in fact Root is a subclass of Document with a link to the Behaviors operational on all Documents.

Behavior implement all the user-level functionality of the system. They are grouped in Layers, and the list of Behavior in a hub is read from a XML hub document. System.hub, most important hub, is associated with the Root and establishes all those Behaviors that are always active no matter if the document displayed is HTML, PDF, or something else. Associated with a particular Document is a genre hub, such as for one for HTML or scanned paper. Often genre hubs share via inclusion other hubs; for instance, DVI and scanned paper share the "fixed format" hub.

To read new documents, format them, paint them on the screen, and respond to events, the system initiates a protocol. Depending on the protocol, the system passes control through tree Nodes (a tree-based protocol) or through the Behaviors in the Root and the current Document (a round-robin protocol). Behaviors can also be invoked during tree-based protocols by observing the tree at any node; when control passes through this node in the tree walk, the observing Behavior is notified before and after the node itself is traversed. An example of such a Behavior is one that implements key bindings; it listens for key events (java.awt.event.KeyEvent) that matches a binding and takes action on a match, and then has the option of short-circuiting further propagation of the event.

A widely use Behavior type, the Span is often used for hyperlinks and highlight regions. It hooks into the tree at two points and can affect appearance and receive events at any point inbetween, cutting across structural boundaries. In the diagram, the generic base class Span is used to associate a label, such as "i", "tt", or "emph" in HTML, over some text for the stylesheet to determine its appearance.