AST Node System

/**
 * Base class for all CommonMark AST nodes
 */
public abstract class Node {
    /**
     * Accept a visitor (visitor pattern implementation)
     * @param visitor the visitor to accept
     */
    public abstract void accept(Visitor visitor);
    
    /**
     * Get the next sibling node
     * @return the next sibling, or null if this is the last child
     */
    public Node getNext();
    
    /**
     * Get the previous sibling node  
     * @return the previous sibling, or null if this is the first child
     */
    public Node getPrevious();
    
    /**
     * Get the first child node
     * @return the first child, or null if no children
     */
    public Node getFirstChild();
    
    /**
     * Get the last child node
     * @return the last child, or null if no children
     */
    public Node getLastChild();
    
    /**
     * Get the parent node
     * @return the parent node, or null if this is the root
     */
    public Node getParent();
    
    /**
     * Append a child node to this node
     * @param child the child node to append
     */
    public void appendChild(Node child);
    
    /**
     * Prepend a child node to this node
     * @param child the child node to prepend
     */
    public void prependChild(Node child);
    
    /**
     * Remove this node from its parent
     */
    public void unlink();
    
    /**
     * Insert this node after the specified sibling
     * @param sibling the sibling to insert after
     */
    public void insertAfter(Node sibling);
    
    /**
     * Insert this node before the specified sibling
     * @param sibling the sibling to insert before
     */
    public void insertBefore(Node sibling);
    
    /**
     * Get source spans for this node (available since 0.16.0)
     * @return list of source spans, empty if not included by parser
     */
    public List<SourceSpan> getSourceSpans();
    
    /**
     * Replace current source spans with provided list (available since 0.16.0)
     * @param sourceSpans the new source spans to set
     */
    public void setSourceSpans(List<SourceSpan> sourceSpans);
    
    /**
     * Add a source span to the end of the list (available since 0.16.0)
     * @param sourceSpan the source span to add
     */
    public void addSourceSpan(SourceSpan sourceSpan);
}

/**
 * Base class for block-level nodes
 */
public abstract class Block extends Node {
    /**
     * Get the parent block (overridden for type safety)
     * @return the parent block
     */
    public Block getParent();
}

/**
 * Root document node - the top level of any parsed document
 */
public class Document extends Block {
    public void accept(Visitor visitor);
}

/**
 * Heading nodes (# ## ### etc.)
 */
public class Heading extends Block {
    /**
     * Get the heading level (1-6)
     * @return the heading level
     */
    public int getLevel();
    
    /**
     * Set the heading level
     * @param level the heading level (1-6)
     */
    public void setLevel(int level);
    
    public void accept(Visitor visitor);
}

/**
 * Paragraph blocks
 */
public class Paragraph extends Block {
    public void accept(Visitor visitor);
}

/**
 * Block quotes (> text)
 */
public class BlockQuote extends Block {
    public void accept(Visitor visitor);
}

/**
 * Horizontal rules (---, ***, ___)
 */
public class ThematicBreak extends Block {
    public void accept(Visitor visitor);
}

/**
 * Fenced code blocks (``` code ```)
 */
public class FencedCodeBlock extends Block {
    /**
     * Get the fence character (` or ~)
     * @return the fence character
     */
    public char getFenceChar();
    
    /**
     * Set the fence character
     * @param fenceChar the fence character
     */
    public void setFenceChar(char fenceChar);
    
    /**
     * Get the fence length (number of fence characters)
     * @return the fence length
     */
    public int getFenceLength();
    
    /**
     * Set the fence length
     * @param fenceLength the fence length
     */
    public void setFenceLength(int fenceLength);
    
    /**
     * Get the fence indentation
     * @return the fence indentation
     */
    public int getFenceIndent();
    
    /**
     * Set the fence indentation
     * @param fenceIndent the fence indentation
     */
    public void setFenceIndent(int fenceIndent);
    
    /**
     * Get the info string (language identifier)
     * @return the info string, or null if not specified
     */
    public String getInfo();
    
    /**
     * Set the info string
     * @param info the info string
     */
    public void setInfo(String info);
    
    /**
     * Get the literal code content
     * @return the code content
     */
    public String getLiteral();
    
    /**
     * Set the literal code content
     * @param literal the code content
     */
    public void setLiteral(String literal);
    
    public void accept(Visitor visitor);
}

/**
 * Indented code blocks (4+ spaces)
 */
public class IndentedCodeBlock extends Block {
    /**
     * Get the literal code content
     * @return the code content
     */
    public String getLiteral();
    
    /**
     * Set the literal code content
     * @param literal the code content
     */
    public void setLiteral(String literal);
    
    public void accept(Visitor visitor);
}

/**
 * Base class for list blocks (ordered and unordered)
 */
public abstract class ListBlock extends Block {
    // Base class for BulletList and OrderedList
}

/**
 * Unordered lists (*, -, +)
 */
public class BulletList extends ListBlock {
    /**
     * Get the bullet marker character
     * @return the bullet marker (*, -, or +)
     */
    public char getBulletMarker();
    
    /**
     * Set the bullet marker character
     * @param bulletMarker the bullet marker
     */
    public void setBulletMarker(char bulletMarker);
    
    public void accept(Visitor visitor);
}

/**
 * Ordered lists (1. 2. 3.)
 */
public class OrderedList extends ListBlock {
    /**
     * Get the starting number
     * @return the starting number
     */
    public int getStartNumber();
    
    /**
     * Set the starting number
     * @param startNumber the starting number
     */
    public void setStartNumber(int startNumber);
    
    /**
     * Get the delimiter character (. or ))
     * @return the delimiter character
     */
    public char getDelimiter();
    
    /**
     * Set the delimiter character
     * @param delimiter the delimiter character
     */
    public void setDelimiter(char delimiter);
    
    public void accept(Visitor visitor);
}

/**
 * Individual list items
 */
public class ListItem extends Block {
    public void accept(Visitor visitor);
}

/**
 * Plain text nodes
 */
public class Text extends Node {
    /**
     * Create an empty text node
     */
    public Text();
    
    /**
     * Create a text node with the given literal content
     * @param literal the text content
     */
    public Text(String literal);
    
    /**
     * Get the literal text content
     * @return the text content
     */
    public String getLiteral();
    
    /**
     * Set the literal text content
     * @param literal the text content
     */
    public void setLiteral(String literal);
    
    public void accept(Visitor visitor);
}

/**
 * Inline code (`code`)
 */
public class Code extends Node {
    /**
     * Get the literal code content
     * @return the code content
     */
    public String getLiteral();
    
    /**
     * Set the literal code content
     * @param literal the code content
     */
    public void setLiteral(String literal);
    
    public void accept(Visitor visitor);
}

/**
 * Interface for nodes with delimiters (emphasis, strong emphasis)
 */
public interface Delimited {
    /**
     * Get the opening delimiter
     * @return the opening delimiter string
     */
    String getOpeningDelimiter();
    
    /**
     * Get the closing delimiter
     * @return the closing delimiter string
     */
    String getClosingDelimiter();
}

/**
 * Italic text (*text* or _text_)
 */
public class Emphasis extends Node implements Delimited {
    /**
     * Create an empty emphasis node
     */
    public Emphasis();
    
    /**
     * Create an emphasis node with the given delimiter
     * @param delimiter the delimiter string (* or _)
     */
    public Emphasis(String delimiter);
    
    /**
     * Get the opening delimiter
     * @return the opening delimiter
     */
    public String getOpeningDelimiter();
    
    /**
     * Get the closing delimiter
     * @return the closing delimiter
     */
    public String getClosingDelimiter();
    
    /**
     * Set the delimiter for both opening and closing
     * @param delimiter the delimiter string
     */
    public void setDelimiter(String delimiter);
    
    public void accept(Visitor visitor);
}

/**
 * Bold text (**text** or __text__)
 */
public class StrongEmphasis extends Node implements Delimited {
    /**
     * Create an empty strong emphasis node
     */
    public StrongEmphasis();
    
    /**
     * Create a strong emphasis node with the given delimiter
     * @param delimiter the delimiter string (** or __)
     */
    public StrongEmphasis(String delimiter);
    
    /**
     * Get the opening delimiter
     * @return the opening delimiter
     */
    public String getOpeningDelimiter();
    
    /**
     * Get the closing delimiter
     * @return the closing delimiter
     */
    public String getClosingDelimiter();
    
    /**
     * Set the delimiter for both opening and closing
     * @param delimiter the delimiter string
     */
    public void setDelimiter(String delimiter);
    
    public void accept(Visitor visitor);
}

/**
 * Links [text](url "title")
 */
public class Link extends Node {
    /**
     * Create an empty link
     */
    public Link();
    
    /**
     * Create a link with destination and title
     * @param destination the link destination
     * @param title the link title (may be null)
     */
    public Link(String destination, String title);
    
    /**
     * Get the link destination
     * @return the destination URL
     */
    public String getDestination();
    
    /**
     * Set the link destination
     * @param destination the destination URL
     */
    public void setDestination(String destination);
    
    /**
     * Get the link title
     * @return the title, or null if not specified
     */
    public String getTitle();
    
    /**
     * Set the link title
     * @param title the title
     */
    public void setTitle(String title);
    
    public void accept(Visitor visitor);
}

/**
 * Images ![alt](src "title")
 */
public class Image extends Node {
    /**
     * Create an empty image
     */
    public Image();
    
    /**
     * Create an image with destination and title
     * @param destination the image source
     * @param title the image title (may be null)
     */
    public Image(String destination, String title);
    
    /**
     * Get the image destination (src)
     * @return the image source URL
     */
    public String getDestination();
    
    /**
     * Set the image destination
     * @param destination the image source URL
     */
    public void setDestination(String destination);
    
    /**
     * Get the image title
     * @return the title, or null if not specified
     */
    public String getTitle();
    
    /**
     * Set the image title
     * @param title the title
     */
    public void setTitle(String title);
    
    public void accept(Visitor visitor);
}

/**
 * Link reference definitions [label]: url "title"
 */
public class LinkReferenceDefinition extends Node {
    /**
     * Get the reference label
     * @return the label
     */
    public String getLabel();
    
    /**
     * Set the reference label
     * @param label the label
     */
    public void setLabel(String label);
    
    /**
     * Get the destination URL
     * @return the destination
     */
    public String getDestination();
    
    /**
     * Set the destination URL
     * @param destination the destination
     */
    public void setDestination(String destination);
    
    /**
     * Get the title
     * @return the title, or null if not specified
     */
    public String getTitle();
    
    /**
     * Set the title
     * @param title the title
     */
    public void setTitle(String title);
    
    public void accept(Visitor visitor);
}

/**
 * HTML block elements
 */
public class HtmlBlock extends Block {
    /**
     * Get the literal HTML content
     * @return the HTML content
     */
    public String getLiteral();
    
    /**
     * Set the literal HTML content
     * @param literal the HTML content
     */
    public void setLiteral(String literal);
    
    public void accept(Visitor visitor);
}

/**
 * Inline HTML elements
 */
public class HtmlInline extends Node {
    /**
     * Get the literal HTML content
     * @return the HTML content
     */
    public String getLiteral();
    
    /**
     * Set the literal HTML content
     * @param literal the HTML content
     */
    public void setLiteral(String literal);
    
    public void accept(Visitor visitor);
}

/**
 * Soft line breaks (newlines in text)
 */
public class SoftLineBreak extends Node {
    public void accept(Visitor visitor);
}

/**
 * Hard line breaks (  \n or \)
 */
public class HardLineBreak extends Node {
    public void accept(Visitor visitor);
}

/**
 * Visitor pattern interface for traversing AST nodes
 */
public interface Visitor {
    void visit(Document document);
    void visit(Heading heading);
    void visit(Paragraph paragraph);
    void visit(BlockQuote blockQuote);
    void visit(BulletList bulletList);
    void visit(OrderedList orderedList);
    void visit(ListItem listItem);
    void visit(FencedCodeBlock fencedCodeBlock);
    void visit(IndentedCodeBlock indentedCodeBlock);
    void visit(HtmlBlock htmlBlock);
    void visit(ThematicBreak thematicBreak);
    void visit(Text text);
    void visit(Code code);
    void visit(Emphasis emphasis);
    void visit(StrongEmphasis strongEmphasis);
    void visit(Link link);
    void visit(Image image);
    void visit(HtmlInline htmlInline);
    void visit(SoftLineBreak softLineBreak);
    void visit(HardLineBreak hardLineBreak);
    void visit(LinkReferenceDefinition linkReferenceDefinition);
    void visit(CustomBlock customBlock);
    void visit(CustomNode customNode);
}

/**
 * Abstract visitor that visits all children by default.
 * Provides default implementations of all visit methods that call visitChildren.
 * Override specific visit methods to customize behavior for particular node types.
 */
public abstract class AbstractVisitor implements Visitor {
    /**
     * Visit all children of the given parent node. 
     * This method is called by all default visit method implementations.
     * Override specific visit methods and call visitChildren if you want to continue traversal.
     * 
     * @param parent the parent node whose children to visit
     */
    protected void visitChildren(Node parent);
    
    // Default implementations for all visit methods in Visitor interface:
    // public void visit(Document document) { visitChildren(document); }
    // public void visit(Heading heading) { visitChildren(heading); }
    // ... and so on for all node types
}

/**
 * Base class for custom inline nodes
 */
public abstract class CustomNode extends Node {
    // Base class for creating custom inline node types
}

/**
 * Base class for custom block nodes
 */
public abstract class CustomBlock extends Block {
    // Base class for creating custom block node types
}

/**
 * Utility class for working with multiple nodes
 */
public class Nodes {
    /**
     * Get all nodes between start and end (exclusive)
     * @param start the starting node
     * @param end the ending node
     * @return iterable of nodes between start and end
     */
    public static Iterable<Node> between(Node start, Node end);
}

import org.commonmark.node.*;
import org.commonmark.parser.Parser;

// Parse and traverse AST
Parser parser = Parser.builder().build();
Node document = parser.parse("# Title\n\nHello **world**!");

// Using visitor pattern
document.accept(new AbstractVisitor() {
    @Override
    public void visit(Heading heading) {
        System.out.println("Found heading level " + heading.getLevel());
        super.visit(heading);
    }
    
    @Override
    public void visit(StrongEmphasis strongEmphasis) {
        System.out.println("Found bold text");
        super.visit(strongEmphasis);
    }
});

// Manual tree traversal
Node child = document.getFirstChild();
while (child != null) {
    System.out.println("Child node: " + child.getClass().getSimpleName());
    child = child.getNext();
}

// Modify AST - add a new paragraph
Paragraph newParagraph = new Paragraph();
Text text = new Text("This is a new paragraph.");
newParagraph.appendChild(text);
document.appendChild(newParagraph);

// Create links programmatically
Link link = new Link("https://example.com", "Example Site");
link.appendChild(new Text("Visit our site"));