🚶 Visitor Design Pattern

// Element hierarchy
public interface DocumentElement {
    void accept(DocumentVisitor visitor);
}

public class Paragraph implements DocumentElement {
    private final String text;
    private final String style; // "heading", "body", "quote"

    public Paragraph(String text, String style) {
        this.text = text;
        this.style = style;
    }

    @Override
    public void accept(DocumentVisitor visitor) {
        visitor.visitParagraph(this);
    }

    public String getText() { return text; }
    public String getStyle() { return style; }
}

public class Image implements DocumentElement {
    private final String url;
    private final int width;
    private final int height;
    private final String altText;

    public Image(String url, int width, int height, String altText) {
        this.url = url;
        this.width = width;
        this.height = height;
        this.altText = altText;
    }

    @Override
    public void accept(DocumentVisitor visitor) {
        visitor.visitImage(this);
    }

    public String getUrl() { return url; }
    public int getWidth() { return width; }
    public int getHeight() { return height; }
    public String getAltText() { return altText; }
}

public class Table implements DocumentElement {
    private final String[][] data;
    private final String[] headers;

    public Table(String[] headers, String[][] data) {
        this.headers = headers;
        this.data = data;
    }

    @Override
    public void accept(DocumentVisitor visitor) {
        visitor.visitTable(this);
    }

    public String[][] getData() { return data; }
    public String[] getHeaders() { return headers; }
}

// Visitor interface
public interface DocumentVisitor {
    void visitParagraph(Paragraph paragraph);
    void visitImage(Image image);
    void visitTable(Table table);
}

// Concrete Visitor — HTML Export
public class HtmlExportVisitor implements DocumentVisitor {
    private final StringBuilder html = new StringBuilder();

    @Override
    public void visitParagraph(Paragraph p) {
        String tag = p.getStyle().equals("heading") ? "h1" : "p";
        html.append("<").append(tag).append(">")
            .append(p.getText())
            .append("</").append(tag).append(">\n");
    }

    @Override
    public void visitImage(Image img) {
        html.append(String.format("<img src=\"%s\" width=\"%d\" height=\"%d\" alt=\"%s\"/>\n",
            img.getUrl(), img.getWidth(), img.getHeight(), img.getAltText()));
    }

    @Override
    public void visitTable(Table table) {
        html.append("<table>\n<tr>");
        for (String h : table.getHeaders()) {
            html.append("<th>").append(h).append("</th>");
        }
        html.append("</tr>\n");
        for (String[] row : table.getData()) {
            html.append("<tr>");
            for (String cell : row) {
                html.append("<td>").append(cell).append("</td>");
            }
            html.append("</tr>\n");
        }
        html.append("</table>\n");
    }

    public String getHtml() { return html.toString(); }
}

// Concrete Visitor — Markdown Export
public class MarkdownExportVisitor implements DocumentVisitor {
    private final StringBuilder md = new StringBuilder();

    @Override
    public void visitParagraph(Paragraph p) {
        if (p.getStyle().equals("heading")) {
            md.append("# ").append(p.getText()).append("\n\n");
        } else if (p.getStyle().equals("quote")) {
            md.append("> ").append(p.getText()).append("\n\n");
        } else {
            md.append(p.getText()).append("\n\n");
        }
    }

    @Override
    public void visitImage(Image img) {
        md.append(String.format("![%s](%s)\n\n", img.getAltText(), img.getUrl()));
    }

    @Override
    public void visitTable(Table table) {
        md.append("| ").append(String.join(" | ", table.getHeaders())).append(" |\n");
        md.append("| ").append("--- | ".repeat(table.getHeaders().length)).append("\n");
        for (String[] row : table.getData()) {
            md.append("| ").append(String.join(" | ", row)).append(" |\n");
        }
        md.append("\n");
    }

    public String getMarkdown() { return md.toString(); }
}

// Concrete Visitor — Statistics
public class StatisticsVisitor implements DocumentVisitor {
    private int wordCount = 0;
    private int imageCount = 0;
    private int tableCount = 0;

    @Override
    public void visitParagraph(Paragraph p) {
        wordCount += p.getText().split("\\s+").length;
    }

    @Override
    public void visitImage(Image img) { imageCount++; }

    @Override
    public void visitTable(Table table) { tableCount++; }

    public void printStats() {
        System.out.println("📊 Document Statistics:");
        System.out.println("  Words: " + wordCount);
        System.out.println("  Images: " + imageCount);
        System.out.println("  Tables: " + tableCount);
    }
}

// Usage
public class Main {
    public static void main(String[] args) {
        List<DocumentElement> document = List.of(
            new Paragraph("Design Patterns", "heading"),
            new Paragraph("Visitor pattern separates operations from objects.", "body"),
            new Image("diagram.png", 800, 600, "Pattern diagram"),
            new Table(
                new String[]{"Pattern", "Type"},
                new String[][]{{"Visitor", "Behavioral"}, {"Factory", "Creational"}}
            )
        );

        // Export to HTML
        HtmlExportVisitor htmlVisitor = new HtmlExportVisitor();
        document.forEach(el -> el.accept(htmlVisitor));
        System.out.println(htmlVisitor.getHtml());

        // Export to Markdown
        MarkdownExportVisitor mdVisitor = new MarkdownExportVisitor();
        document.forEach(el -> el.accept(mdVisitor));
        System.out.println(mdVisitor.getMarkdown());

        // Gather statistics
        StatisticsVisitor statsVisitor = new StatisticsVisitor();
        document.forEach(el -> el.accept(statsVisitor));
        statsVisitor.printStats();
    }
}

// Elements — different item types
public interface ShopItem {
    void accept(PricingVisitor visitor);
}

public class Book implements ShopItem {
    private final String title;
    private final double price;

    public Book(String title, double price) {
        this.title = title;
        this.price = price;
    }

    @Override
    public void accept(PricingVisitor visitor) {
        visitor.visitBook(this);
    }

    public double getPrice() { return price; }
    public String getTitle() { return title; }
}

public class Electronics implements ShopItem {
    private final String name;
    private final double price;
    private final double weight; // kg

    public Electronics(String name, double price, double weight) {
        this.name = name;
        this.price = price;
        this.weight = weight;
    }

    @Override
    public void accept(PricingVisitor visitor) {
        visitor.visitElectronics(this);
    }

    public double getPrice() { return price; }
    public double getWeight() { return weight; }
}

// Visitor — different pricing strategies as visitors
public interface PricingVisitor {
    void visitBook(Book book);
    void visitElectronics(Electronics electronics);
    double getTotal();
}

public class TaxCalculator implements PricingVisitor {
    private double total = 0;

    @Override
    public void visitBook(Book book) {
        total += book.getPrice() * 1.05; // 5% tax on books
    }

    @Override
    public void visitElectronics(Electronics e) {
        total += e.getPrice() * 1.18; // 18% tax on electronics
    }

    @Override
    public double getTotal() { return total; }
}