JavaExercism/exercises/markdown/src/main/java/Markdown.java

class Markdown {
  
    String parse(String markdown) {

        String[] lines = markdown.split("\n");

        String result = "";

        boolean activeList = false;

        for (int i = 0; i < lines.length; i++) {

            String theLine = parseHeader(lines[i]);
          
            if (theLine == null) {
              theLine = parseListItem(lines[i]);
            }
    
            if (theLine == null) 
            {
                theLine = parseParagraph(lines[i]);
            }

            if (theLine.matches("(<li>).*") && !theLine.matches("(<h).*") && !theLine.matches("(<p>).*") && !activeList) {
                activeList = true;
              result = result + "<ul>";
                result = result + theLine;
            } 
            
            else if (!theLine.matches("(<li>).*") && activeList) {
                activeList = false;
                result = result + "</ul>";
                result = result + theLine;
            } else {
              result = result + theLine;
            }
        }

        if (activeList) {
            result = result + "</ul>";
        }

        return result;
    }

    private String parseHeader(String markdown) {
        int count = 0;

        for (int i = 0; i < markdown.length() && markdown.charAt(i) == '#'; i++) 
        {
            count++;
        }

        if (count == 0) { return null; }

        return "<h" + Integer.toString(count) + ">" + markdown.substring(count + 1) + "</h" + Integer.toString(count)+ ">";
    }

    private String parseListItem(String markdown) {
        if (markdown.startsWith("*")) {
            String skipAsterisk = markdown.substring(2);
            String listItemString = parseSomeSymbols(skipAsterisk);
            return "<li>" + listItemString + "</li>";
        }

        return null;
    }

    private String parseParagraph(String markdown) {
        return "<p>" + parseSomeSymbols(markdown) + "</p>";
    }

    private String parseSomeSymbols(String markdown) {

        String lookingFor = "__(.+)__";
        String update = "<strong>$1</strong>";
        String workingOn = markdown.replaceAll(lookingFor, update);

        lookingFor = "_(.+)_";
        update = "<em>$1</em>";
        return workingOn.replaceAll(lookingFor, update);
    }
}