IDS50-JG. Properly encode or escape output

public class BadOutput {
  // description and input are String variables containing values obtained from a database
  // description = "description" and input = "<script> executable code </script>"
  public static void display(String description, String input) {
    // Display to the user or pass description and input to another system
  }
}

public class ValidateOutput {
  // Allows only alphanumeric characters and spaces
  private Pattern pattern = Pattern.compile("^[a-zA-Z0-9\\s]{0,20}$");

  // Validates and encodes the input field based on a whitelist
  private String validate(String name, String input) throws ValidationException {
    String canonical = normalize(input);

    if(!pattern.matcher(canonical).matches()) {
      throw new ValidationException("Improper format in " + name + " field");
    }
    
    // Performs output encoding for non valid characters 
    canonical = HTMLEntityEncode(canonical);
    return canonical;
  }

  // Normalizes to known instances 	
  private String normalize(String input) {
    String canonical = java.text.Normalizer.normalize(input, Normalizer.Form.NFKC);
    return canonical;
  }

  // Encodes non valid data
  public static String HTMLEntityEncode(String input) {
    StringBuffer sb = new StringBuffer();

    for (int i = 0; i < input.length(); i++) {
      char ch = input.charAt(i);
        if (Character.isLetterOrDigit(ch) || Character.isWhitespace(ch)) {
          sb.append(ch);
        } else {
          sb.append("&#" + (int)ch + ";");
        }
    }
    return sb.toString();
  }

  // description and input are String variables containing values obtained from a database
  // description = "description" and input = "2 items available"
  public static void display(String description, String input) throws ValidationException {
    ValidateOutput vo = new ValidateOutput();
    vo.validate(description, input);
    // Pass to another system or display to the user
  }
}