code: public static String camelToSnake(String camel, boolean upper) {
          StringBuilder stringBuilder = new StringBuilder();
          for (char c : camel.toCharArray()) {
              char nc = upper ? Character.toUpperCase(c) : Character.toLowerCase(c);
              if (Character.isUpperCase(c)) {
                  stringBuilder.append('_').append(nc);
              } else {
                  stringBuilder.append(nc);
              }
          }
          return stringBuilder.toString();
      }
docs: Converts camel case string (lower or upper/Pascal) to snake case,
  for example 'helloWorld' or 'HelloWorld' -> 'hello_world'.
  
  @param camel Input string.
  @param upper True if result snake cased string should be upper cased like 'HELLO_WORLD'.