Java converting from camel case to snake case

CaseConverter.java

    /**
     * Camel case to underscores (examles CamelCase -> CAMEL_CASE)
     * @param camel camel case
     * @return underscores
     */
    public static String camelCaseToUnderscores(String camel)
    {
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < camel.length(); i++)
        {
            Character actualChar = camel.charAt(i);
            Character prevChar = i > 0 ? camel.charAt(i - 1) : null;
            Character nextChar = i < camel.length() - 1 ? camel.charAt(i + 1) : null;

            if (i > 0 && (isUpperCase(actualChar) && isLowerCase(nextChar)
                || isUpperCase(actualChar) && isLowerCase(prevChar)))
            {
                sb.append("_");
                sb.append(actualChar);
            }
            else
            {
                sb.append(actualChar);
            }
        }
        return sb.toString().toUpperCase();
    }

    private static boolean isUpperCase(Character character)
    {
        if (character == null)
        {
            return false;
        }
        
        return Character.isDigit(character) || Character.isUpperCase(character);
    }
    
    private static boolean isLowerCase(Character character)
    {
        if (character == null)
        {
            return false;
        }
        
        return !Character.isDigit(character) && Character.isLowerCase(character);
    }