code: public boolean getBoolean(String key, boolean defaultValue) {
      String value = get(key);
      // We can't use `Boolean.parseBoolean` here, as it returns false for invalid strings.
      if (value == null) {
        return defaultValue;
      } else if (value.equalsIgnoreCase("true")) {
        return true;
      } else if (value.equalsIgnoreCase("false")) {
        return false;
      } else {
        throw new IllegalArgumentException(value + " is not a boolean string.");
      }
    }
docs: Returns the boolean value to which the specified key is mapped,
  or defaultValue if there is no mapping for the key. The key match is case-insensitive.