code: public static void checkState(boolean expression, Object errorMessage) {
      com.google.common.base.Preconditions.checkState(expression, errorMessage);
    }
docs: Ensures the truth of an expression involving the state of the calling instance, but not
  involving any parameters to the calling method.
  
  @param expression a boolean expression
  @param errorMessage the exception message to use if the check fails; will be converted to a
  string using {@link String#valueOf(Object)}
  @throws IllegalStateException if {@code expression} is false