code: public static void checkState(boolean expression, @NullableDecl Object errorMessage) {
      if (!expression) {
        throw new IllegalStateException(String.valueOf(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
  @see Verify#verify Verify.verify()