code: private void readObject(java.io.ObjectInputStream s)
          throws java.io.IOException, ClassNotFoundException {
          s.defaultReadObject();     // read in all fields
          // validate possibly bad fields
          if (precision < MIN_DIGITS) {
              String message = "MathContext: invalid digits in stream";
              throw new java.io.StreamCorruptedException(message);
          }
          if (roundingMode == null) {
              String message = "MathContext: null roundingMode in stream";
              throw new java.io.StreamCorruptedException(message);
          }
      }
docs: Reconstitute the {@code MathContext} instance from a stream (that is,
  deserialize it).
  
  @param s the stream being read.