You may have noticed the class ObjectOutputStream in the previous diagram. Streams of that class write objects to a destination such as a disk file.
ObjectInputStream streams read objects from a source.
So far in our programs objects have existed only in main memory and lasted only as long the program that created them was running. With an ObjectOutputStream an object can be written to disk and will remain there even after the program stops. This is the topic of object serialization , an advanced topic not yet covered in these notes.