17.5. Object Serialization

In Section 17.4, we demonstrated how to write the individual fields of an AccountRecord object into a file as text, and how to read those fields from a file and place their values into an AccountRecord object in memory. In the examples, AccountRecord was used to aggregate the information for one record. When the instance variables for an AccountRecord were output to a disk file, certain information was lost, such as the type of each value. For instance, if the value "3" is read from a file, there’s no way to tell whether it came from an int, a String or a double. We have only data, not type information, on a disk. If the program that’s going to read this data “knows” what object type the data corresponds to, then the data is simply read into objects of that type. For example, in Section 17.4.2, we know that we’re inputting an int (the account number), followed by two Strings (the first and last name) and a double (the balance). We also know that these values are separated by spaces, with only one record on each line. Sometimes we’ll not know exactly how the data is stored in a file. In such cases, we want to read or write an entire object from a file. Java provides such a mechanism, called object serialization. A so-called serialized object is an object represented as a sequence of bytes that includes the object’s data as well as information about the object’s type and the types of data stored in the object. After a serialized object has been written into a file, it can be read from the file and deserialized—that is, the type information and bytes that represent the object and its data can be used to recreate the object in memory.