The storage of objects is a primary concern in the development of object-orientated software. An object is a software unit which is closed off from the exterior and whose attributes are fixed internally by a series of local variables. The content of these local variables defines the state of the object but outside the object neither the local variables nor their content can be seen. The state of an object can only be changed by running one of its local subroutines. These subroutines are also known as methods. An object is requested to run a method by another object as a result of the object making the request sending a message to this effect. The object addressed will, assuming it understands the message, cause the appropriate method (local subroutine) to be run. Storing objects consists of storing both the attributes and the state in which methods/algorithms are, so that the latter can be continued seamlessly when the object is restored. Persistance, CORBA and ORB are jargon terms currently used to describe this.
For chip cards, there is no way of achieving this. Data is usually stored on them in conventional file structures. For the application data relating to the host application, there are various simple file types and file directories available on the card. Data is exchanged between host and card by means of individual command sequences (APDU's) to which the data is attached in the form of bytes. It is file-orientated cards of this kind which are most widely used today.
A widely used Java method of storing object states is known as object serializing. What object serializing means is that any desired object is converted into a sequence of bytes which represent the object and its present state. For this purpose all the attributes and their names and all the other objects to which the object makes reference are stored. Given this information, the object can be restored unchanged. The restoration is performed in a similar way: the object, and all the other objects to which it makes reference, is instanced and, using the data read from the chip card, initialized. The byte array which represents the object can be stored without any problems both on JavaCards and on conventional file-orientated cards.
The methods known hitherto for serializing and storing objects were designed for powerful computers. However, because they require too much storage space they are not suitable for storing objects on chip cards or storage media on which there is little storage space or else the only objects which can be stored in serialized form on chip cards are very small ones. For example, in Java object serialization several hundred bytes are required even for small objects with only a few attributes.
The object of the invention is therefore to provide a method whereby data packages can be easy and quickly stored in a very compact form on a storage medium having only limited storage space or can be transmitted over a transmission channel of narrow band-width, in such a way that the data package can be restored at the receiving end.