Type Safe Programming of XML-based Applications

There is an emerging amount of software for generating and manipulating XML documents. This paper addresses the problem of guaranteeing the validity of dynamically generated XML structures statically at compile time of an XML-based application. In the XOBE (XML OBJECTS) project we extend the object-oriented programming language Java by new language constructs. XML Schema is used for describing sets of valid XML documents. An XML schema provides a vehicle to define new classes, i.e. each element declaration in a schema defines a new class of objects (XML objects). Each object within a class represents an XML structure which is valid according to the underlying XML schema. XML objects are created by a new language construct called XML object constructor. XML object constructors are expressed in XML syntax. Previously generated XML objects can be inserted according to the declared XML schema. The main focus of the paper is the type system of XOBE. Among others, this type system provides the basis for checking the validity of assignments of XML objects to variables. The type system will be described and we present formally a type checking algorithm based on this type system.