Verification Support for Object Database Design

boolean Element::isConnectedTo(Element n); An implementation, with the same signature (i.e., name and input/output types), should be defined for all concrete classes that inherit from Element, namely for the classes ANode, CNode and Link. We define different implementations for Link and Node (i.e., for both ANode and CNode). Here is the implementation for the abstract class Node: boolean Node::isConnectedTo(Element n) { return (n in incomingLinks) or (n in outgoingLinks) } 3.1. The object definition language: OASIS-ODL 21 The implementation for Link is very different, although the signature should not change. Here is the code for Link: boolean Link::isConnectedTo(Element n) { return (from == n) or (to == n) } The isConnectedTo method can be invoked for any object of type Element. In this case, the actual run-time type of the receiver object determines which implementation of the method is used. This mechanism is called late binding. For example, in the definition of method removeElement, the method isConnectedTo is applied to each object in the collection of elements. Depending on the actual run-time type of the Element at hand (i.e., ANode, CNode, or Link), the right implementation of isConnectedTo is applied. Methods such as the isConnectedTo method above are called retrieval methods. Retrieval methods are methods that do not update any objects. The body of a retrieval methods consists of only a single return statement, where the return value is specified by an OQL expression. These expressions are side-effect free (see Section 3.2). Retrieval methods can be invoked in queries (see Section 3.2), methods that perform updates cannot. An extended class may add new methods, or it may override some of the methods inherited from one of its superclasses. A method defined in a superclass is redefined in a subclass by defining a method with the same name and the same number of parameters and parameter/return types as specified in the superclass. Again, late binding determines which implementation should actually be used. In Java it is also possible to hide some of the attributes from a superclass. Hiding an attribute means that if we declare an attribute in a class and that attribute has the same name as another attribute in one of the inherited superclasses, then the inherited attribute is hidden and can only be accessed using the keyword super, instead of this. In OASIS, this feature has not been taken into account.