Querying objects

Informally, an accessor A is characterized by a set of tuples: A=((M₁, P₁), ..., (M_k, P_k)), where every M_i is a method and P_i=(P_i,1, ...,P_i,l(i)) its corresponding argument list. Whenever a method M_i is applied to an object, this subsumes that it is invoked with its arguments P_i.

An accessor can be seen as a function, which applied to a message object returns another object: A(o: obj): obj. When such an accessor A is evaluated for a message object m, M₁ is invoked on m and every method M_i+1 (0 < i < k) is recursively invoked on the result of M_i. Finally, the result of M_k is returned.

In JAVA, an accessor object implements the interface Accessor given in Figure 4, and is evaluated by calling the get() method with the message object as argument. This method can throw exceptions raised when evaluating the method chain, which enables the reaction to exceptions. Returning null in case of exceptions would contradict the use of null as matching criterion.

public interface Accessor {

  public Object get(Object m) throws Exception;
}

Figure 4: Accessor Interface