Primitive of the tree regular expression. most of the derived class is immutable (except ReferenceExp, ElementExp and OtherExp).
By making it immutable, it becomes possible to share subexpressions among expressions. This is very important for regular-expression-derivation based validation algorithm, as well as for smaller memory footprint. This sharing is automatically achieved by ExpressionPool.
ReferebceExp, ElementExp, and OtherExp are also placed in the pool, but these are not unified. Since they are not unified, application can derive classes from these expressions and mix them into AGM. This technique is heavily used to introduce schema language specific primitives into AGM. See various sub-packages of this package for examples.
The equals method must be implemented by the derived type. equals method will be used to unify the expressions. equals method can safely assume that its children are already unified (therefore == can be used to test the equality, rather than equals method).
To achieve unification, we overload the equals method so that o1.equals(o2)
is true if o1 and o2 are identical. There, those two objects must return the same hash code. For this purpose, the hash code is calculated statically and cached internally.
@author Kohsuke KAWAGUCHI