HP Integrity NonStop J-Series - Manual (Page 156)

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©Copyright 1996 Rogue Wave Software
Choosing a Smalltalk-like Collection Class
Now that you have reviewed the list of Smalltalk-like collection classes, their class hierarchy,
and an example of how they work, you may be wondering how you can use them. This section
gives an overview of the various Smalltalk-like collection classes to help you choose an
appropriate one for your problem. You can also see Appendix A, on choosing.
Bags Versus Sets Versus Hash Tables
Class RWHashTable is the easiest Smalltalk-like collection class to understand. It uses a simple
hashed lookup to find the bucket where a particular object occurs, then does a linear search of
the bucket to find the object. A key concept is that multiple objects that test isEqual to each
other can be inserted into a hash table.
Class RWBag is similar to RWHashTable , except that it counts occurrences of multiple objects
with the same value; that is, it retains only the first occurrence and merely increments an
occurrence count for subsequent ones. RWBag is implemented as a dictionary, where the key is
the inserted object and the value is the occurrence count. This is the same way the Smalltalk
Bag object is implemented. Note that this implementation differs significantly from many other
C++ Bag classes which are closer to the RWHashTable class and not true Bags.
Class RWSet is similar to its base class RWHashTable , except that it doesn't allow duplicates. If
you try to insert an object that isEqual to an object already in RWSet, the object will be rejected.
Class RWIdentitySet , which inherits from RWSet, retrieves objects on the basis of identity
instead of value. Because RWIdentitySet is a Set, it can take only one instance of a given object.
Note that the ordering of objects in any of these classes based on hash tables is not meaningful.
If ordering is important, you should choose a sequenceable class.
Sequenceable Classes
Classes inheriting from RWSequenceable have an innate ordering. You can speak meaningfully
of their first or last object, or of their 6th or ith object.
These classes are implemented generally as either a vector, or a singly-linked or doubly-linked
list. Vector-based classes make good stacks and queues, but poor insertions in the middle. If