The Collection Classes

Some potentially very useful classes for programmers are those for defining collections of objects that require manipulation. A collection is an object that contains a number of items, which can be any objects. These manipulations might include counting objects, organizing them, or assigning them a supplier (for example, to indicate that a specific assortment of baked goods is supplied by the Pie-by-Night Bakery). The classes that enable such manipulations are called collection classes.

REXX includes classes for arrays, lists, queues, tables, directories, and the like. Every item stored in a REXX collection has an associated index that you can use to retrieve the item from the collection with the AT or [] (left and right bracket) methods, and each collection defines its own acceptable index types:

Array

A sequenced collection of objects ordered by whole number indexes.
List
A sequenced collection that lets you add new items at any position in the sequence. A list generates and returns an index value for each item placed in the list. The returned index remains valid until the item is removed from the list.
Queue
A sequenced collection with the items ordered as a queue. You can remove items from the head of the queue and add at either its tail or its head. Queues index the items with whole number indexes, in the order in which the items would be removed. The current head of the queue has index 1, the item after the head item has index 2, up to the number of items in the queue.
Table
A collection with indexes that can be any object. For example, string objects, array objects, alarm objects, or any user-created object can be a table index. The Table class determines index match by using the == comparison method to test for strict equality. A table contains no duplicate indexes.
Directory
A collection with character string indexes. Index comparisons are performed using the string == comparison method to test for strict equality.
Relation
A collection with indexes that can be any object (as with the Table class). A relation can contain duplicate indexes.
Set
A collection where the indexes and the values are the same object. Set indexes can be any object (as with the Table class) and each index is unique.
Bag
A collection where the index and the value are the same object. Bag indexes may be any object (as with the Table class) and each index may appear more than once.

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