Editing Object REXX (section)

=== Collection classes ===
A collection is an object that contains multiple items with associated indexes that enable items to be retrieved using the <code>AT</code> or <code>[]</code> methods. There are MapCollection, SetCollection and OrderedCollection classes, all of which allow manipulation of a specific collection type.<ref name=":9" /> 

A MapCollection is a mixin class that defines the basic set of methods implemented by all collections that map from an index to a value. The <code>Directory</code>, <code>StringTable</code>, <code>IdentityTable</code>, <code>Properties</code>, <code>Table</code>, <code>Relation</code> and <code>Stem</code> classes inherit these methods.<ref name=":2" /> A Directory or a StringTable object is a collection of unique string indexes. In an IdentityTable object, each item is associated with a single index, and there can only be one item for each index. The Properties object provides specific methods for saving and loading properties into files. Unlike a Table object, which cannot contain duplicate indexes, a Relation object is a collection in which elements can have the same index, which can be of any object type.  

A Stem object is created automatically when a compound variable is used. As in classic Rexx, such a variable consists of a stem and a tail, separated by a dot (<code>.</code>). While the stem must begin with a letter, the tail can be any character. Using a single numeric tail creates the same effect as an [[Array (data type)|array]], while multiple numeric tails can be used to create a multidimensional array.<ref name=":2" /><ref name=":9" /><syntaxhighlight lang="oorexx" style="background-color: #ffffff; !important" line="1">fruits.1 = "Apple"          /* assigning to stem variable */
fruits.4 = "Orange"         /* assigning to stem variable */
say fruits.4                /* output: Orange             */
say fruits.[1]              /* output: Apple              */</syntaxhighlight>SetCollections are special types of MapCollections where the index and the element are the same object. While the indexes in a <code>Set</code> object are unique, each index in a <code>Bag</code> object can appear more than once.<ref name=":2" />

An OrderedCollection is a mixin class that defines the basic methods for all collections that have an inherent index order, such as the <code>List</code>, <code>Queue</code>, <code>CircularQueue</code> and <code>Array</code> classes. A List object allows new items, for which a new index is created, to be added at any position in a collection. The associated index remains valid for that item regardless of other additions or removals. A Queue object allows items to be removed from the head and added to the tail or head of the queue. A CircularQueue object is a queue with a predefined size. Once the end of the circular queue is reached, new elements are inserted from the beginning to replace the previous items.<ref name=":2" /><ref name=":9" />

An Array is sequenced collection ordered by whole-number indexes. Like some other collection classes, the <code>Array</code> class provides the <code>MAKESTRING</code> method to encode its elements as a string object.<ref name=":2" /> <syntaxhighlight lang="oorexx" style="background-color: #ffffff; !important" line="1">
ArrayObj = .array~of("One", "Two", "Three") /* array with 3 items      */
say ArrayObj~at(2)                          /* output: Two             */
say ArrayObj~makeString(,"; ")              /* output: One; Two; Three */
</syntaxhighlight>