Each metadata field commonly stores only one value, e.g. one integer, one string, one date, etc. However, you can also set one field to carry more values of the same type.

	Note
	Multivalue fields is a new feature available as of version 3.3.

As implied above, there are two types of structures:

list - is a set containing elements of a given data type (any you want). In source code, lists are marked by the [] brackets, for example:

integer[] list1 = [1, 367, -1, 20, 5, 0, -79]; // a list of integer elements
boolean[] list2 = [true, false, randomBoolean()]; // a list of three boolean elements
string[] list3; // a just-declared empty list to be filled by strings

map - is a pair of keys and their values. A key is always a string while a value can be any data type - but you cannot mix them (remember a map holds values of the same type). Example:

map[string,date] dateMap; // declaration

// filling the map with values
dateMap["a"] = 2011-01-01;
dateMap["b"] = 2012-12-31;
dateMap["c"] = randomDate(2011-01-01,2012-12-31);

For more information about maps and lists, see Data Types in CTL2.

	Important
	To change a field from single-value to multi-value: Go to Metadata Editor. Click a field or create a new one. In Property → Basic, switch Container Type either to list or map. (You will see an icon appears next to the field Type in the left hand record pane.)

Joining on Lists and Maps (Comparison Rules)

You can specify fields that are lists or maps as Join keys (see Join Types) just like any other fields. The only question is when two maps (lists) equal.

A list/map can:

• be null - it is not specified;

  map[string,date] myMap; // a just-declared map - no keys, no values

• contain empty elements

  string[] myList = ["hello", ""]; // a list whose second element is empty

• contain n elements - an ordinary case described, e.g. in Example 31.3, “Example situations when you could take advantage of multivalue fields”

Two maps (lists) are equal if both of them are not null, they have the same data type, element count and all element values (keys-values in maps) are equal.

Two maps (lists) are not equal if either of them is null.

	Important
	When comparing two lists, the order of their elements has to match, too. In maps, there is no 'order' of elements, therefore you cannot use them in Sort key.

Example 31.4. Integer lists which are (not) equal - symbolic notation

[1,2] == [1,2]
[null] != [1,2]
[1] != [1,2]
null != null // two unspecified lists
[null] == [null] // an extra case: lists which are not empty but whose elements are null

Note: Maps are implemented as LinkedHashMap, thus their properties derive from it.