Each metadata field commonly stores only one value (e.g. one integer, one string, one date). 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 Clover v. 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. 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);

You will find out more about maps and lists if you go to 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.)

Lists and Maps Support in Components

An alphabetically sorted list of components which you can use multivalue fields in:

Component	List	Map	Note
CloverDataReader
CloverDataWriter
ParallelSimpleGather			Round robin
			Merge by key
			Simple gather
ParallelPartition			Ranges
			Partition key
			Partition class
Concatenate
DataGenerator
DataIntersection
			Map is not a part of key
DBJoin			Map is not a part of key
Dedup
			Map is not a part of key
Denormalizer
			Map is not a part of key
Filter
ExtHashJoin
			Map is not a part of key
ExtMergeJoin			Map is not a part of key
ExtSort
			Map is not a part of key.
JavaBeanReader
JavaBeanWriter
JavaMapWriter
JSONExtract
JSONReader
JSONWriter
LookupJoin
LookupTableReaderWriter
Merge
			Map is not a part of key
Normalizer
			Map is not a part of key
Partition			Ranges
			Partition key
			Partition class
Reformat
RelationalJoin			Map is not a part of key
Rollup			Sorted input
			Sorted input, map not part of key
			Unsorted input
SequenceChecker
			Map is not a part of key
SimpleCopy
SimpleGather
Sleep
SortWithinGroups
XMLExtract
XMLReader
XMLWriter

At the moment, neither map nor list structures can be extracted as metadata from flat files.

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.

First of all, let us clarify this. 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 32.4, “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 and therefore you cannot use them in Sort key.

Example 32.5.  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 and thus their properties derive from it.