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## Multivalue Fields

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.

Example 32.4. Example situations when you could take advantage of multivalue fields

- A record containing an employee's `ID`, `Name` and `Address`. Since employees move from time to time, you might need to keep track of all their addresses, both current and past. Instead of creating new metadata fields each time an employee moves to a new house, you can store a list of all addresses into one field.

- You are processing an input stream of `CSV` files, each containing a different column count. Normally, that would imply creating new metadata for each file (each column count). Instead, you can define a generic map in metadata and append fields to it each time they occur.

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.

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