Collection Selection
Selection is a powerful expression language feature that lets you transform a source collection into another collection by selecting from its entries.
Selection uses a syntax of .?[selectionExpression]
. It filters the collection and
returns a new collection that contains a subset of the original elements. For example,
selection lets us easily get a list of Serbian inventors, as the following example shows:
-
Java
-
Kotlin
List<Inventor> list = (List<Inventor>) parser.parseExpression(
"members.?[nationality == 'Serbian']").getValue(societyContext);
val list = parser.parseExpression(
"members.?[nationality == 'Serbian']").getValue(societyContext) as List<Inventor>
Selection is supported for arrays and anything that implements java.lang.Iterable
or
java.util.Map
. For an array or Iterable
, the selection expression is evaluated
against each individual element. Against a map, the selection expression is evaluated
against each map entry (objects of the Java type Map.Entry
). Each map entry has its
key
and value
accessible as properties for use in the selection.
Given a Map
stored in a variable named #map
, the following expression returns a new
map that consists of those elements of the original map where the entry’s value is less
than 27:
-
Java
-
Kotlin
Map newMap = parser.parseExpression("#map.?[value < 27]").getValue(Map.class);
val newMap = parser.parseExpression("#map.?[value < 27]").getValue() as Map
In addition to returning all the selected elements, you can retrieve only the first or
the last element. To obtain the first element matching the selection expression, the
syntax is .^[selectionExpression]
. To obtain the last element matching the selection
expression, the syntax is .$[selectionExpression]
.