scalatestCoreDotty/org.scalatest/LoneElement

LoneElement

Trait that provides an implicit conversion that adds to collection types a loneElement method, which will return the value of the lone element if the collection does indeed contain one and only one element, or throw TestFailedException if not.

This construct allows you to express in one statement that a collection should contain one and only one element and that the element value should meet some expectation. Here's an example:

set.loneElement should be > 9

Or, using an assertion instead of a matcher expression:

assert(set.loneElement > 9)

The loneElement syntax can be used with any collection type C for which an implicit Collecting[C] is available. ScalaTest provides implicit Collecting instances for scala.collection.GenTraversable, Array, and java.util.Collection. You can enable the loneElement syntax on other collection types by defining an implicit Collecting instances for those types.

If you want to use loneElement with a java.util.Map, first transform it to a set of entries with entrySet, and if helpful, use ScalaTest's Entry class:

scala> import org.scalatest._
import org.scalatest._

scala> import LoneElement._
import LoneElement._

scala> import Matchers._
import Matchers._

scala> val jmap = new java.util.HashMap[String, Int]
jmap: java.util.HashMap[String,Int] = {}

scala> jmap.put("one", 1)
res0: Int = 0

scala> jmap.entrySet.loneElement should be (Entry("one", 1))

Companion:: object

class Object

trait Matchable

class Any

object LoneElement.type

Type members

Classlikes

Wrapper class that adds a loneElement method to any collection type C for which an implicit Collecting[C] is available.

Through the implicit conversion provided by trait LoneElement, this class allows you to make statements like:

trav.loneElement should be > 9

Type parameters:

CTC: the "collection type constructor" for the collection on which to add the loneElement method
E: the element type of the collection on which to add the loneElement method

Value parameters:

collecting: a typeclass that enables the loneElement syntax
collection: a collection to wrap in a LoneElementCollectionWrapper, which provides the loneElement method.

Wrapper class that adds a loneElement method to Java Map for which an implicit Collecting[org.scalatest.Entry, java.util.Map] is available.

Through the implicit conversion provided by trait LoneElement, this class allows you to make statements like:

jmap.loneElement.getKey should be > 9

Type parameters:

JMAP: the "Java Map type constructor" for the collection on which to add the loneElement method
K: the element type of the Java Map key on which to add the loneElement method
V: the element type of the Java Map value on which to add the loneElement method

Value parameters:

collecting: a typeclass that enables the loneElement syntax

Wrapper class that adds a loneElement method to Map for which an implicit Collecting[(K, V), scala.collection.GenTraversable[(K, V)]] is available.

Through the implicit conversion provided by trait LoneElement, this class allows you to make statements like:

map.loneElement._1 should be > 9

Type parameters:

K: the element type of the Map key on which to add the loneElement method
MAP: the "Map type constructor" for the collection on which to add the loneElement method
V: the element type of the Map value on which to add the loneElement method

Value parameters:

collecting: a typeclass that enables the loneElement syntax

Wrapper class that adds a loneElement method to String for which an implicit Collecting[C] is available.

Through the implicit conversion provided by trait LoneElement, this class allows you to make statements like:

"9".loneElement should be ('9')

Value parameters:

collecting: a typeclass that enables the loneElement syntax
s: the String to wrap

Implicits

Implicit conversion that adds a loneElement method to any collection type C for which an implicit Collecting[C] is available.

Type parameters:

CTC: the "collection type constructor" for the collection on which to add the loneElement method
E: the element type of the collection on which to add the loneElement method

Value parameters:

collecting: a typeclass that enables the loneElement syntax
collection: the collection on which to add the loneElement method

Implicit conversion that adds a loneElement method to String for which an implicit Collecting[C] is available.

Value parameters:

collecting: a typeclass that enables the loneElement syntax
s: the String to wrap