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An extractor object is an object with an
unapply
method. Whereas the
apply
method is like a constructor which takes arguments and creates an object, the
unapply
takes an object and tries to give back the arguments. This is most often used in pattern matching and partial functions.
def
apply
(
name
:
String
)
=
s
"$name--${Random.nextLong()}"
def
unapply
(
customerID
:
String
)
:
Option
[
String
]
=
{
val
stringArray
:
Array
[
String
]
=
customerID
.
split
(
"--"
)
if
(
stringArray
.
tail
.
nonEmpty
)
Some
(
stringArray
.
head
)
else
None
val
customer1ID
=
CustomerID
(
"Sukyoung"
)
// Sukyoung--23098234908
customer1ID
match
{
case
CustomerID
(
name
)
=>
println
(
name
)
// prints Sukyoung
case
_
=>
println
(
"Could not extract a CustomerID"
)
def
apply
(
name:
String
)
=
s
"$name--${Random.nextLong()}"
def
unapply
(
customerID
:
String
)
:
Option
[
String
]
=
val
stringArray
:
Array
[
String
]
=
customerID
.
split
(
"--"
)
if
stringArray
.
tail
.
nonEmpty
then
Some
(
stringArray
.
head
)
else
None
val
customer1ID
=
CustomerID
(
"Sukyoung"
)
// Sukyoung--23098234908
customer1ID
match
case
CustomerID
(
name
)
=>
println
(
name
)
// prints Sukyoung
case
_
=>
println
(
"Could not extract a CustomerID"
)
The
apply
method creates a
CustomerID
string from a
name
. The
unapply
does the inverse to get the
name
back. When we call
CustomerID("Sukyoung")
, this is shorthand syntax for calling
CustomerID.apply("Sukyoung")
. When we call
case CustomerID(name) => println(name)
, we’re calling the unapply method with
CustomerID.unapply(customer1ID)
.
Since a value definition can use a pattern to introduce a new variable, an extractor can be used to initialize the variable, where the unapply method supplies the value.
If it is just a test, return a
Boolean
. For instance
case even()
.
If it returns a single sub-value of type T, return an
Option[T]
.
If you want to return several sub-values
T1,...,Tn
, group them in an optional tuple
Option[(T1,...,Tn)]
.
Sometimes, the number of values to extract isn’t fixed and we would like to return an arbitrary number of values, depending on the input. For this use case, you can define extractors with an
unapplySeq
method which returns an
Option[Seq[T]]
. Common examples of these patterns include deconstructing a
List
using
case List(x, y, z) =>
and decomposing a
String
using a regular expression
Regex
, such as
case r(name, remainingFields @ _*) =>
.
