An Extractor<S, T> may be seen as a partial function from S to T: it can only “extract” a value of type T from a value of type S if such a value (or the material from which one can be created) is present. In Octarine, an Extractor<T, V> is a cross between a Function<T, Optional<V>> and a Predicate<T> . It has three methods:
- V extract(T source) – extracts a value of type V directly from the source (or fails with an exception).
- Optional<V> apply(T source) – returns either a value of type V extracted from the source and wrapped in an Optional, or Optional.empty if no such value is available.
- boolean test(T source) – returns true if the source contains the kind of value that we want, and false otherwise.
The obvious example is a Record , which might or might not contain a value for a given Key<T> . We have:
|
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Key<Integer>
age
=
Key.named("age"); Record recordWithAge
=
Record.of(age.of(23)); age.extract(recordWithAge);
// returns 23. age.apply(recordWithAge);
// returns Optional.of(23). age.test(recordWithAge);
// returns true. Record emptyRecord
=
Record.empty(); age.extract(emptyRecord);
// throws an exception age.apply(emptyRecord);
// returns Optional.empty(). age.test(emptyRecord);
// return false. |
We can enhance extractors with predicates to look for values matching additional criteria besides existence – for example:
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Key<Integer>
age
=
Key.named("age"); Extractor<Record,
Integer>
ageOverForty
=
age.is(i
->
i
>=
40); Extractor<Record,
Integer>
ageUnderForty
=
age.is(i
->
i
<
40); Record record
=
Record.of(age.of(23)); ageOverForty.extract(record);
// throws an exception ageUnderForty.extract(record);
// returns 23 ageOverForty.apply(record);
// returns Optional.empty() ageUnderForty.apply(record);
// returns Optional.of(23) ageOverForty.test(record);
// returns false. ageUnderForty.test(record);
// returns true. |
This makes them useful when composing tests on a record:
|
1 |
boolean
isOldArthur
=
name.is("Arthur").and(age.is(i
->
i
>=
40)).test(record)); |
Which in turn makes them useful when filtering a collection of records:
|
1 |
Stream<Record>
oldArthurs
=
records.stream().filter(name.is("Arthur").and(age.is(i
->
i
>=
40)));
|
Any OptionalLens<T, V> in Octarine is also an Extractor<T, V> , and any plain old Lens<T, V> can be turned into an Extractor<T, V> by calling Lens::asOptional on it.