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Porting Scala For-Comprehension usage examples in pure Java 8+ examples ( at least some interesting examples )
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import java.util.List; | |
import java.util.Optional; | |
import java.util.function.Function; | |
import java.util.stream.Stream; | |
import static java.util.stream.Collectors.toList; | |
/** | |
* | |
* @author Stefano fago | |
* | |
* Here are not used Records to be compatible with Java 8+ | |
* Using Records and .toList() with newer Java for smaller code! ;-) | |
* Inside every example the related Scala code. | |
* | |
*/ | |
public class ForComp_ScalaPorting { | |
public static void main(String[] args) { | |
for_comprehension_with_filtering(); | |
for_comprehension_inner_elements_with_filtering(); | |
for_comprehension_twoOptionalsSameType(); | |
for_comprehension_twoOptionalsSameType_with_filtering("a"); | |
for_comprehension_twoOptionalsMixed(); | |
for_comprehension_moreOptionalsMixed(); | |
for_comprehension_listOfOptionals(); | |
} | |
// ======================================================================== | |
// | |
// | |
// | |
// ======================================================================== | |
private static void for_comprehension_with_filtering() { | |
class Book{ | |
String title; | |
String author; | |
Book(String title, String author){} | |
} | |
var books = List.of(new Book("Title1","Auth1"),new Book("Title2","Auth2")); | |
/* | |
for { | |
book <- books | |
if book.author startsWith "bob" | |
} yield book.title | |
*/ | |
Stream<String> titlesFromBob = | |
books.stream() | |
.filter( | |
book-> book.author.startsWith("bob")) | |
.map( | |
book->book.title); | |
} | |
private static void for_comprehension_inner_elements_with_filtering() { | |
class Book{ | |
String title; | |
List<String> authors; | |
Book(String title, List<String> authors){} | |
} | |
var books = List.of(new Book("Title1",List.of("Auth1","Auth2")), | |
new Book("Title2",List.of("Auth3"))); | |
/* | |
for { | |
book <- books | |
author <- book.authors | |
if author startsWith "bob" | |
} yield book.title | |
*/ | |
Stream<String> titlesFromBookWithAlsoBob = | |
books.stream() | |
.flatMap( | |
book->book.authors.stream() | |
.filter( | |
author->author.startsWith("bob")) | |
.map( | |
author->book.title)); | |
} | |
private static void for_comprehension_twoOptionalsSameType() { | |
/* | |
for { | |
a <- optA | |
b <- optB | |
} yield (a,b) | |
*/ | |
class Result { | |
Result(String a, String b) { | |
} | |
} | |
var optA = Optional.ofNullable("VAL_1"); | |
var optB = Optional.ofNullable("VAL_2"); | |
Optional<Result> result = optA.flatMap(a -> optB.map(b -> new Result(a, b))); | |
} | |
private static void for_comprehension_twoOptionalsSameType_with_filtering(String prefix) { | |
/* | |
for { | |
a <- optA | |
if(a startsWith "a") | |
b <- optB | |
} yield (a,b) | |
*/ | |
class Result { | |
Result(String a, String b) { | |
} | |
} | |
var optA = Optional.ofNullable("VAL_1"); | |
var optB = Optional.ofNullable("VAL_2"); | |
Optional<Result> result = | |
optA.filter( | |
a->a.startsWith(prefix)) | |
.flatMap( | |
a->optB.map(b->new Result(a,b))); | |
} | |
private static void for_comprehension_twoOptionalsMixed() { | |
class Result { | |
Result(String a, int b) { | |
} | |
} | |
var optA = Optional.ofNullable("VAL_1"); | |
var optB = Optional.ofNullable(1000); | |
Optional<Result> result = optA.flatMap(a -> optB.map(b -> new Result(a, b))); | |
} | |
private static void for_comprehension_moreOptionalsMixed() { | |
class Result { | |
Result(String a, String b, int c) { | |
} | |
} | |
var optA = Optional.ofNullable("VAL_1"); | |
var optB = Optional.ofNullable("VAL_2"); | |
var optC = Optional.ofNullable(1000); | |
Optional<Result> result = | |
optA.flatMap( | |
a -> optB.flatMap( | |
b -> optC.map( | |
c -> new Result(a, b, c)))); | |
} | |
private static void for_comprehension_listOfOptionals() { | |
List<Optional<Integer>> listOfMaybeNumbers = | |
List.of( Optional.of(1), | |
Optional.empty(), | |
Optional.of(2), | |
Optional.of(3), | |
Optional.empty()); | |
/* | |
for { | |
optNumber <- optNumbers | |
value <- optNumber | |
} yield value +1 | |
*/ | |
List<Integer> filteredNumbers = | |
listOfMaybeNumbers.stream() | |
.flatMap( | |
maybeNumber->maybeNumber | |
.map( | |
value->value+1).stream()) | |
.collect(toList()); | |
} | |
}//END |
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