How to convert the following scala to java code?
for {
x <- List(1, 2)
y <- List(3, 4)
} yield (x, y)
Is it possible? what is yield?
I guess it's possible to convert any scala code to java...
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3 Answers
@Tim already explained what is for..yield.
The Java code to do the same would be:
// Java doesn't have tuples, create your own type
record Pair(int x, int y) {}
List<Pair> result =
List.of(1, 2)
.stream()
.flatMap(x -> List.of(3, 4)
.stream()
.map(y -> new Pair(x, y))
)
.toList();
IMHO this is a great example of the conciceness and exprenessiveness of Scala, especially when working with collections.
8 Comments
Tim
Thank you! I didn't know the Java equivalent. I have also learned where rust gets its "functional" coding style from. And, yes, Scala is SO much more clear and expressive.
markspace
You could make this a bit more concise by using
Stream.of(1,2) instead of List.of(1,2).stream().markspace
Finally
.collect(Collectors.toList()); can be replaced by toList(); in newer versions of Java (v 21 here).
Gaël J
I assumed the inputs are two
Lists. Regarding using a list to represent a pair, it sounds quite unsafe to me, a record is a great fit as it can also be destructured in pattern matching in recent Java version I believe.
stefanobaghino
I just wanted to point out one important difference between the Scala Collections API and the Java Streams API: the latter is always lazy for intermediate operations (like map and flatMap), the former requires a little bit extra to make it so: docs.scala-lang.org/overviews/collections-2.13/views.html
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You need to read up on this, but for is just syntactic sugar that is converted to map, flatMap, filter, and foreach calls.
Your for statement
for {
x <- List(1, 2)
y <- List(3, 4)
} yield (x, y)
becomes
List(1,2).flatMap(x => List(3,4).map(y => (x, y)))
This is just function calls which can easily be converted to Java.
2 Comments
markspace
I don't about "easily." What going on with
map? Does this produce a map of the pairs (1,3), (2,4)?Gaël J
@markspace
map is for transforming each item of the input to something else. Pretty sure it exists also in Java on Streams. Here List(3, 4).map(..) returns List((x,3),(x,4)), so a list of pairs. Then the flatMap does that for each x and flatten the list of lists into a list. End result is the list of all pairs (1,3),(1,4),(2,3),(2,4).All the existing replies are spot on, but I wanted to give a slightly different perspective on how to translate that into imperative Java code:
record Pair(int x, int y) {}
var yielded = new ArrayList<Pair>();
for (var x: List.of(1, 2)) {
for (var y: List.of(3, 4)) {
yielded.add(new Pair(x, y));
}
}
I'm not necessarily suggesting to do any of this, but maybe it can help people with no prior exposure to functional programming match Scala for-comprehensions with their experience.
1 Comment
Gaël J
This is actually a great "mapping" of the
for..yield from Scala for someone that is used to imperative code. I had never really thought of it this way :)