これでうまくいきますか？これは、圧縮するストリームに対して遅延評価する短い関数なので、無限ストリームを提供できます（圧縮するストリームのサイズをとる必要はありません）。
ストリームが有限の場合、ストリームの1つが要素を使い果たすとすぐに停止します。
import java.util.Objects;
import java.util.function.BiFunction;
import java.util.stream.Stream;
class StreamUtils {
static <ARG1, ARG2, RESULT> Stream<RESULT> zip(
Stream<ARG1> s1,
Stream<ARG2> s2,
BiFunction<ARG1, ARG2, RESULT> combiner) {
final var i2 = s2.iterator();
return s1.map(x1 -> i2.hasNext() ? combiner.apply(x1, i2.next()) : null)
.takeWhile(Objects::nonNull);
}
}
ここにいくつかの単体テストコードがあります（コード自体よりもはるかに長いです！）
import org.junit.jupiter.api.Test;
import org.junit.jupiter.params.ParameterizedTest;
import org.junit.jupiter.params.provider.Arguments;
import org.junit.jupiter.params.provider.MethodSource;
import java.util.List;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.function.BiFunction;
import java.util.stream.Collectors;
import java.util.stream.Stream;
import static org.junit.jupiter.api.Assertions.assertEquals;
class StreamUtilsTest {
@ParameterizedTest
@MethodSource("shouldZipTestCases")
<ARG1, ARG2, RESULT>
void shouldZip(
String testName,
Stream<ARG1> s1,
Stream<ARG2> s2,
BiFunction<ARG1, ARG2, RESULT> combiner,
Stream<RESULT> expected) {
var actual = StreamUtils.zip(s1, s2, combiner);
assertEquals(
expected.collect(Collectors.toList()),
actual.collect(Collectors.toList()),
testName);
}
private static Stream<Arguments> shouldZipTestCases() {
return Stream.of(
Arguments.of(
"Two empty streams",
Stream.empty(),
Stream.empty(),
(BiFunction<Object, Object, Object>) StreamUtilsTest::combine,
Stream.empty()),
Arguments.of(
"One singleton and one empty stream",
Stream.of(1),
Stream.empty(),
(BiFunction<Object, Object, Object>) StreamUtilsTest::combine,
Stream.empty()),
Arguments.of(
"One empty and one singleton stream",
Stream.empty(),
Stream.of(1),
(BiFunction<Object, Object, Object>) StreamUtilsTest::combine,
Stream.empty()),
Arguments.of(
"Two singleton streams",
Stream.of("blah"),
Stream.of(1),
(BiFunction<Object, Object, Object>) StreamUtilsTest::combine,
Stream.of(pair("blah", 1))),
Arguments.of(
"One singleton, one multiple stream",
Stream.of("blob"),
Stream.of(2, 3),
(BiFunction<Object, Object, Object>) StreamUtilsTest::combine,
Stream.of(pair("blob", 2))),
Arguments.of(
"One multiple, one singleton stream",
Stream.of("foo", "bar"),
Stream.of(4),
(BiFunction<Object, Object, Object>) StreamUtilsTest::combine,
Stream.of(pair("foo", 4))),
Arguments.of(
"Two multiple streams",
Stream.of("nine", "eleven"),
Stream.of(10, 12),
(BiFunction<Object, Object, Object>) StreamUtilsTest::combine,
Stream.of(pair("nine", 10), pair("eleven", 12)))
);
}
private static List<Object> pair(Object o1, Object o2) {
return List.of(o1, o2);
}
static private <T1, T2> List<Object> combine(T1 o1, T2 o2) {
return List.of(o1, o2);
}
@Test
void shouldLazilyEvaluateInZip() {
final var a = new AtomicInteger();
final var b = new AtomicInteger();
final var zipped = StreamUtils.zip(
Stream.generate(a::incrementAndGet),
Stream.generate(b::decrementAndGet),
(xa, xb) -> xb + 3 * xa);
assertEquals(0, a.get(), "Should not have evaluated a at start");
assertEquals(0, b.get(), "Should not have evaluated b at start");
final var takeTwo = zipped.limit(2);
assertEquals(0, a.get(), "Should not have evaluated a at take");
assertEquals(0, b.get(), "Should not have evaluated b at take");
final var list = takeTwo.collect(Collectors.toList());
assertEquals(2, a.get(), "Should have evaluated a after collect");
assertEquals(-2, b.get(), "Should have evaluated b after collect");
assertEquals(List.of(2, 4), list);
}
}