If you don't want to collect the elements
The title of the question says collect pairs from a stream, so I'd assume that you want to actually collect these, but you commented:
Your solution works, the problem is that it loads the data from file to PairList and then I may use stream from this collection to process pairs. I can't do it because the data might be too big to store in the memory.
so here's a way to do this without collecting the elements.
It's relatively straightforward to transform an Iterator<T> into an Iterator<List<T>>, and from that to transform a stream into a stream of pairs.
/**
* Returns an iterator over pairs of elements returned by the iterator.
*
* @param iterator the base iterator
* @return the paired iterator
*/
public static <T> Iterator<List<T>> paired(Iterator<T> iterator) {
return new Iterator<List<T>>() {
@Override
public boolean hasNext() {
return iterator.hasNext();
}
@Override
public List<T> next() {
T first = iterator.next();
if (iterator.hasNext()) {
return Arrays.asList(first, iterator.next());
} else {
return Arrays.asList(first);
}
}
};
}
/**
* Returns an stream of pairs of elements from a stream.
*
* @param stream the base stream
* @return the pair stream
*/
public static <T> Stream<List<T>> paired(Stream<T> stream) {
return StreamSupport.stream(Spliterators.spliteratorUnknownSize(paired(stream.iterator()), Spliterator.ORDERED),
false);
}
@Test
public void iteratorAndStreamsExample() {
List<String> strings = Arrays.asList("a", "b", "c", "d", "e", "f");
Iterator<List<String>> pairs = paired(strings.iterator());
while (pairs.hasNext()) {
System.out.println(pairs.next());
// [a, b]
// [c, d]
// [e, f]
}
paired(Stream.of(1, 2, 3, 4, 5, 6, 7, 8)).forEach(System.out::println);
// [1, 2]
// [3, 4]
// [5, 6]
// [7, 8]
}
If you want to collect the elements...
I'd do this by collecting into a list, and using an AbstractList to provide a view of the elements as pairs.
First, the PairList. This is a simple AbstractList wrapper around any list that has an even number of elements. (This could easily be adapted to handle odd length lists, once the desired behavior is specified.)
/**
* A view on a list of its elements as pairs.
*
* @param <T> the element type
*/
static class PairList<T> extends AbstractList<List<T>> {
private final List<T> elements;
/**
* Creates a new pair list.
*
* @param elements the elements
*
* @throws NullPointerException if elements is null
* @throws IllegalArgumentException if the length of elements is not even
*/
public PairList(List<T> elements) {
Objects.requireNonNull(elements, "elements must not be null");
this.elements = new ArrayList<>(elements);
if (this.elements.size() % 2 != 0) {
throw new IllegalArgumentException("number of elements must have even size");
}
}
@Override
public List<T> get(int index) {
return Arrays.asList(elements.get(index), elements.get(index + 1));
}
@Override
public int size() {
return elements.size() / 2;
}
}
Then we can define the collector that we need. This is essentially shorthand for collectingAndThen(toList(), PairList::new):
/**
* Returns a collector that collects to a pair list.
*
* @return the collector
*/
public static <E> Collector<E, ?, PairList<E>> toPairList() {
return Collectors.collectingAndThen(Collectors.toList(), PairList::new);
}
Note that it could be worthwhile defining a PairList constructor that doesn't defensively copy the list, for the use case that we know the backing list is freshly generated (as in this case). That's not really essential right now, though. But once we did that, this method would be collectingAndThen(toCollection(ArrayList::new), PairList::newNonDefensivelyCopiedPairList).
And now we can use it:
/**
* Creates a pair list with collectingAndThen, toList(), and PairList::new
*/
@Test
public void example() {
List<List<Integer>> intPairs = Stream.of(1, 2, 3, 4, 5, 6)
.collect(toPairList());
System.out.println(intPairs); // [[1, 2], [2, 3], [3, 4]]
List<List<String>> stringPairs = Stream.of("a", "b", "c", "d")
.collect(toPairList());
System.out.println(stringPairs); // [[a, b], [b, c]]
}
Here's a complete source file with a runnable example (as a JUnit test):
package ex;
import java.util.AbstractList;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.Objects;
import java.util.stream.Collector;
import java.util.stream.Collectors;
import java.util.stream.Stream;
import org.junit.Test;
public class PairCollectors {
/**
* A view on a list of its elements as pairs.
*
* @param <T> the element type
*/
static class PairList<T> extends AbstractList<List<T>> {
private final List<T> elements;
/**
* Creates a new pair list.
*
* @param elements the elements
*
* @throws NullPointerException if elements is null
* @throws IllegalArgumentException if the length of elements is not even
*/
public PairList(List<T> elements) {
Objects.requireNonNull(elements, "elements must not be null");
this.elements = new ArrayList<>(elements);
if (this.elements.size() % 2 != 0) {
throw new IllegalArgumentException("number of elements must have even size");
}
}
@Override
public List<T> get(int index) {
return Arrays.asList(elements.get(index), elements.get(index + 1));
}
@Override
public int size() {
return elements.size() / 2;
}
}
/**
* Returns a collector that collects to a pair list.
*
* @return the collector
*/
public static <E> Collector<E, ?, PairList<E>> toPairList() {
return Collectors.collectingAndThen(Collectors.toList(), PairList::new);
}
/**
* Creates a pair list with collectingAndThen, toList(), and PairList::new
*/
@Test
public void example() {
List<List<Integer>> intPairs = Stream.of(1, 2, 3, 4, 5, 6)
.collect(toPairList());
System.out.println(intPairs); // [[1, 2], [2, 3], [3, 4]]
List<List<String>> stringPairs = Stream.of("a", "b", "c", "d")
.collect(toPairList());
System.out.println(stringPairs); // [[a, b], [b, c]]
}
}
