Google Guava “zip” two lists

Question

Using Google Guava (Google Commons), is there a way to merge two equally sized lists into one list, with the new list containing composite objects of the two input lists?

Answer 1

You can refer to underscore-java library.

Underscore-java is a port of Underscore.js for Java, and the zip method can achieve the goal.

Following is a sample code & output :

$.zip(Arrays.asList("moe", "larry", "curly"), Arrays.asList("30", "40", "50"));

=> [[moe, 30], [larry, 40], [curly, 50]]

Answer 2

Looks like this is not currently in Guava, but is a desired feature. See this github issue, in particular Iterators.zip().

Answer 3

Here's a version with no explicit iteration, but it's getting pretty ugly.

List<Person> persons = ImmutableList.copyOf(Iterables.transform(
    ContiguousSet.create(Range.closedOpen(0, names.size()),
        DiscreteDomain.integers()),
    new Function<Integer, Person>() {
      @Override
      public Person(Integer index) {
        return new Person(names.get(index), ages.get(index));
      }
    }));

It's really not much better than having explicit iteration, and you probably want some level of bounds checking to ensure that the two inputs are indeed of the same size.

Answer 4

Just pretend this is a Guava method:

for (int i = 0; i < names.size(); i++) {
    persons.add(new Person(names.get(i), ages.get(i)));
}

Answer 5

Here's a generic way to zip lists with vanilla Java. Lacking tuples, I opted to use a list of map entries (If you don't like to use map entries, introduce an additional class ZipEntry or something).

public static <T1,T2> List<Map.Entry<T1,T2>> zip(List<T1> zipLeft, List<T2> zipRight) {
    List<Map.Entry<T1,T2>> zipped = new ArrayList<>();
    for (int i = 0; i < zipLeft.size(); i++) {
        zipped.add(new AbstractMap.SimpleEntry<>(zipLeft.get(i), zipRight.get(i)));
    }
    return zipped;
}

To support arrays as well:

@SuppressWarnings("unchecked")
public static <T1,T2> Map.Entry<T1,T2>[] zip(T1[] zipLeft, T2[] zipRight) {
    return zip(asList(zipLeft), asList(zipRight)).toArray(new Map.Entry[] {});
}

To make it more robust add precondition checks on list sizes etc, or introduce left join / right join semantics similar to SQL queries.

Answer 6

As of Guava 21, this is possible via Streams.zip():

List<Person> persons = Streams.zip(names.stream(), ages.stream(), Person::new)
                              .collect(Collectors.toList());