Zipping Collections in Java

1. Introduction

In this tutorial, we’ll illustrate how to zip two collections into one logical collection.

The “zip” operation is slightly different from the standard “concat” or “merge”. While the “concat” or “merge” operations will simply add the new collection at the end of the existing collection, “zip” operation will take an element from each collection and combine them.

The core library does not support “zip” implicitly, but there are certainly third-party libraries which do feature this useful operation.

Consider two lists, one having names of people, other contains their ages.

List<String> names = new ArrayList<>(Arrays.asList("John", "Jane", "Jack", "Dennis"));

List<Integer> ages = new ArrayList<>(Arrays.asList(24, 25, 27));

After zipping, we end up with name-age pairs constructed from corresponding elements from those two collections.

2. Using Java 8 IntStream

Using core Java, we could generate indexes using IntStream and then use them to extract corresponding elements from two collections:

IntStream
  .range(0, Math.min(names.size(), ages.size()))
  .mapToObj(i -> names.get(i) + ":" + ages.get(i))
  // ...

3. Using Guava Streams

Starting version 21, Google Guava provides a zip helper method in the Streams class. This removes all the fuss of creating and mapping indexes and reduces the syntax to inputs and operations:

Streams
  .zip(names.stream(), ages.stream(), (name, age) -> name + ":" + age)
  // ...

4. Using jOOλ (jOOL)

jOOL also provides some of the fascinating additions over Java 8 Lambda, and with the support of Tuple1 to Tuple16, the zip operation becomes much more interesting:

Seq
  .of("John","Jane", "Dennis")
  .zip(Seq.of(24,25,27));

This will produce a result of a Seq containing Tuples of zipped elements:

(tuple(1, "a"), tuple(2, "b"), tuple(3, "c"))

jOOL’s zip method gives the flexibility to provide custom transformation function:

Seq
  .of(1, 2, 3)
  .zip(Seq.of("a", "b", "c"), (x, y) -> x + ":" + y);

or if one wishes to zip with index only, he can go with the zipWithIndex method provided by jOOL:

Seq.of("a", "b", "c").zipWithIndex();

5. Conclusion

In this quick tutorial, we had a look at how to perform the zip operation.

As always, the code examples in the article can be found over on GitHub.