What is the Java 8 Stream API equivalent for LINQ Join?

Question

In C#/.Net it is possible to join IEnumerable sequences with the extension method Enumerable.Join in a SQL 'JOIN ... ON' way.

Is there something similar in Java 8 (Stream API)? Or what is the best way to simulate Enumerable.Join?

See: https://msdn.microsoft.com/en-us/library/bb534675%28v=vs.100%29.aspx

Answer 1

join is just syntactic sugar for Stream.flatMap() as explained in this article. Consider this example:

List<Integer> l1 = Arrays.asList(1, 2, 3, 4);
List<Integer> l2 = Arrays.asList(2, 2, 4, 7);

l1.stream()
  .flatMap(i1 -> l2.stream()
                   .filter(i2 -> i1.equals(i2)))
  .forEach(System.out::println);

The result is:

2
2
4

In the above example, flatMap() corresponds to (INNER) JOIN whereas the filter() operation of the nested stream corresponds to the ON clause.

jOOλ is a library that implements innerJoin() and other join types to abstract over this, e.g. also to buffer stream contents in case you want to join two Stream instances, instead of two Collection instances. With jOOλ, you would then write:

Seq<Integer> s1 = Seq.of(1, 2, 3, 4);
Seq<Integer> s2 = Seq.of(2, 2, 4, 7);

s1.innerJoin(s2, (i1, i2) -> i1.equals(i2))
  .forEach(System.out::println);

... which prints (the output are tuples, which is more like SQL's semantics semantics):

(2, 2)
(2, 2)
(4, 4)

(disclaimer, I work for the company behind jOOλ)