Java Stream Reduce of array of objects

With JDK-12, you can use

Object[] array = list.stream()
    .collect(Collectors.teeing(
        Collectors.reducing(1, a -> (Integer)a[0], (a,b) -> a * b),
        Collectors.mapping(a -> (String)a[1], Collectors.joining()),
        (i,s) -> new Object[] { i, s}
    ));

but you really should rethink your data structures.

This answer shows a version of the teeing collector which works under Java 8.

You can achieve this with reduce() :

public void testStacko() {
    List<Object[]> list = new ArrayList<>();
    list.add(new Object[] {1, "foo"});
    list.add(new Object[] {6, "|bar"});
    list.add(new Object[] {15, "|baz"});
    Object[] array = list.stream()
                         .reduce(
                                  (obj1, obj2) -> 
                                   new Object[] {(int) obj1[0] * (int) obj2[0], 
                                                 (String) obj1[1] + (String) obj2[1]
                                                }
                                )
                         .get();
    System.out.println(array[0]); // 90
    System.out.println(array[1]); // foo|bar|baz
}