Find all the indexes of an item within a list using stream API

Question

I am trying sequential search using Java 8 streams and lambda expressions. Here is my code

List<Integer> list = Arrays.asList(10, 6, 16, 46, 5, 16, 7);
int search = 16;
list.stream().filter(p -> p == search).forEachOrdered(e -> System.out.println(list.indexOf(e)));

Output: 2
        2

I know list.indexOf(e) always prints the index of the first occurrence. How do I print all the indexes?

Answer 1

For a start, using Lambdas is not the solution to all problems... but, even then, as a for loop, you would write it:

List<Integer> results = new ArrayList<>();
for (int i = 0; i < list.size(); i++) {
    if (search == list.get(i).intValue()) {
        // found value at index i
        results.add(i);
    }
}

Now, there's nothing particularly wrong with that, but note that the critical aspect here is the index, not the value. The index is the input, and the output of the 'loop'.

As a stream::

List<Integer> list = Arrays.asList(10, 6, 16, 46, 5, 16, 7);
int search = 16;
int[] indices = IntStream.range(0, list.size())
                .filter(i -> list.get(i) == search)
                .toArray();
System.out.printf("Found %d at indices %s%n", search, Arrays.toString(indices));

Produces output:

Found 16 at indices [2, 5]

Answer 2

To find the indexes of every value in a List as a Map, we can use an IntStream of indexes with Collectors.groupingBy.

import java.util.stream.Collectors;
import java.util.stream.IntStream;
//...
final Map<Integer, List<Integer>> indexMap = IntStream.range(0, list.size()).boxed()
        .collect(Collectors.groupingBy(list::get));
//{16=[2, 5], 5=[4], 6=[1], 7=[6], 10=[0], 46=[3]}
//Map of item value to List of indices at which it occurs in the original List

Demo

Now, if you want to get the list of indices for search, you can do so as follows:

System.out.println(indexMap.get(search));