How to get index of findFirst() in java 8?

Question

I have the following code:

ArrayList <String> entries = new ArrayList <String>();

entries.add("0");
entries.add("1");
entries.add("2");
entries.add("3");

String firstNotHiddenItem = entries.stream()
                    .filter(e -> e.equals("2"))
                    .findFirst()
                    .get();

I need to know what is the index of that first returned element, since I need to edit it inside of entries ArrayList. As far as I know get() returns the value of the element, not a reference. Should I just use

int indexOf(Object o)

instead?

Answer 1

You can't in a straightforward way - streams process elements without context of where they are in the stream.

However, if you're prepared to take the gloves off...

int[] position = {-1};

String firstNotHiddenItem = entries.stream()
        .peek(x -> position[0]++)  // increment every element encounter
        .filter("2"::equals)
        .findFirst()
        .get();

System.out.println(position[0]); // 2

The use of an int[], instead of a simple int, is to circumvent the "effectively final" requirement; the reference to the array is constant, only its contents change.

Note also the use of a method reference "2"::equals instead of a lambda e -> e.equals("2"), which not only avoids a possible NPE (if a stream element is null) and more importantly looks way cooler.

---

A more palatable (less hackalicious) version:

AtomicInteger position = new AtomicInteger(-1);

String firstNotHiddenItem = entries.stream()
        .peek(x -> position.incrementAndGet())  // increment every element encounter
        .filter("2"::equals)
        .findFirst()
        .get();

position.get(); // 2