Java 8: Find index of minimum value from a List

Question

Say I have a list with elements (34, 11, 98, 56, 43).

Using Java 8 streams, how do I find the index of the minimum element of the list (e.g. 1 in this case)?

I know this can be done easily in Java using list.indexOf(Collections.min(list)). However, I am looking at a Scala like solution where we can simply say List(34, 11, 98, 56, 43).zipWithIndex.min._2 to get the index of minimum value.

Is there anything that can be done using streams or lambda expressions (say Java 8 specific features) to achieve the same result.

Note: This is just for learning purpose. I don't have any problem in using Collections utility methods.

Answer 1

import static java.util.Comparator.comparingInt;

int minIndex = IntStream.range(0,list.size()).boxed()
            .min(comparingInt(list::get))
            .get();  // or throw if empty list

As @TagirValeev mentions in his answer, you can avoid boxing by using IntStream#reduce instead of Stream#min, but at the cost of obscuring the intent:

int minIdx = IntStream.range(0,list.size())
            .reduce((i,j) -> list.get(i) > list.get(j) ? j : i)
            .getAsInt();  // or throw

Answer 2

You could do it like this:

int indexMin = IntStream.range(0, list.size())
                .mapToObj(i -> new SimpleEntry<>(i, list.get(i)))
                .min(comparingInt(SimpleEntry::getValue))
                .map(SimpleEntry::getKey)
                .orElse(-1);

If the list is a random access list, get is a constant time operation. The API lacks of a standard tuple class, so I used the SimpleEntry from the AbstractMap class as a substitute.

So IntStream.range generates a stream of indexes from the list from which you map each index to its corresponding value. Then you get the minimum element by providing a comparator on the values (the ones in the list). From there you map the Optional<SimpleEntry<Integer, Integer>> to an Optional<Integer> from which you get the index (or -1 if the optional is empty).

As an aside, I would probably use a simple for-loop to get the index of the minimum value, as your combination of min / indexOf does 2 passes over the list.

You might also be interested to check Zipping streams using JDK8 with lambda (java.util.stream.Streams.zip)