Stream Way to get index of first element matching boolean

Question

I have a List<Users>. I want to get the index of the (first) user in the stream with a particular username. I don't want to actually require the User to be .equals() to some described User, just to have the same username.

I can think of ugly ways to do this (iterate and count), but it feels like there should be a nice way to do this, probably by using Streams. So far the best I have is:

int index = users.stream()     .map(user -> user.getName())     .collect(Collectors.toList())     .indexOf(username); 

Which isn't the worst code I've ever written, but it's not great. It's also not that flexible, as it relies on there being a mapping function to a type with a .equals() function that describes the property you're looking for; I'd much rather have something that could work for arbitrary Function<T, Boolean>

Anyone know how?

Answer 1

Occasionally there is no pythonic zipWithIndex in java. So I came across something like that:

OptionalInt indexOpt = IntStream.range(0, users.size())      .filter(i -> searchName.equals(users.get(i)))      .findFirst(); 

Alternatively you can use zipWithIndex from protonpack library

Note

That solution may be time-consuming if users.get is not constant time operation.

Answer 2

Try This:

IntStream.range(0, users.size())     .filter(userInd-> users.get(userInd).getName().equals(username))     .findFirst()     .getAsInt();