Stream Way to get index of first element matching boolean
JavaJava 8Java StreamJava Problem Overview
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?
Java Solutions
Solution 1 - Java
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.
Solution 2 - Java
Try This:
IntStream.range(0, users.size())
.filter(userInd-> users.get(userInd).getName().equals(username))
.findFirst()
.getAsInt();
Solution 3 - Java
Using Guava library: int index = Iterables.indexOf(users, u -> searchName.equals(u.getName()))
Solution 4 - Java
You can try StreamEx library made by Tagir Valeev. That library has a convenient #indexOf method.
This is a simple example:
List<User> users = asList(new User("Vas"), new User("Innokenty"), new User("WAT"));
long index = StreamEx.of(users)
.indexOf(user -> user.name.equals("Innokenty"))
.getAsLong();
System.out.println(index);
Solution 5 - Java
A solution without any external library
AtomicInteger i = new AtomicInteger(); // any mutable integer wrapper
int index = users.stream()
.peek(v -> i.incrementAndGet())
.anyMatch(user -> user.getName().equals(username)) ? // your predicate
i.get() - 1 : -1;
peek increment index i while predicate is false hence when predicate is true i is 1 more than matched predicate => i.get() -1
Solution 6 - Java
There is the detectIndex
method in the Eclipse Collections library which takes a Predicate
.
int index = ListIterate.detectIndex(users, user -> username.equals(user.getName()));
If you have a method on User
class which returns boolean
if username
matches you can use the following:
int index = ListIterate.detectIndexWith(users, User::named, username);
Note: I a committer for Eclipse Collections