Kotlin Java 8 Stream Equivalents

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Kotlin provides many extension methods on collections and iterables for applying functional-style operations. A dedicated Sequence type allows for lazy composition of several such operations.


About laziness

If you want to lazy process a chain, you can convert to a Sequence using asSequence() before the chain. At the end of the chain of functions, you usually end up with a Sequence as well. Then you can use toList(), toSet(), toMap() or some other function to materialize the Sequence at the end.

// switch to and from lazy
val someList = items.asSequence().filter { ... }.take(10).map { ... }.toList()

// switch to lazy, but sorted() brings us out again at the end
val someList = items.asSequence().filter { ... }.take(10).map { ... }.sorted()

Why are there no Types?!?

You will notice the Kotlin examples do not specify the types. This is because Kotlin has full type inference and is completely type safe at compile time. More so than Java because it also has nullable types and can help prevent the dreaded NPE. So this in Kotlin:

val someList = people.filter { it.age <= 30 }.map { it.name }

is the same as:

val someList: List<String> = people.filter { it.age <= 30 }.map { it.name }

Because Kotlin knows what people is, and that people.age is Int therefore the filter expression only allows comparison to an Int, and that people.name is a String therefore the map step produces a List<String> (readonly List of String).

Now, if people were possibly null, as-in a List<People>? then:

val someList = people?.filter { it.age <= 30 }?.map { it.name }

Returns a List<String>? that would need to be null checked (or use one of the other Kotlin operators for nullable values, see this Kotlin idiomatic way to deal with nullable values and also Idiomatic way of handling nullable or empty list in Kotlin)

Reusing Streams

In Kotlin, it depends on the type of collection whether it can be consumed more than once. A Sequence generates a new iterator every time, and unless it asserts "use only once" it can reset to the start each time it is acted upon. Therefore while the following fails in Java 8 stream, but works in Kotlin:

// Java:
Stream<String> stream =
Stream.of("d2", "a2", "b1", "b3", "c").filter(s -> s.startsWith("b"));

stream.anyMatch(s -> true);    // ok
stream.noneMatch(s -> true);   // exception
// Kotlin:  
val stream = listOf("d2", "a2", "b1", "b3", "c").asSequence().filter { it.startsWith('b' ) }

stream.forEach(::println) // b1, b2

println("Any B ${stream.any { it.startsWith('b') }}") // Any B true
println("Any C ${stream.any { it.startsWith('c') }}") // Any C false

stream.forEach(::println) // b1, b2

And in Java to get the same behavior:

// Java:
Supplier<Stream<String>> streamSupplier =
    () -> Stream.of("d2", "a2", "b1", "b3", "c")
          .filter(s -> s.startsWith("a"));

streamSupplier.get().anyMatch(s -> true);   // ok
streamSupplier.get().noneMatch(s -> true);  // ok

Therefore in Kotlin the provider of the data decides if it can reset back and provide a new iterator or not. But if you want to intentionally constrain a Sequence to one time iteration, you can use constrainOnce() function for Sequence as follows:

val stream = listOf("d2", "a2", "b1", "b3", "c").asSequence().filter { it.startsWith('b' ) }

stream.forEach(::println) // b1, b2
stream.forEach(::println) // Error:java.lang.IllegalStateException: This sequence can be consumed only once. 

See also:

Got any Kotlin Question?