There are two types of sequence and stream operations:
- Intermediate operations, which return another sequence or stream
- Terminal operations, which return something else
Intermediate operations are lazy, meaning they aren’t actually executed until and unless a terminal sequence operation is performed on their
results. Consequently, if the result of an intermediate sequence operation is not fed to a terminal operation, it serves no purpose, which is almost
certainly an error.
Noncompliant code example
val sequence = sequenceOf("Hello", " ", "World")
var totalLength = 0
sequence.map { totalLength += it.length } // Noncompliant, do nothing, "map" is not a terminal operation
// here totalLength == 0
val stream = listOf("Hello", " ", "World").stream()
stream.filter { it.isNotBlank() } // Noncompliant, do nothing, "filter" is not a terminal operation
// the following "forEach" throws an exception, only one method can operate on a stream instance
// here "filter" has already operated on it.
stream.forEach { println(it) }
Compliant solution
val sequence = listOf("Hello", " ", "World").asSequence()
var totalLength = sequence.map { it.length }.sum() // Compliant, the sequence ends with the terminal operation "sum"
// here totalLength == 11
listOf("Hello", " ", "World").stream()
.filter { it.isNotBlank() }
.forEach { println(it) } // Compliant, the stream ends with the terminal operation "forEach"
