This post looks at the Java 9 Streams API using JShell. The Streams API changes build on the success of Streams in Java 8, and introduce a number of utility methods – takeWhile, dropWhile and iterate. This post continues My Top Java 9 Features, and explores these methods using Jshell.
The Streams API and Lambda’s were the most successful features of Java 8, and the changes in Java 9 build on this with some new utility methods
Lets now return all values greater than 3, and we see the predicate instantly returns false and we get nothing returned
jshell> Stream.of(1,2,3,4,5).takeWhile(p -> p > 3).forEach(System.out::print); jshell>
We can see this below where the list only returns 2, even though the final element is 1, while the ordered list would have returned the 1 and the 2 -
jshell> Stream.of(2,3,6,5,1).takeWhile(p -> p < 3).forEach(System.out::print); 2
dropWhile provides the opposite behaviour of takeWhile, so records are dropped while a predicate is true. As before we have similar considerations for sorted and unsorted lists.
jshell> Stream.of(1,2,3,4,5).dropWhile(p -> p < 3).forEach(System.out::print); 345
jshell> Stream.of(2,3,6,5,1).dropWhile(p -> p < 3).forEach(System.out::print); 3651 jshell> Stream.of(1,2,3,5,6).dropWhile(p -> p < 3).forEach(System.out::print); 365
The conclusion is that you need to take care when working with unordered list, unless the side effects are acceptable in your code. Although I cant think of a use case where I could accept the random element of unordered lists, I am sure some exist.
This operates in a similar way to a for-loop. Taking a start value(T seed), exit condition(Predicate super T> hasNext) and whether we have a next value(Predicate super T> hasNext)
The iterate method has a exit condition attached -
jshell> Stream.iterate(1, i -> i < 6, i -> i + 1).forEach(System.out::println); 1 2 3 4 5
dropWhile and takeWhile present some useful utility methods for the Java Streams API. The major implication being whether your streams is ordered or unordered. The Stream.iterate method allows us to have for-loop functionality inside a Stream. I look forward to hearing peoples experiences with these new methods.