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 lambdas were the most successful features of Java 8, and the changes in Java 9 build on those foundations with some new utility methods:
jshell> Stream.of(1,2,3,4,5).takeWhile(p -> p < 3).forEach(System.out::print); 12
Let's now return all values greater than 3. 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>
- Note for Unordered Lists: Longest list of values until predicate fails, although there may be values downstream that satisfy the predicate. These won't be returned.
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 (Predicate predicate)
dropWhile provides the opposite behavior of takeWhile, so records are dropped while a predicate is true. As before, we have similar considerations for sorted and unsorted lists.
- Ordered Lists: Will return the longest list of records excluding those elements that satisfy the predicate:
jshell> Stream.of(1,2,3,4,5).dropWhile(p -> p < 3).forEach(System.out::print); 345
- Unordered Lists: Will drop the first records that satisfy the predicate:
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 lists unless the side effects are acceptable in your code. Although I can't think of a use case where I could accept the random element of unordered lists, I am sure some exist.
Iterate (T seed, Predicate hasNext, UnaryOperator next)
This operates in a similar way to a for-loop. Take a start value (T seed), exit condition (Predicate hasNext), and whether we have a next value (Predicate hasNext)
The iterate method has an 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 are ordered or unordered. The Stream.iterate method allows us to have for-loop functionality inside a Stream. I look forward to hearing people's experiences with these new methods.