jshell> Stream.of(1,2,3,4,5).takeWhile(p -> p < 3).forEach(System.out::print);
12
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>
- Unordered Lists - Longest list of values until predicate fails, although there may be values down Stream that satisfy the predicate and these wont 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 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.
- 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
dropWhile/takeWhile Conclusions
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
Conclusion
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.
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