Hi Peter, FWIW, Python has some very powerful but idiosyncratic "slicing" idioms. For > example (if I recall my python correctly), > > 0:-1 means from start to end (i.e. the whole array) > 1:-2 means from the second to second-last element > 0:-1:2 means every other element from start to end > -1:0:-1 means the whole array, but reversed!
In fact, -1 in Python refers to the last element, not the end of the list. So 0:-1 means all elements of the array except the last one, not the whole array. If you like reverse indices, you can check out the kala-common library I developed. In this library, I use bitwise negated indices to represent reverse indices. Here is the documentation: https://github.com/Glavo/kala-common/blob/main/docs/Tutorial.md#use-reverse-indexes-in-kala-common In kala-common, you can use the reverse indices like this: var seq = Seq.of(1, 2, 3); // Get the last elementvar _ = seq.get(~1); // ===> 3 // Get a slice of Seq except the first elementvar _ = seq.slice(1, ~0); // ===> [2, 3] // Get a slice of Seq except the last elementvar _ = seq.slice(0, ~1); // ===> [1, 2] I'm very proud of this design. Since reverse indexing uses bitwise negation instead of a minus sign, ~0 makes sense. I think this is better than Python's design. Glavo On Mon, Jul 28, 2025 at 6:45 AM Peter Burka <pe...@quux.net> wrote: > I agree that this feels way too pythonic to be in commons. > > FWIW, Python has some very powerful but idiosyncratic "slicing" idioms. For > example (if I recall my python correctly), > > 0:-1 means from start to end (i.e. the whole array) > 1:-2 means from the second to second-last element > 0:-1:2 means every other element from start to end > -1:0:-1 means the whole array, but reversed! > > The optional third term is the "step", which would more traditionally be > called "stride". > > For further fun, the slice syntax actually creates slice objects. You can > eschew the built-in slice type, and implement your own slice object using > completely arbitrary indices. Want a slice based on the Fibonacci sequence? > Not a problem! > > With a bit of clever stream chaining, these can all be in done in base > Java, albeit more verbosely. > > Peter > > On Sun, Jul 27, 2025 at 3:20 PM Gary Gregory <garydgreg...@gmail.com> > wrote: > > > On Sun, Jul 27, 2025 at 12:46 PM Yokesh Chowdary < > > bollineniyok...@gmail.com> > > wrote: > > > > > Hi Gary, > > > > > > Thanks for the quick reply! > > > > > > You're absolutely right, *ArrayUtils.subarray()* and > > > *StringUtils.substring() > > > *covers basic slicing. However, my proposal aims to enhance these > methods > > > by introducing: > > > > > > 1) Support for step > > > ArrayUtils.slice(new int[]{1, 2, 3, 4, 5, 6}, 0, -1, 2); // returns [1, > > 3, > > > 5] > > > > > > 2) Support for negative indices > > > StringUtils.slice("abcdef", -4, -1); // returns "cde" > > > > > > These enhancements bring Python-like expressiveness to Java, making > > slicing > > > more intuitive and concise for developers, especially when dealing with > > > dynamic ranges or reverse slicing. > > > Would love to hear thoughts on whether extending existing utils or > > > introducing a SliceUtils would be preferable. > > > > > > > Hello Yokesh, > > > > I think you're going to need to take a Java POV here, not Python, because > > this: > > > > StringUtils.slice("abcdef", -4, -1); // returns "cde" > > > > feels nonsensical for this Commons component. > > > > In StringUtils, CharSequenceUtils, and ArrayUtils, negative indices are > > usually (always?) errors. > > > > I don't think we should adopt the naming convention ("slice/SliceUtils") > of > > another language. Java and Commons Lang on top of it both have their own > > vocabulary of APIs. In this case, Java uses "substring", and we're > reusing > > that term in StringUtils. In ArrayUtils, we derived that term into > > "subarray". > > > > Adopting Python naming would set the expectation that this would maintain > > the Python-ness behavior over time, and encourage further Python creep, > > which is not what Commons components are about. They are about providing > > Java-centric APIs, not "it works like X in language Y, so let's implement > > that". > > > > There are odd bits in Lang to be sure, but that's mostly because the code > > base has been around for a long time. > > > > WRT: ArrayUtils.slice(new int[]{1, 2, 3, 4, 5, 6}, 0, -1, 2); // returns > > [1, 3, > > 5] > > > > I have no idea how -1 makes sense or what these parameters mean. You'll > > want to provide actual descriptions of each parameter so we know what > > you're talking about. Assuming "it works like Python" is not a good idea. > > Think about the whole class and how a new API fits in. Saying this lives > in > > a new class SliceUtils is not good IMO (see above on terminology). > > > > HTH, > > Gary > > > > > > > Regards, > > > Yokesh > > > > > > On Sun, Jul 27, 2025 at 10:02 PM Gary Gregory <garydgreg...@gmail.com> > > > wrote: > > > > > > > Hello Yokesh, > > > > > > > > This functionality already exists in ArrayUtils (subarray()) and > > > > StringUtils (substring()). > > > > > > > > Gary > > > > > > > > On Sun, Jul 27, 2025, 12:06 Yokesh Chowdary < > bollineniyok...@gmail.com > > > > > > > wrote: > > > > > > > > > Hi All, > > > > > > > > > > I'd like to propose adding a new utility method slice() to Apache > > > Commons > > > > > Lang that brings Python-style slicing capabilities to Java for > > > > Collections, > > > > > arrays (primitive and wrapper types), and String. > > > > > The proposed method would follow the form: *slice(input, start, > end, > > > > step)* > > > > > > > > > > This would be a highly reusable utility that simplifies what is > > > currently > > > > > verbose or error-prone logic in standard Java. It could live in a > new > > > > class > > > > > like *SliceUtils *or be integrated into existing utility classes > such > > > > > as *ArrayUtils > > > > > *and *StringUtils*. > > > > > > > > > > *Highlights:* > > > > > 1) Supports positive and negative indices for start,end and step > > > > > 2) Works on Collections, Arrays and Strings > > > > > > > > > > *Examples:* > > > > > List<String> names = Arrays.asList("Hello","Hi","Bye"); > > > > > SliceUtils.slice(names, 0,2); // returns ["Hello", "Hi"] > > > > > > > > > > String s = "abcdef"; > > > > > SliceUtils.slice(s, 1, 4); // returns "bcd" > > > > > > > > > > int[] numbers = {1, 2, 3, 4, 5, 6}; > > > > > SliceUtils.slice(numbers, 0, -1, 2); // returns [1, 3, 5] > > > > > > > > > > If the community agrees this is in scope for Commons Lang, I’d be > > happy > > > > to > > > > > implement the feature with full test coverage and documentation. > > > > > > > > > > Looking forward to your thoughts. > > > > > > > > > > Regards, > > > > > Yokesh > > > > > [bollineniyok...@gmail.com] > > > > > > > > > > > > > > >
