Could we add headList and tailList methods to j.u.List?

[Stuart Marks]

On 11/16/25 10:51 AM, David Alayachew wrote:
> Could we add headList(int) and tailList(int) to j.u.List? I searched JBS and found 
> nothing.
> 
> It's commonly what people want when doing subList(int, int), so this should be 
> pretty well received.

Maybe. :-)

There's a nexus of diffuse concerns here.

One concern is subranges of things in general, such as Strings, CharSequences, 
Lists, and arrays. Subranges are all expressed as methods with (start, end) 
parameters. These are powerful enough to do anything you need to do, but they're 
sometimes inconvenient.

One reason these can be inconvenient is related to the second concern, which is that 
sometimes the APIs require the creation of a local variable, which in turn forces an 
expression to turn into a statement. For example, consider a task of getting a 
String from somewhere, taking the first three characters, and passing that elsewhere 
to perform further work. One can write that as

     furtherWork(getString().substring(0, 3));

But if you want to take the *last* three characters and pass them along, you have to 
do this:

     var tmp = getString();
     furtherWork(tmp.substring(tmp.length() - 3));

This isn't terrible, but sometimes it disrupts an expression to declare a local 
variable.

At least there is a one-arg substring method that takes characters to the end of the 
string. If this method weren't there, you'd have to call length() twice (or store 
the length in another local variable). Again, not terrible, but yet another thing 
that adds friction.

A third concern is that people come along and ask whether we can have something in 
Java that's rather like Python's slices. For a variety of reasons I don't think we 
should accept negative indexes or step values other than 1, but there are some 
things that probably occur frequently. For example, to get the last three characters 
of a string, one can write s[-3:]. That's pretty nice.

You had asked about List, not String, but I think similar issues apply. I'd guess 
that String and substring operations are a lot more frequent than subList 
operations. But maybe you have some subList use cases in mind. Could you give more 
details about what you're thinking of?

> In that case, would be nice if we could add an entry to JBS with a Won't Fix, to 
> make it easier for those looking to see why not. Maybe even link this thread for 
> further reading.

Heh, yeah I've done that a couple times -- filed a bug just to close it out with an 
explanation why. I was wondering whether anybody would find that useful. I think 
this topic is worth some discussion, though, so I won't do that quite yet. :-)

s'marks