Proposed API for JEP 259: Stack-Walking API

Timo Kinnunen timo.kinnunen at
Fri Nov 6 02:16:58 UTC 2015


I tested all 6 combinations of Functions that a simple sanity-check StackWalker can call:

Function<? super Stream<? extends  StackFrame>, [any]> 
Function<? super Stream<? super StackFrame>, [any]> 
Function<? super Stream<StackFrame>, [any]> 
Function<Stream<? extends StackFrame>, [any]> 
Function<Stream<? super StackFrame>, [any]> 
Function<Stream<StackFrame>, [any]> 

Unfortunately there doesn’t appear to be any candidate for the walk-method that doesn’t have at least one counterexample which is well-typed and doesn’t compile. Remi’s Function<Stream<Object>, Void> is one example which doesn’t compile against any of the six.

	static Function<? extends Stream<?>, Long> meekCounter() {
		return Stream::count;

That one is another such example, a bit surprisingly.

So this means you can’t support all clients at the same time.

Sent from Mail for Windows 10

From: Mandy Chung
Sent: Thursday, November 5, 2015 22:49
To: Remi Forax
Cc: core-libs-dev at
Subject: Re: Proposed API for JEP 259: Stack-Walking API

> On Nov 4, 2015, at 5:00 AM, Remi Forax <forax at> wrote:
>> Good point. Damn, i don’t like wildcards :-)
>> The following works fine:
>>  static <T> Function<Stream<T>, Long> counter() {
>>      return Stream::count;
>>  }
>> But there could also cases where one is stuck using a wildcard:
>>  Function<Stream<?>, Long> f = Stream::count;
> Wildcards are not that complex, but because they are use-side annotations, it's really easy to forget them, and IDEs do not help here :(
> I really hope that the JEP about declaration site variance [1] will be implemented at the beginning of jdk 10, it will remove the need of wildcards for functional interfaces.

Remi, Paul,

The current StackWalker::walk method:
    <T> T walk(Function<Stream<StackFrame>, T> function)

This would mean that the StackWalker API is intended to work with a function taking Stream<StackFrame> but not Stream<?>.

Changing to
    <T> T walk(Function<? super Stream<StackFrame>, ? extends T> function)

will allow use of some existing functions taking Stream<?>.  I don’t object changing it to use wildcard.

However, the StackWalker API is specific for reading StackFrame and I would expect it’s reasonable if it doesn’t work with a function taking Stream<?> as the useful functions such as Stream::count, Stream::collect(Collectors.toList(…)) can be used.

Just want to double confirm the advice which way to go.


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