Draft JLS spec for JEP 305: Pattern matching for instanceof

Gavin Bierman gavin.bierman at oracle.com
Thu Sep 19 09:28:42 UTC 2019

A draft language spec for JEP 305 (Pattern Matching for instanceof) is available at:

http://cr.openjdk.java.net/~gbierman/jep305/jep305-20190918/specs/patterns-instanceof-jls.html <http://cr.openjdk.java.net/~gbierman/jep305/jep305-20190918/specs/patterns-instanceof-jls.html>

Comments are welcomed on all aspects, but I draw your attention to a couple of things that we’d like your feedback on:

1. The instanceof operator restricts the type to be a reifiable reference type. The spec currently keeps that restriction for type test patterns too. But should we go further, i.e. will people expect to be able to say the following (given that this *declares* a pattern variable l)?

if (o instanceof List<Integer> l) {

2. We’d like to keep the possibility open for merging of multiple pattern declarations, where it makes sense. For example:

if (a instanceof Foo f || b instanceof Foo f) {
… // Like to be able to use f here

The current spec explicitly calls out cases like these as compile-time errors, to allow for forwards compatibility if we add this feature. But what do you think of this feature? (We have textually multiple declarations of a pattern variable, but they are “merged”, so they are really the same thing…)

3. [Only for spec nerds] I am proposing to add a new Chapter 16 to discuss patterns (at the moment it’s short, but we’re planning for it to grow). The existing Chapters 16-19 will be renumbered to 17-20. Will this renumbering cause problems for anyone?


More information about the amber-spec-observers mailing list