Summary

Based on experiences with the previous preview of the record patterns feature, we propose a few improvements and simplifications.

Problem

Three issues related to record patterns have been identified:

• Named record patterns (for example R(String s) r) introduce an ambiguity in the grammar, and their value appears to be low.

• Allowing a record pattern in an enhanced for statement would make it easy to loop over a collection of record values and swiftly extract the components of each record. 

• Record patterns for generic record classes currently require type arguments to be given explicitly. But in most circumstances reasonable type arguments could be inferred automatically.

Solution

Named Record Patterns

We propose to drop the named record patterns feature.

Record Patterns in Enhanced For

We propose to permit record patterns to appear in the header of an enhanced for statement. For example:

record Complex(double real, double img) {}

List<Complex> list = ...;

for (Complex(var real, var img) : list) {
    // can use "real" and "img" directly
}

Currently, the grammar for the enhanced for statement requires the header to contain a LocalVariableDeclaration. We propose to extend this and also support a record pattern.

Any pattern variables introduced by the record pattern are in scope in the body of the enhanced for statement.

It is required that a record pattern appearing in the header is exhaustive for the type of the elements being iterating over.

At runtime, if an iteration element is null, and a record pattern appears in the header, then the enhanced for statement raises a MatchException that wraps NullPointerException.

Record Pattern Type Inference

Previously, record patterns for generic record classes required type arguments to be given explicitly, and the use of a raw type in this case was forbidden. It is proposed in this preview that using a raw type will be allowed and will now entail that inference of the type arguments is performed using the type information from the expression being pattern matched.

record Pair<S,T>(S x, T y) { };

Pair<Integer, String> p = ...;

if (p instanceof Pair(var x, var y)) {  
                   // inferred to be Pair<Integer, String>(var x, var y)
    ...
}

If a type can not be inferred then a compile-time error occurs.

Specification

The specification draft is available for convenience here: http://cr.openjdk.java.net/~gbierman/jep432%2b433/jep432%2b433-20221028/specs/patterns-switch-record-patterns-jls.html and is attached as jls-jep432+433-20221028.zip. Please note that, for technical reasons, the specification draft also includes changes for JEP-433 (https://openjdk.org/jeps/433), which are reviewed under CSR JDK-8294946.

The specdiff for API changes is available for convenience here: http://cr.openjdk.java.net/~jlahoda/8294945/specdiff.00/overview-summary.html, and is attached as specdiff.00.zip.