redhat-developer / redhat-developer/vscode-java

Java type inference fails with this program

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Description

Environment
  • Operating System: macOS Sonoma
  • JDK version: 21
  • Visual Studio Code version: 1.90.2
  • Java extension version: 1.31.0
Steps To Reproduce

Create a file Main.java, paste the following content into it and then open it with VS Code and the Java extension:

import java.util.*;
import java.util.stream.Collectors;

public class Main {

    record Foo(String id, List<Bar> bars) {}

    record Bar(String id) {}

    public static void main(String[] args) {
        var barsByFooIdAndBarId = Collections.<Foo>emptyList()
            .stream()
            .collect(
                Collectors.toMap(
                    Foo::id,
                    foo ->
                        Collections.<String, Bar>emptyMap()
                            .entrySet()
                            .stream()
                            .collect(
                                Collectors.toMap(
                                    Map.Entry::getKey,
                                    entry ->
                                        foo
                                            .bars()
                                            .get(0)
                                )
                            )
                )
            );

        Bar bar = barsByFooIdAndBarId
            .computeIfAbsent("fooId", id -> Collections.emptyMap())
            .get("barId");

        System.out.println(bar);
    }
}
Current Result

The program doesn't compile. barsByFooIdAndBarId's type is inferred as Map<String, Map<String, Object>> instead of Map<String, Map<String, Bar>>. As a result, the assignment to Bar bar is invalid because Object is not assignable to Bar. However, if you replace var with Map<String, Map<String, Bar>> directly, then the program will be just fine. You can even run it and it will print null.

I know that this doesn't happen with javac nor IntelliJ because I noticed this at work and my colleagues using IntelliJ don't have the issue and manual compilation with Maven is also fine.

I suspect that the issue comes from the Eclipse Java language server which the Java extension is based on. But I cannot confirm this for now as I'm still trying to build and run the language server locally. And this is harder than I thought.

Expected Result

The program compiles and runs with VS Code and the Java extension. I understand that this may not be easy to solve. I would already appreciate some insights on the issue. If you think that I should open an issue on Eclipse's side, please let me know!

Additional Informations

Last but not least, thank you for your amazing work! 💪 I'm so happy to be able to write Java in VS Code thanks to the extension. I was able to move away from IntelliJ which I genuinely dislike. So thank you so much for the dedication in making this possible! ❤️

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Reproduce the inference failure with the provided Main.java program in VS Code 1.90.2 using the Java extension 1.31.0 and JDK 21. Start by investigating the Eclipse Java language server, then verify that barsByFooIdAndBarId is inferred as Map<String, Map<String, Bar>> and the program compiles and runs as expected.

Written by the indexing model from the issue text.

Assessment

Tech stack
java
Domain
devtools
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
32/100

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