Would a program using local type inference work on lower Java versions?

Question

Java 10 introduced local type inference, which is really cool:

var list = new ArrayList<String>(); // Compiles fine

However, it appears people are reluctant to update their Java versions the day (week, month, year) a new release is out.

This means that I need to wait till the EOL of Java 8 before I can even use List.of(1, 2, 3) if want to support the majority of users using old Java versions.

Local type inference on the other hand, would likely be inferred when it is compiled to bytecode, so presumably programs using it would likely run on older Java versions (9 or 8). On the other hand, Java seems to like doing optimizations at runtime, which could mean that type-inference requires JRE version 10.

Has anyone tried this out? Do programs using type inference run on older Java versions?

Your input is appreciated.

Check this https://stackoverflow.com/questions/15492948/javac-source-and-target-options/25273329 — Victor Gubin, Mar 26 '18 at 13:22
People will be using Java 8 long after it is EOL’d. We still regularly get questions on Stack Overflow from people using Java 6. — VGR, Mar 26 '18 at 13:22
@VGR Mostly out of necessity. We just upgraded from Java 6 at work. — Strikegently, Mar 26 '18 at 13:26
Ah, so essentially if I specify `-source 1.10 -target 1.8` to `javac` then my program would run on a Java 8 JVM? — Horsey, Mar 26 '18 at 13:34
@Horsey Do you mean you would compile for Java10 and execute on Java8? — Naman, Mar 26 '18 at 14:35
You can find a good explanation [here](https://stackoverflow.com/questions/49376007/can-java-10-compiled-classes-run-on-9#answer-49387212) — Oleksandr Pyrohov, Mar 26 '18 at 15:21
"Java seems to like doing optimizations at runtime" - type inference is not an optimisation, it is a "syntactic sugar". It is done at compile time. — davmac, Mar 26 '18 at 15:45

Holger · Accepted Answer · 2018-03-26T15:42:09.150

5

The type inference doesn’t change the bytecode, so in principle, compiled code using var would be compatible with older JVMs.
However, like always, javac doesn’t allow you to combine the -source option with a higher release version than the -target option.

So, the only way to use var in source code, to run with an older version, would be compiling it for Java 10, followed by downporting the compiled classes to an earlier version. The simplest approach would be just changing the class file version. I just verified that a simple test case using var worked on Java 9 after decrementing the class version number. But, of course, a more sophisticated downporting tool should verify that no other code features or API references of the newer platform features slipped through.

Perhaps, an alternative compiler will show up, allowing the flexibility to use a newer source version with an older target version. As said at the beginning, javac never did this.

edited Mar 26 '18 at 15:42

answered Mar 26 '18 at 15:28

Holger

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really? you can say `53` at the byte code level and it would just work? very interesting – Eugene Mar 26 '18 at 17:49
@Eugene Interesting but useless. Very few people are using Java 9. It is simpler just to upgrade to 10. – ZhekaKozlov Mar 27 '18 at 13:51
1

@Eugene when you don’t use modules, even `52` works without problems. There weren’t much changes on the bytecode level. Did you manage to download the diffs from [this answer](https://stackoverflow.com/a/49460187/2711488) by now? – Holger Mar 27 '18 at 16:40
@Holger no time for SO these days :( hammered at work, but will try later – Eugene Mar 27 '18 at 16:44

Would a program using local type inference work on lower Java versions?

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