Avoid `Boolean.box`

Question

I am calling a java method from io.netty.bootstrap.BootStrap that has the following signature:

public <T> B option(ChannelOption<T> option, T value)

I am using the following code to call this method:

b.option(ChannelOption.SO_KEEPALIVE, true);

And this fails to compile with the following error:

Error:(57, 30) type mismatch;
 found   : io.netty.channel.ChannelOption[Boolean]
 required: io.netty.channel.ChannelOption[Any]
Note: Boolean <: Any, but Java-defined class ChannelOption is invariant in type T.
You may wish to investigate a wildcard type such as `_ <: Any`. (SLS 3.2.10)
      b.option(ChannelOption.SO_KEEPALIVE, true); // (4)
                         ^

I don't fully understand what this says, but I understand it's complaining about getting a boolean since it was parametrized with Any instead of Boolean. So I tried the following code and it works:

b.option(ChannelOption.SO_KEEPALIVE, Boolean.box(true));

This compiles and works. Is there a way to make this prettier without the box call?

Can anyone translate that compiler error?

Thank you.

For the record, when you post to the ML you should mention that you cross-posted to SO to avoid duplicated effort. — som-snytt, Nov 11 '14 at 20:15

score 8 · Accepted Answer · answered Nov 11 '14 at 16:56

8

Java generics are invariant in scala, so you cannot pass a scala Boolean and have the type inferred as you would expect. Explicitly annotating the type should fix it:

b.option[java.lang.Boolean](ChannelOption.SO_KEEPALIVE, true)

answered Nov 11 '14 at 16:56

Gabriele Petronella

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Could you explain a bit more what "java generics are invariant"? Or point me some link where I can read about this. Thank you – simao Nov 11 '14 at 17:06
Here's a nice writeup: http://stackoverflow.com/questions/8481301/covariance-invariance-and-contravariance-explained-in-plain-english – Gabriele Petronella Nov 11 '14 at 17:10
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Or annotate the true. https://groups.google.com/d/msg/scala-user/T93aNh6PYTA/_7wSBGT3phUJ – som-snytt Nov 11 '14 at 20:18
I don't understand how variance matters to type inference here. A Scala Boolean is not a java.lang.Boolean, right? `implicitly[Boolean <:< java.lang.Boolean]` If you need a java boolean you can get it by virtue of autoboxing, but you have to induce the implicit conversion. – som-snytt Nov 11 '14 at 20:37
btw, could I define some implicit conversion here instead? – simao Nov 11 '14 at 20:39

Avoid `Boolean.box`

1 Answers1