[Java] Exception on BytBuffer position()

[krojew]

After updating to Java 2.0.0, I've encountered https://stackoverflow.com/questions/58120357/jeromq-on-android-no-virtual-method-clearljava-nio-bytebuffer on Android. The fix is simple: replace all bb.position() calls with ((Buffer) bb).position(). Unfortunately, I don't see a client-side workaround.

[aardappel]

Your link seems to suggest it is about the return value of position, i.e. the version of position that takes an argument. The solution would then be to simply not use that return value, but I see no use of that in the code.

Can you point to a use of position that causes this exception?

[krojew]

I actually observed the behavior on the overload setting the position in createString() in FlatBufferBuilder.

[aardappel]

That does bb.position(space -= length);, and the return value the link talks about is unused.

[krojew]

The actual return value is not relevant - the issue is with the VM not finding the right method to call. It tries to look for a one returning a Buffer, but only finds one returning a ByteBuffer, hence the exception. It simply doesn't handle covariant return types. Upcasting forces it to use the one returning Buffer.

[aardappel]

Ok, thanks for the explanation. That is pretty bad bad breakage of the Java APIs, surprised they thought that was a good idea.

This should affect the setter version of position, not the getter, and you are also changing the latter?

Remains the issue of what the costs is of all these extra casts. If its only the setter it may be ok?

Yes, and don't understand why this is only now an issue..

@paulovap

[krojew]

I may have gone a bit overboard with replacing calls. I'll revert changes to the getters in ~9h.

[krojew]

Just pushed an update. Seems to work.