Again, this is not a bug. There are many, way too many places in standard library which are thread unsafe. Some of these are actual bugs, but not this one. If Java originally moved in Odersky's direction that won't be the case but now it's way too late to point fingers at these "quirks". In order to properly fix these issues the whole standard library has to be redesigned around immutability and, probably, immutable arrays must be supported at VM level. This is not going to happen.

But it's specified that standard Java classes are thread unsafe by default. So this is a specified behaviour.

This is pretty much the same as breaking a Map by mutating a key during insertion.