It's quite good, but e.g. Java SDK has stuff that runs circles around Go's standard library regarding breadth and maturity, especially stuff added since 1.4 (nio, etc).
And some aspects of the Go SKD are horrible in practical use. Case in point, most things math related.
Well, yes. But Java's standard library is also huge and full of deprecated stuff. I have not done a lot of Java programming, but when I did play around with Java, I found myself spending most of the time actually browsing through the standard library's documentation (which is, to be fair, really, really good) looking for stuff.
Also, Java has a head start of nearly 15 years on Go, and the Java community is (or used to be, at least) pretty huge.
Add an extra 30 years to that. Java's designers didn't history of computer science and language development for the of sake "simplicity". (Generics, error handling, etc.)
And some aspects of the Go SKD are horrible in practical use. Case in point, most things math related.