It is, but combining a sequence of flags is POSIX.
it's a simple to learn gnuism that most good tools follow but (mostly) java apps for whatever reason disrupted it and added a lot of mess.
The Java tools predate the GNU convention (or at best were developed at the same time, unbeknownst to each other). Golang uses single-dash as well (but also accepts double-dash), as do PowerShell and Unix commands like find. In early Unix, there wasn’t a clear convention to use a dash for options at all (dd, tar, ps).