In that code, after generateKey, during the sensitive operation, the system might need to do a garbage collection, at which point this array might have been copied to a different location in memory before your call to zero. You have to also "pin" (afaik this is the usual terminology for this) that array to a fixed location (which would then have to be a feature of that runtime and garbage collector) after allocating it but before generating the key.
Java also has no mfence or clflush support, so crypto.zero would never be secure. It might overwrite the key in the store buffer only, but not on the heap immediately, so prone to sidechannel attacks.
In .NET you can pin memory in the GC heap, either using something like the fixed expression in C# or GCHandle. A quick Googleing did not turn up something usable directly by Java, except ByteBuffer.
