CareerCup

I didn't understand this question. But optimum java code is the one that doesn't rely that doesn't crash due to memory issues. So, make sure that you use string objects instead of string etc..

reusability/modularity, data/functional abstraction, complete code coverage, fail-safe (crashes safely.. no one can totally avoid crashing).. most imp of all.. meet the user requirements..
add more if needed

To add: Takes care of garbage collection.

Time complexity is not a feature of java code but of algorithms.
Space complexity you cannot control in java because you don't know what it is doing behind the scenes.
Eg. in C++, if you just have 8B space,you could do a int x = new int(23); int y = new int(30); delete x; int p = new int...
i mean that memory is totally in user's control in C++, but not in java.

optimum java code may be bytecode...its machine independent(can be run on any machine having jvm), secured, robust etc

Optimum Java code is something which does have best code practises, Avoid pre instantiation of classes, unnecessary variables, low complexity alogorithms, dead code avoidings , all that...

I will add a few more

Time Complexity
Space Complexity
concurrency