Does unused import and objects have a performance impact

Its a very common question.

Like most performance questions the best approach is to write the clearest and simplest code you can as this improves the maintainability of the code and helps ensure it performs reasonably well even after it is changed. (Clever/Obtuse/Needlessly Verbose code can run fast to start with but as it is changed by mere mortals it can get much slower)

Unused imports have a trivial impact on the compiler, but there are no imports in the byte code or at runtime.

Unused objects can be optimised away, but its best to avoid these as they almost always cause some performance impact, but more importantly make reading and maintaining your code more difficult.

Unused imports have no performance impact at runtime. It is purely a namespace mechanism. Nonetheless, you should always import only what you need for readability and avoid namespace collisions which are a nuisance.

Apart from code readability and hence maintainability of code, there may be faster compilation of java code (however, unnoticeable) by tidying up imports, but runtime performance is not impacted, since byte code generated is not impacted by untidy imports. Byte code generated remains the same.

While impact in compilation is minimal, the impact in deployment can be bad. I've just come across an unused import that required a separate library which became a maven dependency. A further transitive dependency problem was fortunately not found, but the .war file was thicker for no reason. Add to that a superfluous jar in the webapp classloader.