问题
I have a doubt, whether the unused imports and unused objects in Java code creates any performance impact?
Suppose an object is initialized and never used, what happens? And what is the cost of unused imports
回答1:
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.
回答2:
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.
回答3:
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 was thicker for no reason. Add to that a superfluous jar in the webapp classloader.
回答4:
Though unused imports in Java file do not create any harm, it unnecessarily increases the length and size of the Java source file.
回答5:
Yes it impact a bit on performance, if we are referring unused import statement in our java class. The Java compiler will check for references mentioned into the import statement and at minute level it impact on the performance of the your class.
Thanks
来源：https://stackoverflow.com/questions/8724045/does-unused-import-and-objects-have-an-performance-impact