Question

What is effected by redundant java import statements?

Do they effect the compiled runtime (performance/size)?
or just stuff like intellisense?

To ask differently:
how important is it to remove them?

Answer 1

Import statements only affect what happens during compile time.

The compiler takes this code, and creates a .class file that represents your code in an executable format (something in binary).

In the end, the binaries are exactly the same, but the method by which they are made are different.

Let’s look at a simple case:

import java.util.*;

vs

import java.util.ArrayList;
import java.util.List;

when used in:

//...
List <String> someList = new ArrayList <String> ();
//...

When the compiler hits the word List, in the first case, it will need to figure out if List exists in that set of classes or not. In the second case, it is already given it explicitly, so its much easier.

In essence, what happens is the compiler must take all the classes existing in the import statements and keep track of their names so that, if you use it, the compiler can then retrieve the appropriate functions that you are calling.

Sometimes, there are classes that have the same name in multiple packages. It is in this case (which Thomas is referring to) that you should not use the * to select all the classes in the directory.

It is best practice to explicitly describe your class usage.