Performance difference between a wild card import and the required class import

Question

What is the complexity in terms of performance between

java.io.* 

and

java.io.File 

PS.

I know that the first one will include every file in java.io.* and the next one only the selected class file.

Answer 1

At runtime 0.

Both generate the same byte code

Answer 2

Imports are resolved to fully qualified names at compile time. There is no runtime performance difference. If you look at the generated bytecodes, they will be identical.

There might be a small compile time overhead in using one or the other form, but it is likely to be so small that nobody should notice it, let alone care about it.

I know that the first one will include every file in java.io.* and the next one only the selected class file.

Not exactly. What a star import does is to make all of the class names available. The actual classes themselves are not "included" ... in the sense of the C or C++ programming languages.

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The real reasons that many people use explicit imports rather than wildcard imports are:

• Explicit imports clearly document what external classes a class is directly using, provided that you don't leave redundant imports in your code.

• Explicit imports avoid problems with name collisions arising when you import two packages that contain classes with the same (simple) class name.

• Explicit imports avoid fragility problems where someone adds a new class to some package that you have wildcard imported. This can lead to new compilation errors in code that previously used to compile, due to a name collision (see previous).

Modern IDEs have accelerators, code elision and other features that help you keep your imports under control if you use explicit imports.