behaviour of import statement

[Siva Prasad]

Hi Friends
I have simple question for those who knows C/C++. I know most of the people who read and post messages in this forum are with this background only.
Anyways my question is...
in C/C++ you use #include directives to refer the class definitions and function declarations.
in Java we use import <packagename> to use refer the libraries.
What is the actual behaviour of this import statement?
When u say import this package what does it meann exactly?
Siva Prasad

[Mark Savory]

'import java.util;' means that you can declare say Hashtable without the package name - that's all. If you don't use the import statement, you can still declare the same using java.util.Hashtable. 'import' is not like #include in C/C++ in that it doesn't actually reference anything external. All that matters is that a class is referenced in the classpath with the JRE starts. 'import' does not load classes.

[Cindy Glass]

Which means that java import does not have the overhead associated with c++ include. There is no penalty for importing unneeded stuff.

[Mark Savory]

Cindy,
I'm guessing that there is a penalty when you compile the java code because everywhere that the compiler finds Hashtable it needs to replace it with java.util.Hashtable and I don't see any other way of doing that except trying each of the packages specified in the import statements.

[Matts Smith]

Mark is right,
but the overhead is only for the first time the class is referenced.

[Amit Badle]

Originally posted by Matts Smith:
Mark is right,
but the overhead is only for the first time the class is referenced.

hi matts,
cud u clarify on ur quote. does the include in c/c++ have a overhead evertime the program is run in c/c++ and import is completely different in java. thanx. bye.

[Cindy Glass]

Yes, there is compile time overhead for both java and C++. There is no Runtime penalty in Java, only the classes used in the application are involved.
It is my understanding that in C++ the included classes are all loaded whether used or not.

[Siva Prasad]

Originally posted by Cindy Glass:
Yes, there is compile time overhead for both java and C++. There is no Runtime penalty in Java, only the classes used in the application are involved.
It is my understanding that in C++ the included classes are all loaded whether used or not.

There is a very clear overhead when we talk about #include directive in C++ in the form of unnecessary inclusion of code at the time of compilation. And also we get an .Obj file too which will be used by the linker inorder to generate the .exe file. Even then C++ program runs faster than Java program. When there is no runtime penalty in Java why its so slow?
Any thoughts?
As per my understanding about import statement, its just the way to tell Java to look for those specific classes in the specific directories/packages. Am I rite?
Siva

[Matts Smith]

well well well.
both #include and import are directive to the compiler.
import is just to tell the compiler where to look for classes. Vector for example is 'replaced' by java.util.Vector.
#include pastes the content of the included file at the place the directive is. usually it's only prototypes so it's just used to make your call to functions defined in another source file or later in the same source file to be 'accepted' (i.e. link might still fail but the compiler will shut up)

hope it helps.

[Steve Fahlbusch]

The other key point of #include is to include the definition
of inline functions so they can be inlined in the object.