I know Java does not have pre-processor, but I struggle to find a way to do this.
I am looking to create macros to improve code readibility, to specify which of the functions or parameters are for input or output specifically. In C, this would be done like this :
#define IN
#define OUT
And these would just be used before the variables and functions. Can I simulate the same behavior in Java?
I did not manage to find a way to do this. The only way I found was including defines in the code and first using the cpp pre-processor to replace them, and then run the java code, but maybe there is a better way of doing this?
Java indeed does not have a pre-processor, and your use case doesn't require that either: you aren't actually preprocessing the code, you just put in some tags that the compiler can ignore.
What you are looking for are "annotations" - you basically want to annotate your code with some nice text that will not affect the compiler.
This basically requires defining some specialized Java types for this using the @interface keyword and then you can use them to write things like:
public void doStuff(@Input invar, @Output outvar) {
...
These annotations can be simply:
@interface Input {}
@interface Output {}
or you can add more features and even use reflection to examine them in runtime.
See the linked documentation above for more details.