What's the logic of putting @interface in .m file? [duplicate]

Question

Possible Duplicate:
Difference between @interface definition in .h and .m file
What is the @interface declaration in .m files used for in iOS 5 projects?

I've seen code like this:

// In Header.h
@interface Header{}
@end

// In Header.m
@interface Header()
@end

My questions are:

1. What's the difference in putting it in 2 files?
2. Why put {} after class name in ".h" file and why "()" in ".m" file?

Answer 1

@interface MyClass(){
    NSInteger aInt;

}
@property(nonatomic,strong) NSString *name;
@end

is a class extension

with modern compilers this is a great way, to decrale methods, ivars and properties only for private use in the class MyClass.

Class extensions have to be declared in the main implementation file (not in a category).

So you can hide implementation details from the header file, they are private.

Answer 2

This has become a common practice for declaring "private" properties/methods for a given class. By declaring them in an anonymous class extension inside the .m, these properties/methods are not exposed to consuming objects.