> BOOL, YES, NO – boolean, true, false
In Java, the boolean type is boolean. In Objective-C, it is BOOL, which can be set to YES or NO.
> nil, NULL and null
nil is the equivalent of NULL for a pointer to an object. nil and NULL should not be interchangeable.
use nil to set and test Objective-C object references exactly as you would use null in Java.
NULL is C’s empty pointer constant and is used to set and test pointers.
id is object type.
> +, –
an instance method is declared with a minus sign (-) prefix.
similar as instance method sign (+) defines a class method.
> namespace and source code
use #import and a preﬁx to identify the origin of a class, no package and namespace, eg. NS for Cocoa NSObject, CC for Cocos2D CCNode.
Objective-C uses .h ﬁles for headers, and .m ﬁles for the code; .mm ﬁles are used for Objective-C++.