The IOS rewrite isequal method needs to rewrite the hash method

In a project, you sometimes compare two objects for equality, but instead of having to be the same object, you compare the content.

NSObject provides a isequal to determine whether it is the same object. System NSString, Nsarray, etc., have been overridden isequal methods.

When you override the IsEqual method generally, you need to rewrite the hash method, because: if you don't, Violates the general convention of Object.hashcode, which causes the class to not work properly with all collection classes based on hash values (hashes), including HashMap (Nsdictionary), HashSet (Nsset), Hashtable (Nsarray).

What is hash, what is the role.

See http://blog.csdn.net/kafeidev/article/details/6224407