I am just like a normal Dictionary, except that I am implemented differently. Each Class has an instance of MethodDictionary to hold the correspondence between selectors (names of methods) and methods themselves.
In a normal Dictionary, the instance variable 'array' holds an array of Associations. Since there are thousands of methods in the system, these Associations waste space.
Each MethodDictionary is a variable object, with the list of keys (selector Symbols) in the variable part of the instance. The variable 'array' holds the values, which are CompiledMethods.
