Microsoft Word OpenIssuesInOO. doc

Separation of interface and implementation 

An argument for distinguishing between type and class is the separation of interface and 

implementation. The type defines the interface and the class defines the implementation. It 

is definitely necessary to be able to separate the interface of a class from its implementation, 

and most object-oriented languages supports such a separation. Basically there are three 

ways: 

The abstract data types (ADT) approach.There are pre-defined rules for what is visible 

via the interface. Often operations (methods) are visible whereas (instance) variables 

are hidden. This is called the ADT-approach since an ADT is a type characterised in 

terms of its operations. Smalltalk is an example of a language using the ADT-approach.