Object-Oriented Programming


Michael L. Collard, Ph.D.

Department of Computer Science, The University of Akron


Application Programming Interface

  • Modern design is the design of APIs and how they interact with each other
  • Developers build large programs using existing APIs as much as possible
  • For missing functionality, create new APIs
  • Final application is integrated into existing APIs
  • Large-scale mechanism for separation of concerns

Role in Programming Languages [Koenig, Moo]

Language design is library design

Library design is language design

Existing APIs

  • Standard library of programming language and platform
  • E.g., std::*
  • External libraries for a programming language
  • E.g., boost::*
  • Custom APIs for specific purposes:
  • libxml2 - XML parser and toolkit
  • libarchive - Multi-format archive and compression library
  • wxWidgets - Toolkit and tools library for creating cross-platform GUIs
  • cpp-netlib - C++ network programming library

Good APIs

  • Low complexity
  • High degree of safety
  • Flexible enough
  • Efficient enough


  • Low external complexity often means high internal complexity
  • Flexibility adds external complexity
  • Partial solution: Good defaults
  • Partial solution: Staged features

Staged Feature: Reading a file into a std::vector

How to Decide

  • Carefully name any parts of the API
  • Methods are actions, and naming should reflect that
  • Look at existing code and see if anything is not in the API that should be
  • Make sure that the API does not contain concerns for a specific client, e.g., loc is an srcML concern
  • Imagine separate developers working on the different parts (even if you are doing both parts)
  • Why should they know this concern?
  • Where would they have learned this concern?

API Taxonomy

  • Platform
  • Framework
  • Toolkit
  • Library


  • Functions/classes typically for a single purpose
  • E.g., libxml2, libarchive
  • Used with many other libraries and client code
  • Control: client (small-scale inversion of control)


  • Functions/classes for a set of related purposes
  • Used with other toolkits and client code
  • Control: client (small-scale inversion of control)


  • Large set of classes for a broad set of purposes
  • E.g., MFC, Qt
  • Client code is integrated into classes
  • Control: Heavy use of inversion of control


  • Entire programming environment, often with a custom language and IDE
  • E.g., iOS, .NET, Android, UNIX/Linux
  • Client code is integrated into multiple frameworks and supporting tools
  • Control: Typically complete inversion of control

Design Using APIs

  • Use existing APIs as much as possible
  • Create new APIs for any functionality that is missing
  • Start with libraries
  • Then move into toolkits
  • Finally, if significant enough, framework
  • Minimal chance to create a platform

Order of Magnitude

of 10
Order of
1 100 0
10 101 1
100 102 2
1,000 103 3
10,000 104 4
  • Large programs are more complex than small programs by an order of magnitude
  • Essential skill of development is to (temporarily) "forget" and concentrate on the current task
  • If you do not develop this essential skill, you are limited in what you can develop and the types of jobs you can handle. This will impact your career
  • The composition and frequency of commits indicate how tasks are isolated