{"id":1138,"date":"2023-09-12T13:25:33","date_gmt":"2023-09-12T11:25:33","guid":{"rendered":"http:\/\/oopm.org\/?page_id=1138"},"modified":"2025-01-15T10:45:20","modified_gmt":"2025-01-15T09:45:20","slug":"5-3-representing-classification-hierarchies","status":"publish","type":"page","link":"https:\/\/oopm.org\/?page_id=1138","title":{"rendered":"8.6 Representing classification hierarchies"},"content":{"rendered":"
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Classification<\/em> is a fundamental means for understanding and organizing knowledge about the real world, and is used in many disciplines like biology, physics, health, etc. Classification is a means for organising concepts in a hierarchy called a classification hierarchy.<\/em><\/p>\n\n\n\n

Class Account<\/code> and the subclasses SavingsAccount<\/code> and CreditAccount<\/code> and class Booking<\/code> and its subclasses TravelBooking<\/code>, FlightBooking<\/code>, TrainBooking<\/code> and HotelBooking<\/code> are examples representing classification hierarchies.<\/p>\n\n\n\n

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We use classification in our everyday language when we deal with various types of concepts. We use general terms like animal, and more specific terms like mammal, fish, bird, and insects. We may use terms like dog and cat to be more specific than just mammal. Even though dogs like golden retriever, puddle, and greyhound are breeds of dogs, one may also put these into such a classification. The figure to the right shows a partial animal classification hierarchy.<\/p>\n<\/div>

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The terms generalization<\/em> and specialization<\/em> are often associated with classification hierarchies. A generalization<\/em> is a more general concept that captures the commonalities of and suppresses some of the differences of a number of more special concepts. A specialization<\/em> is the inverse of a generalization. In the above figure, Account<\/code> is a generalization of SavingsAccount<\/code> and CreditAccount<\/code>,  and these are specializations of Account<\/code>. Booking<\/code> is a generalisation of TravelBooking<\/code>, and HotelBooking<\/code>, and FlightBooking<\/code>, and TrainBooking<\/code> are specializations of TravelBooking<\/code>, etc.<\/p>\n\n\n\n

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The vehicle hierarchy is another example of a classification hierarchy. The need for being able to represent such a hierarchy of vehicles lead the designers of SIMULA, Ole-Johan Dahl and Kristen Nygaard to invent the subclass mechanism of SIMULA.<\/p>\n<\/div>\n\n\n\n

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In section