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Object-Oriented Vocabulary: An Introduction

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Version of 16 January 20101

In reading about Object-Oriented Programming, one is likely to encounter many words with special meaning in an OO context. Sometimes, the meaning of these words may seem apparent from the context, since they are mostly special uses of existing English words, but at times their meaning may be unclear or the true meaning may be different from the meaning one might intuit in that particular context. In order to provide those starting out with OOABL2 a reference by which they can check the meanings of such words, I have compiled the following list. This is not to be considered a substitute for systematic reading and study, but might help the newcomer to OO thinking to get started in their reading.

Abstract Method: A method which is defined only by a Signature, i.e., its parameters and return type, but no code to implement the method. A Class containing an Abstract Method cannot itself be instantiated3. Instead, a Class which Inherits from the Class with the Abstract method must provide a Concrete Implementation if it is to be Instantiated. Any Class which is to be Instantiated directly must have resolved all Abstract Methods in its Class Hierarchy.

Abstraction: Knowledge4, behavior5, or relationships not associated with a specific instance or implementation.

Aggregation: A relationship between one Object and one or more other Objects in which one object “contains” the other objects or which one would describe by the phrase “part of”, i.e., some group of teachers is “part of” a school. Aggregation differs from Composition in that in Aggregation, the contained Object has the potential to stand alone, i.e., if the school is disbanded, the teachers still exist.

Association: A structural relationship between two objects indicating that they are or can be used together.

Cast: A programming language method for converting data types, e.g., casting an object passed as a Superclass or Interface into the actual specific Class of the original object.

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