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Chapter 1: Object Technology 1.2.1 Objects: combined services
Chapter 1: Object Technology 1.2.1 Objects: combined services and data An object-oriented system comprises a number of software objects that interact to achieve the system objective. The software objects usually mimic the real-world objects of the application domain. The real-world objects may have a physical presence or may represent some well-understood conceptual entity in the application. For example, in a university application we might have software objects that represent students. Equally, we may have software objects representing programmes of study at a university even though they have no physical existence. Objects are characterized by having both state and behaviour. The state of an object is the information an object has about itself. For example, a student object may have a name, a date of birth and a university matriculation number. Equally, a programme of study object might have the name of the programme, its duration and the name of the programme leader. The behaviour of an object describes the actions the object is prepared to engage in. For example, we might ask a programme of study object for its duration. We might ask a student object for its age. This would involve the student object performing a calculation based on its date of birth and today s date. The behaviour of an object is described by the set of operations it is prepared to perform. One object interacts with another by asking it to perform one of its advertised operations. This interaction is achieved by one object sending a message to another. The first object is known as the sender object and the second object is known as the receiver or recipient object. The only messages an object can receive belong to the set of operations it can accept. In the UML, objects and message passing are usually captured by a sequence diagram as shown in figure 1.1. Here a university object is shown sending the message getAge to a student object. : University : Student getAge Figure 1.1 Message passing in a sequence diagram When an object receives a message it performs some action. This action is described by a method. A method is the processes the receiving object follows when servicing the message. For example, if a university object sends a message to a student object asking for its name, then the student object simply replies to the sender with one part of its state, namely, the name. However, if a university object sends a student object the message asking for its age, then the method that student object must follow is more elaborate. First, it must obtain today s date. This might be achieved by the student object
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