lecture33 - COMP 250 Winter 2010 33 - abstract classes,...

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COMP 250 Winter 2010 33 - abstract classes, casting April 9, 2010 Abstract classes It often occurs that one wants to deFne a class with some methods fully speciFed, but some methods speciFed only by their signature. Think of this as a hybrid between a full class and an interface. In Java, this hybrid is called an abstract class . One adds the modiFer abstract to the deFnition of the class and to each method that is missing its body. Here is an example: public abstract class Circular{ private double radius; Circular(){}; Circular(double radius){ this.radius = radius; }; public double getRadius(){ return radius; } public void setRadius(double radius){ this.radius = radius; } public abstract double getArea(); } This abstract class has one abstract method could be extended by concrete classes, e.g. Circle , Cylinder , Sphere which would each implement the method getArea() in a di±erent way. Note that the subclass Circle might also have a method getPerimeter() . Such a method would make no sense for a Sphere or Cylinder . Similarly, getVolume() would make sense for a Sphere and Cylinder , but not for a Circle . An abstract class cannot be instantiated. Despite this, abstract classes do have constructors. This seems like a contradiction, but it is not. Abstract classes are extended by concrete subclasses which provide the missing method bodies. When these subclasses are instantiated, they must inherit the Felds and methods of the superclass – in particular, the values of the Felds are set by the superclass constructor (either via an explicit super()
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This note was uploaded on 09/25/2011 for the course COMP 250 taught by Professor Blanchette during the Spring '08 term at McGill.

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lecture33 - COMP 250 Winter 2010 33 - abstract classes,...

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