Subclasses
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    subclass A class that inherits variables and methods from an existing class. 
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    superclass The class from which variables and methods are inherited. 
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    Object The class that is the root of the tree-structured class hier- archy. 
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    overriding Specifying a method in a subclass for the same message as a method in a superclass. 
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    super A pseudo-variable that refers to the receiver of a message; differs from self in where to start the search for methods. 
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    abstract class A class that specifies protocol, but is not able to fully im- plement it; by convention, instances are not created of this kind of class. 
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    subclassResponsibility A message to report the error that a subclass should have implemented one of the superclass’s messages. 
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    shouldNotImplement A message to report the error that this is a message inherited from a superclass but explicitly not available to instances of the subclass.