Abstract Classes and Methods

An operation should be declared to exist at the highest class in the hierarchy where it makes sense

• The operation may be abstract (lacking implementation) at that level
• If so, the class also must be abstract

  • No instances can be created
  • The opposite of an abstract class is a concrete class
• If a superclass has an abstract operation then its subclasses at some level must have a concrete method for the operation

  • Leaf classes must have or inherit concrete methods for all operations
  • Leaf classes must be concrete

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