308203A Introduction to Computing II Lecture 3: Interfaces and Inheritance

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308-203AIntroduction to Computing IILecture 3
Interfaces and Inheritance
Fall Session 2000
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Interfaces

• More than one class may perform the same service
• e.g. Planes, trains and automobiles
• Interfaces describe what classes can do

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Example
interface Vehicle float costOfTrip(
) ...
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Example
class Car implements Vehicle float
costOfTrip( )
interface Vehicle float costOfTrip(
) ...
class Train implements Vehicle float
costOfTrip( )
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Example
Vehicle chooseLeastExpensive(Vehicle
choices) for (int j 0 j lt
choices.length j) if (choicesj.costOfTrip(
) lt best) bestVehicle choicesj best
choicesj.costOfTrip( )
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Whats the point

• We can manipulate objects without knowing
• their exact class
• Code we write will work, even with new
• kinds of objects
• Weve abstracted a kind of behavior

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Polymorphism
Definition Polymorphism is the ability of a
method call to invoke an underlying method
apropriate to the type of the object.
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Any questions?
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Interface shortcomings

• What if two classes share not only behavior
• but data? E.g. all vehicles have
  wheels
• What if two classes have methods which are
• implemented in the same way?
• What if we want to change the behavior of a
• class slightly?

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Inheritance

• Make new classes based on a previous class
• Behavior remains the same unless overriden
• Code can be modified incrementally, without
• restarting from scratch for each class
• Code can be shared

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Terminology
Superclass - a class on which another class is
based Subclass - a class derived from another
class Override - the replacement of a method
inherited from a superclass by
some new method
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Related Java Keywords

• extends - what class do we build on
• this - lets an object refer to itself
• super - lets an object refer to methods
• as defined in its superclass
• final - prevents a method from being overriden
• abstract - indicates that a method is undefined
  and
• must be overriden in the
  subclass

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Public/private/protected
Keywords to restrict access to methods/data

• Public - visible by any other class
• Private - visible only by this class (not
  subclasses!)
• Protected - visible to this class and subclasses

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A Simple Example
public class Workaholic extends Worker
public doWork( ) super.doWork(
) drinkCoffee( ) super.doWork( )
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Any questions?