Abstract Classes

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Abstract Classes
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Recall

• We have the ability to create hierarchies of
  classes.
• Good design practice suggests that we move common
  methods as high as possible in the tree
• Why?
• Minimize code duplication
• Minimize maintenance efforts
• Good abstraction
• Example?

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But

• What if we know that every subclass will have a
  certain method but the methods will all be
  different???
• What then?
• Example We are designing a calculator that will
  have as a feature the ability to make
  calculations involving geometric shapes
• We want it to handle rectangle, squares,
  triangles, equilateral triangles, ellipses and
  circles
• Each shape should know how to calculate its area
  and perimeter

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Shapes
Shape
getArea getPerim
Ellipse
Rectangle
Triangle
major minor
altitude base
length width
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Need

• What we would like is the ability to say in the
  Shape class that all Shapes will have getArea
  and getPerimeter methods.
• We obviously dont know what they will be when
  all we know is that something is a shape but...
• The basic idea is for the compiler to provide as
  a service the ability to force all subclasses
  of shape to have a getArea and getPerimeter
  method
• The underlying philosophy is simple
• Catch errors when compiling the program not when
  running the program

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Abstract Methods

• We have the ability to define a method with no
  method body.
• We are required to label this method as abstract
• Once a class contains an abstract method it too
  must be labelled abstract
• Every subclass will too be abstract until a
  subclass is written in which every abstract
  method has been defined
• Abstract classes cannot be used to make
  Objects!!!
• But, they can be used to make references!?!?!?

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Shape

• // Comments omitted...
• abstract class Shape
• private String name
• public Shape(String name)
• setname(name)
• public void setName(String name)
• this.name name
• public String getName() return name
• public abstract double getArea()
• public abstract double getPerim()

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Rectangle

• class Rectangle extends Shape
• private double ln
• private double wd
• public Rectangle(String name, double ln, double
  wd)
• super(name)
• this.ln ln
• this.wd wd
• public double getArea()
• return ln wd
• public double getPerim()
• return 2.0 (ln wd)

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In case youre wondering...
If Shape is abstract and ThingsWithStraightSides
doesnt define getArea then it must also be
abstract.
Shape
ThingsWithStraightSides
Triangle
Rectangle
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