CS 162 Spring 2005

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CS 162 Spring 2005

• Chapter 11 Inheritance

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Another Form of Polymorphism

• Inheritance is a different type of Reuse
  (polymorphism) than we have seen
• Interfaces - Reuse of Concept (specifications,
  descriptions)
• Inheritance - Reuse of Code (Behavior, Methods,
  Actual Code)

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Inheritance as Extension

• Inheritance is often described as Extension,
  because when you inherit you get the behavior of
  a parent class, and can extend or add data fields
  or methods.
• Class SavingsAccount extends BankAccount

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Polymorphic Assignment

• Just like with Interfaces, Inheritance brings
  with it the polymorphic assignment. Can assign a
  subclass to a superclass variable
• BankAccount ac new SavingsAccount()
• Ac is "many faced", because it can hold any type
  of BankAccount

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Inheritance

• Re-usable code, maintainable code
• Implemented in Java using classes with extends or
  implements
• Base (super) class / derived class
• Hierarchy of classes
• "parent", "grandparent", "ancestor", etc. (super)
• "child", "grandchild", "descendant", etc.
  (derived)
• transitivity
• Models the "is a kind of" relationship
• Other direction is a generalization of"

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Some features of inheritance

• Derived classes can add new attributes and/or new
  methods
• Derived classes can override base class methods
• Every instantiation of a derived class is an
  instanceof its own class and is an instanceof its
  base class (including parent, grandparent, etc.)

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Relation to Polymorphism

• Suppose class Savings Account overrides method
  interest originally defined in class
  BankAccount
• BankAccount ac new SavingsAccount()
• System.out.println(interest ac.interest())
• // will do method in SavingsAccount,
• // NOT the method in BankAccount

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Some rules of inheritance

• Derived classes may not remove attributes or
  methods.
• public base class methods may not be overridden
  by private derived class methods.
• Multiple inheritance is not supported in Java
• Base class can't "inherit" from derived class.
• protected attributes and methods in a base class
  can be accessed only from the base class and its
  derived classes ( actually the rule is more
  complicated than that)
• Every class automatically inherits from Object
• Primitive types are not objects ("wrappers"
  required).

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Derived classes can

• Inherit method Don't supply a new implementation
  of a method that exists in the base class
• Override method Supply a different
  implementation of a method that exists in the
  base class
• Add method Supply a new method that doesn't
  exist in the base class

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Derived classes can

• Inherit field All fields from the superclass are
  automatically inherited
• Add field Supply a new field that doesn't exist
  in the base class

Derived classes can not
Override field All fields from the base class
are automatically inherited
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public class shape public double area() throws
RunTimeException throw new
RunTimeException( Area not defined for this
shape.)
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public class Circle extends Shape public
Circle( double rad ) radius rad
public double area( ) return
Math.PI radius radius public
double perimeter( ) return 2 Math.PI
radius public String toString(
) return "Circle " radius
private double radius
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public class Rectangle extends Shape public
Rectangle( double len, double wid )
length len width wid public
double area( ) return length width
public double perimeter( )
return 2 ( length width )
public String toString( ) return
"Rectangle " length " " width
public double getLength( ) return
length public double getWidth( )
return width private double
length private double width
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Using super

• public class Square extends Rectangle
• public Square( double side )
• super( side, side )
• // Note We could define area() for square as
• // public double area()
• //
• // return super.area()
• //
• public String toString( )
• return "Square " getLength( )
• public double diagonal()
• return Math.sqrt(2) super.getLength()

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public class ShapeDemo public static void
main( String args ) Shape a new
Circle( 2.0 ), new Rectangle(
1.0, 3.0 ), new Square( 2.0 )
System.out.println( "Total area "
totalArea(a) ) printAll(a) public
static double totalArea( Shape arr )
double total 0 for( int i 0 i lt
arr.length i ) if( arr i ! null )
total arri.area( ) return total
public static void printAll( Shape arr )
for( int i 0 i lt arr.length i )
System.out.println( arri.toString() )
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Using an interface

• public abstract class Shape implements Comparable
  public double area() throws RunTimeException
  throw new RunTimeException
• (Area not defined for this
  shape.) public int compareTo( Object
  rhs ) Shape other (Shape) rhs double
  diff area( ) - other.area( ) if( diff 0
  ) return 0 else if( diff lt 0 )
  return -1 else return 1

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Application

• public class ShapeTest
• public static void main(String args)
• Circle c new Circle(2.5)
• Rectangle r new Rectangle(1.5, 3.5)
• System.out.print(The area of
  c.toString())
• if(c.compareTo(r) 0)
• System.out.print( is equal to )
• else if (c.compareTo(r) 1)
• System.out.print( is greater than )
• else
• System.out.print( is less than )
• System.out.println( the area of r
  .toString())

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Any class can prevent inheritance using final

• public final class Triangle extends Shape
• // etc.

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Converting Subclasses ? Superclasses

• Ok to convert subclass reference to superclass
  reference
• square s new square(10)rectangle r
  sObject anObject s
• Not OK to convert superclass reference to
  subclass reference
• double d r.diagonal() // ERROR

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Access Control Level

• public
• private
• protected (accessible by subclasses and package)
• package access (the default, no modifier)

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Recommended Access Levels

• Fields always private
• except public static final constants
• Methods public or private
• Classes public or package
• Don't use protected
• Beware of accidental package access (forgetting
  public or private)

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Object The Cosmic Superclass

• All classes extend Object
• Most useful methods
• String toString()
• boolean equals(Object otherObject)
• Object clone()

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Overriding the equals method
equals tests for equal contents tests for
equal location Must cast the Object parameter
to subclass public class Rectangle public
boolean equals(Object otherObject)  Rectangle
other (Rectangle)otherObject return (length
other.length) (width
other.width)
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equals Two Objects with same Contents
Rectangle length 3.5 width 1.7
rec1
Rectangle length 3.5 width 1.7
rec2
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Two References to same Object
Rectangle length 3.5 width 1.7
rec1
rec2
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Questions?

• Read Chapter 12
• Now your chance to test your understanding - do
  the anti quiz