Chapter 7: Better Living in Objectville

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Chapter 7 Better Living in Objectville

• Phil FeibishIT 3883

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Overview

• When writing Java code, be sure to write code
  that others can extend easily.
• Write code which can be easily changed and
  updated.
• Flexibility, polymorphism, and inheritance.

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Shape example

• Example

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Inheritance
superclass(more abstract)

subclass(more specific)
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Inheritance - Definition

• Java is a pure object-oriented language.
• In an OOP is that they are able to extend the
  functionality of existing classes.
• This extension of functionality is called
  inheritance.
• Rather than developing from scratch, we can use
  inheritance to add to an existing class whatever
  is needed.
• Syntax derived-class extends base-class

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Inheritance

• Square inherits from Shape, or Circle inherits
  from Shape
• Subclass extends the superclass. Subclass
  inherits the members of the superclass.
• Shape is the superclass of the other four
• The other classes are subclasses
• Subclasses may contain members not included in
  superclass.
• Inherited variables are not overridden.
• "If the Shape class has the functionality, then
  the subclasses automatically get the same
  functionality."

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Inheritance important point

• By default, all Java classes are derived from a
  common class called Object.

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Overriding

• What if the rotate() method for an Amoeba is
  different than the rotate() method for the other
  objects?
• You can write a different rotate() method for
  class Amoeba. This method will override the
  definition used by the Shape class.
• Overriding "just means that a subclass redefines
  one or its inherited methods when it needs to
  change or extend the behavior of that method."

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Inheritance example (p165)

• public class Doctor
• boolean worksAtHospital
• void treatPatient()
• // perform a checkup
• public class FamilyDoctor extends Doctor
• boolean makesHouseCalls
• void giveAdvice()
• // give homespun advice
• public class Surgeon extends Doctor
• void treatPatient()
• // perform surgery

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Implementation - Doctor
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Another example Animals
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Class Hierarchy pg170

• We decide that Canines could use a common roam()
  method.
• We also see that Felines could use a common
  roam() method
• We'll let Hippo continue to use its inherited
  roam() method the generic from Animal

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Which method is called?The lowest one wins.

• Wolf w new Wolf()
• w.makeNoise()
• w.roam()
• w.eat()
• w.sleep()

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Which method is called?

• When you call a method on an object reference,
  you're calling the most specific version of the
  method for that object type.
• The lowest one wins!
• If the JVM doesn't find a version of the method
  in the Wolf class, it starts walking back up the
  inheritance hierarchy until it finds a match.

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Inheritance Class DiagramInheritance Tree
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Using IS-A

• When you want to know if one thing should extend
  another, apply the IS-A test.
• Triangle IS-A Shape? Yes.
• Cat IS-A Feline? Yes.
• Surgeon IS-A Feline? Yes.
• Tub IS-A Bathroom? NO.

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Using HAS-A test

• Use the HAS-A test to determine if a class has an
  instance variable.
• Bathroom HAS-A Tub? Yes.
• Tub HAS-A Bubbles? Yes.
• So bathroom has a reference to a Tub, but
  Bathroom does not extend Tub.

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IS-A in Inheritance Tree

• If class B extends class A, class B IS-A class A.
• This is true anywhere in the inheritance tree.
• If class C extends class B, class C passes the
  IS-A test for both B and A.
• REMEMBER The inheritance IS-A relationship
  works in only one direction!

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Questions?