CSE 114

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CSE 114 Computer Science IPolymorphism
Red Bay, Labrador
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Inheritance Overview

• Inheritance allows you to define a very general
  class then later define more specialized classes
  by adding new detail
• the general class is called the base or parent
  class
• The specialized class inherits all the properties
  of the general class
• specialized classes are derived from the base
  class
• they are called derived or child classes
• After the general class is developed, you only
  have to write the "difference" or
  "specialization" code for each derived class
• A class hierarchy classes can be derived from
  derived classes (child classes can be parent
  classes)
• any class higher in the hierarchy is an ancestor
  class
• any class lower in the hierarchy is a descendent
  class

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Overriding Verses Overloading

• Overloading
• Same method name
• Different signature
• Both methods can be in same class

• Overriding
• Same method name
• Same signature
• One method in ancestor, one in descendant

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Polymorphism with methods/operators

• Polymorphism two or more methods or operators
  with the same name but different definitions
• A polymorphic method or operator is said to be
  "overloaded"
• Examples
• (addition and concatenation)
• multiple constructors
• Example
• public void setUpValue(int setValue)
• if (setValue gt 0 setValue lt numFaces)
• upValue setValue
• public void setUpValue()
• upValue 1

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Polymorphism with classes

• Polymorphic Object an object variable that can
  refer to different types of objects at different
  points in time
• Polymorphic references in Java can be created in
  2 ways
• Using inheritance
• Using interfaces

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Derived Classes a Class Hierarchy
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Apparent vs. Actual Type

• Each constructed object has an apparent type and
  an actual type
• due to the rules of polymorphism
• only methods of an objects apparent type are
  available
• Java guarantees the apparent type of any object
  is an ancestor of its actual type (or the same)
• Ex
• Vector v new Vector()
• // what are vs apparent actual types?
• Object o1 v
• // what are o1s apparent actual types?
• int a 1, 2
• Object o2 a
• // what are o2s apparent actual types?
• Remember where polymorphism is particularly
  important?
• Method arguments return statements in the
  Collections Interface

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java.util.Hashtable

• A Hashtable maps keys to values
• done by mapping Objects (usually Strings) to
  Objects
• http//java.sun.com/j2se/1.5/docs/api/java/util/Ha
  shtable.html
• Ex
• Hashtable dates new Hashtable()
• dates.put("TODAY", new GregorianCalendar())
• dates.put("DDAY", new GregorianCalendar(
• 1944, Calendar.JUNE,6))
• dates.put("ENDOFWORLD", new GregorianCalendar(
• 2004, Calendar.OCTOBER, 27))
• Object obj dates.get("ENDOFWORLD")
• Calendar whatDate (Calendar)obj
• DateFormat df DateFormat.getDateInstance(DateFor
  mat.MEDIUM)
• String dateString df.format(whatDate.getTime())
  
• System.out.println(dateString)

• OUTPUT Oct 27, 2004

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java.util.ArrayList class

• ArrayList implements List
• can be passed to any method that takes a List
  object, like
• Collections.binarySearch
• uses Comparator for comparisons
• Collections.reverseOrder
• Collections.shuffle
• Collections.sort
• uses Comparable for comparisons
• You could also define your own class that
  implements List
• then define the 25 abstract methods in List

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Ex Allowing Employees to be sorted by salary

• public class Employee implements Comparable
• private String name
• private int salary
• public Employee(String initName, int
  initSal)
• name initName
• salary initSal
• public String getName() return name
• public int getSalary() return salary
• public void setSalary(int newSalary)
• salary newSalary
• public int compareTo(Object o)
• Employee otherEmp (Employee)o
• if (this.salary
  otherEmp.salary)
• return 0
• else if (this.salary gt
  otherEmp.salary)
• return 1
• else

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Using ArrayLists

• import java.util.
• public class CollectionsTester
• public static void main(String args)
• ArrayList staff new ArrayList()
• Employee e new Employee("Joe",100000)
• staff.add(e)
• e new Employee("Jane",200000)
• staff.add(e)
• e new Employee("Bob",66666)
• staff.add(e)
• Collections.sort(staff)
• Employee lowestPaid (Employee)staff.get(0)
• System.out.println(lowestPaid)

• Output Bob, 66666

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Assignment Compatibility

• You can not assign an object of an ancestor class
  to a variable of a derived class type
• Person joseph new Person("Joseph")
• Student sophomore joseph

sophomore (Student)joseph

• You can assign an object of a derived class to a
  variable of any ancestor type
• Student jane new Student("Jane Doe")
• Person doe jane
• Student janeDoe (Student)doe

OK
OK?

• Using casting, you can assign an object of a base
  class to an object of a derived class if it was
  previously of that type

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Undergraduate class

• public class Undergraduate extends Student
• private String level
• public Undergraduate(String initName)
• super(initName)
• level "U1"
• public String getLevel()return level
• public void setLevel(String changeLevel)
• level changeLevel

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Polymorphism via Inheritance

• public class PolymorphismExample
• public static void main(String args)
• Undergraduate joeShmo new Undergraduate("Joe
  Shmo")
• Student joe joeShmo
• Person shmo joeShmo
• System.out.println(shmo.getName() ",
• joe.getGpa() ", "
• joeShmo.getLevel())
• joe.setGpa(2.5)
• shmo.setAge(20)
• System.out.println(joeShmo.getName() ", "
• joeShmo.getGpa() ", "
• joeShmo.getAge())

OUTPUT Joe Shmo, 0.0, U1 Joe Shmo, 2.5, 20
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PolymorphismExample (continued)

• Undergraduate jane new Undergraduate("Jane
  Doe")
• jane.setLevel("U4")
• Student doe jane
• Person janeDoe doe
• Undergraduate undergrad (Undergraduate)janeDoe
  
• System.out.println(undergrad.getName() ", "
• undergrad.getGpa() ",
  "
• undergrad.getLevel())

OUTPUT Jane Doe, 0.0, U4
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Polymorphism via Interfaces

• public interface KnowItAll
• public String giveSolution()
• public class Boss implements KnowItAll
• public String name
• public Boss(String initName)
• name initName
• public String giveSolution()
• return "Now listen here..."

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InterfacePolymorphism Example

• public class InterfacePolymorphismExample
• public static void main(String args)
• Boss ceo new Boss("Mr. Burns")
• System.out.println(ceo.name " "
• ceo.giveSolution())
• ceo.name "Mr. Smithers"
• KnowItAll boss ceo
• System.out.println(boss.giveSolution())
• Boss president (Boss)boss
• System.out.println(president.name " "
• president.giveSolution())

OUTPUT Mr. Burns Now listen here... Now
listen here... Mr. Smithers Now listen here...
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Automatic Type Conversion

• public class AutoTypeConversionExample
• public static boolean haveSameName(Person p,
  Student s)
• return (p.getName().equals(s.getName()))
• public static void main(String args)
• Person john new Person("John")
• Person jonny new Person("Jonny Doe")
• Student doe new Student("John")
• Student johnDoe new Student("Doe")
• System.out.println(haveSameName(john, doe))
  
• System.out.println(haveSameName(john, jonny))
• System.out.println(haveSameName(doe, johnDoe))

true
ERROR
false
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Methods returning Objects

• Many times youll see methods that return Object
  values
• For example, inside Vector class
• public Object clone()
• Public Object elementAt(int index)
• You may be able to cast these values into other
  class types to place them in your variables
• For example
• Vector vector new Vector()
• Vector clonedVector (Vector)vector.clone()