Red Bay, Labrador

CSE 114 – Computer Science IPolymorphism

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

Overriding Verses Overloading

Overriding

• Same method name

• Same signature• One method in

ancestor, one in descendant

Overloading

• Same method name

• Different signature

• Both methods can be in same class

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 > 0 && setValue <= numFaces)

upValue = setValue; }

public void setUpValue()

{ upValue = 1; }

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

Derived Classes: a Class Hierarchy

Person

Student Employee

Faculty StaffUndergraduate Graduate

MastersDegree NonDegreePhD

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 v’s apparent & actual types?Object o1 = v;// what are o1’s apparent & actual types?int[] a = {1, 2};Object o2 = a;// what are o2’s apparent & actual types?

• Remember where polymorphism is particularly important?– Method arguments & return statements in the Collections Interface

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/Hashtable.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(DateFormat.MEDIUM);String dateString = df.format(whatDate.getTime());System.out.println(dateString);

•OUTPUT: Oct 27, 2004

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

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 > otherEmp.salary) return 1; else return -1; } public String toString() { return name + ", $" + salary; }}

Using ArrayListsimport 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

• 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;

Assignment Compatibility• You can not assign an object of an ancestor

class to a variable of a derived class typePerson joseph = new Person("Joseph");

Student sophomore = joseph;

OK

Syntax Error

sophomore = (Student)joseph; Runtime ErrorClassCastException

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

Undergraduate classpublic 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; }}

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

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

Polymorphism via Interfacespublic 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..."; }

}

InterfacePolymorphism Examplepublic 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...

Automatic Type Conversionpublic 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

false

ERROR

Automatic type conversion from Student to Person

Methods returning Objects• Many times you’ll 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();

You must know what valid type casts are possible