JAVA
Object Oriented Programming
Objectives
• Be able to implement inheritance using
extends, super, and abstract
• Be able to describe differences between a
concrete and abstract class.
• Be able to recognize polymorphic behavior
3
You can get stuck at the object-based level because
you can quickly get there and you get a lot of benefit
without much mental effort. It’s also easy to feel like
you’re creating data types – you make classes and
objects, you send messages to those objects, and
everything is nice and neat.
But don’t be fooled. If you stop here, you’re missing
out on the greatest part of the language, which is the
jump to true object-oriented programming. You can do
this only with virtual [abstract] functions.
- Bruce Eckel, Thinking in C++,
4
Example: Analysis
• We want to make a payroll program that
lists each employee, some information
about them, and their pay.
...
Employee e = new Employee(0, ”Bashful",
"customer representative”,
10.0, 40.0);
System.out.println(e);
System.out.println(e.computePay());
...
OUTPUT:
0 Bashful
Role: customer representative
Payrate: 10.0
Hours Worked: 40.0
Pay: $400.0
Employee
+myId
+myName
+myRole
+myPayrate
+myHours
+toString()
+computePay()
5
Adding More Employee Types
• Our initial iteration’s design assumes that all employees
are paid in exactly the same way
• If we want a different kind of employee, we need to create
an entirely new type for it
• Bashful was an hourly employee, what about a salaried
employee?
• What would be the same?
• ID, Name, Role
• What would be different?
• No mention of hours, salary instead of payRate
6
Modeling Objects
• Object-Oriented programming models objects and the
relationships between them.
• Examples:
• figures, squares, rectangles, polygons, doodles;
• people, students, teachers, staff members, you;
• animals, birds, penguins, wings,
;
• naval vessels, submarines, carriers, fighter jets.
image from http://www.linux.org/info/penguin.html
7
Modeling and Implementing Relationships
We have seen two inter-object relationships:
• “is a” – instantiation
• implemented using constructor methods (for
reference objects);
• “has a” – aggregation
• implemented using instance data references;
• specifies container-contained relationships
between classes.
• The container references the contained.
Container
+referenceToContained
Contained
8
Modeling and Implementing Relationships
• Inheritance adds another kind of relationship:
• implemented using extends clauses;
9
Class Inheritance
• Inheritance specifies one-directional,
parent-child relationships.
• “is a kind of” – implemented using
extends clause
• The child inherits the parent’s:
• data
• methods
• Each child can polymorphically
“specialize” itself by overriding or adding
data or methods.
Parent
+parent's attributes
+parent's operations()
Child
+parent's attributes
+child's attributes
+parent's operations()
+child's operations()
10
Carl Linneaus (1707-1778)
Taxonomy
• Systema Naturae, 1758
• 7 levels:
• Kingdom
• Phylum
• Class
• Order
• Family
• Genus
• Species
images from www.linnean.org & www.kheper.auz.com
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Example: Java’s Classes
• All Java classes fit into one hierarchy.
●
All classes inherit from the root class:
java.lang.Object
• You can find the full Java hierarchy here:
http://java.sun.com/javase/6/docs/api/overview-tree.html
12
Example: Design of Employees
Employee
+myId
+myName
+myRole
+toString()
HourlyEmployee
SalariedEmployee
+myPayrate
+myHours
+mySalary
+computePay()
+computePay()
13
Implementing Inheritance
• A child class specifies inheritance from a parent class
using the extends clause.
• Java provides three modifiers specifying access for class
variables and methods:
• private
• protected
• public
• It can be safer to declare attributes as private and provide
protected accessor and mutator methods.
14
Super-Class Constructors
• super is a reference to the parent.
• A child can invoke its parent’s constructor.
super(parentConstructorArguments)
• A call to the parent’s constructor:
• must be the first statement in the constructor;
• is added automatically if you don’t add it.
• A child can invoke an overridden method:
super.parentMethod(arguments)
public class Employee {
private int myId;
private String myName, myRole;
public Employee() {
myId = 0;
myName = myRole = "";
}
public Employee(int id, String name, String role) {
myId = id;
myName = name;
myRole = role;
}
//accessors and mutators omitted for space
@Override
public String toString() {
return myId + " " + myName + "\n\tRole: " + myRole;
}
}
public class HourlyEmployee extends Employee {
private static final double WORK_WEEK = 40.0, OVERTIME_RATE = 1.5;
private double myPayrate, myHours;
public HourlyEmployee() {
myPayrate = myHours = 0.0;
}
public HourlyEmployee(int id, String name, String role, double payrate,
double hours) {
super(id, name, role);
myPayrate = payrate;
myHours = hours;
}
//accessor and mutators omitted for space, compute pay on comparison slide
@Override
public String toString() {
return super.toString() + "\n\tPayrate: " + myPayrate +
"\n\tHours worked: " + myHours;
}
}
17
Example: Design of Employees
Employee
+myId
+myName
+myRole
+toString()
HourlyEmployee
SalariedEmployee
+myPayrate
+myHours
+mySalary
+computePay()
+computePay()
public class SalariedEmployee extends Employee {
private double mySalary;
public SalariedEmployee() {
mySalary = 0.0;
}
public SalariedEmployee(int id, String name, String role, double salary) {
super(id, name, role);
mySalary = salary;
}
//accessors, mutators omitted for space, computePay() on following
@Override
public String toString() {
return super.toString() + "\n\tAnnual Salary: " + mySalary;
}
}
Salaried
Hourly
Comparison of computePay()
//Compute pay for hourly employee
@Override
public double computePay() {
if (myHours <= WORK_WEEK) {
return myPayrate * myHours;
} else {
return (myPayrate * WORK_WEEK) +
((myPayrate * OVERTIME_RATE) *
(myHours - WORK_WEEK));
}
}
//Compute pay for salaried employee
@Override
public double computePay() {
return mySalary;
}
public class PayrollConsole {
public static void main(String[] args) {
// 1. Employee only
Employee e = new Employee(0, ”Bashful", "customer representative");
System.out.println(e);
// 2. HourlyEmployee
HourlyEmployee he = new HourlyEmployee();
he.setPayrate(10.0);
he.setHours(50.0);
System.out.println(he.computePay());
he.setId(1);
he.setName("Sneezy");
he.setRole("intern");
System.out.println(he);
// 3. SalariedEmployee
SalariedEmployee se = new SalariedEmployee();
se.setId(2);
se.setName("Doc");
se.setRole("physician");
se.setSalary(100000.0);
System.out.println(se);
System.out.println(se.computePay());
}}
Fruit Example
Fruit
Apple
Orange
VALID OR INVALID??
Fruit fruit = new Gala();
Orange orange = new Orange();
Gala
Fuji
Orange p = new Apple();
Gala p = new Apple();
Apple p = new Fuji();
Examples
public class Test {
public static void main(String[] args) {
B b = new B();
}
}
class A {
public A(){
System.out.println("A's constructor invoked");
}
}
class B extends A{
}
Examples
public class Test {
public static void main(String[] args) {
B b = new B();
}
}
class A {
public A(int x){
System.out.println("A's constructor invoked");
}
}
class B extends A{
public B (){
System.out.println(“B’s constructor invoked”);
}
}
Examples
class Test {
public static void main(String[] args) {
A a = new B();
System.out.println(a.doThis());
}
}
class A {
public A(){
System.out.println("A's constructor invoked");
}
public String doThis(){
return “A’s doThis()”;
}
}
class B extends A{
public B (){
System.out.println(“B’s constructor invoked”);
}
public String doThis(){
return “B’s doThis()”;
}
}
Back to our goal
We want to be able to print out payroll information:
ArrayList<Employee> employees = new ArrayList<Employee>();
employees.add(new HourlyEmployee(1, "Sneezy", "intern", 10.0, 40.0));
employees.add(new SalariedEmployee(2, "Doc", "physician", 100000.0));
employees.add(new SalariedEmployee(3, ”Snow White", ”CEO", 175000.0));
employees.add(new SalariedEmployee(4, "Dopey", "trainer", 65000));
employees.add(new HourlyEmployee(5, "Sleepy", "trainee", 15.00, 40.00));
for (int i = 0; i < employees.size(); i++) {
System.out.println(employees.get(i));
System.out.println("\tPay: $" + employees.get(i).computePay());
}
Problem: There is no computePay() for a generic Employee!
26
Abstract Classes
• Classes can be abstract or concrete.
visibility abstract class className {
classDefinition
}
• Like concrete classes, abstract classes:
• Can have sub-classes;
• Can implement data and methods.
• Unlike concrete classes, abstract classes:
• Cannot be instantiated.
27
Abstract Methods
• As with classes, methods can also be declared as
abstract or concrete.
visibility abstract type methodName(params);
• Abstract classes do not provide definitions for their
abstract methods.
• Classes that contain abstract methods must be declared
as abstract.
28
Implementing Inheritance
• A child class specifies inheritance from a parent class
using the extends clause.
• Concrete sub-classes must define the abstract methods
that they inherit.
Parent
+aMethod()
Child1
+aMethod()
abstract class Parent {
public abstract void aMethod();
}
class Child1 extends Parent {
public void aMethod() {
// define method here...
}
}
29
Abstract Class
public abstract class Employee {
//private instance variables
public Employee(int id, String name, String role) {
myId = id;
myName = name;
myRole = role;
}
public abstract void computePay();
@Override
public String toString() {
return myId + " " + myName + "\n\tRole: " + myRole;
}
}
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Overriding Methods
• When an object to asked to execute a method, Java
searches up the inheritance hierarchy for a matching
method definition.
• Thus, a sub-class that defines its own version of a method
overrides any definitions of the methods that it inherits.
• A concrete sub-class must implement the abstract
methods it inherits using a method with an identical
signature.
31
public
class HourlyEmployee extends Employee {
private static final double WORK_WEEK = 40.0, OVERTIME_RATE = 1.5;
private double myPayrate, myHours;
public HourlyEmployee(int id, String name, String role,
double payrate, double hours) {
super(id, name, role);
myPayrate = payrate;
myHours = hours;
}
@Override
public double computePay() {
if (myHours <= WORK_WEEK) {
return myPayrate * myHours;
} else {
return (myPayrate * WORK_WEEK) +
((myPayrate * OVERTIME_RATE) * (myHours - WORK_WEEK));
}
}
@Override
public String toString() {
return super.toString() + "\n\tPayrate: " + myPayrate +
"\n\tHours worked: " + myHours;
}}
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Polymorphism
Parent
• Polymorphism allows different concrete
sub-classes to provide potentially different
definitions for a given method inherited
from their shared parent.
• Java chooses the appropriate method
definition based upon which child the
object instantiates at either:
• Compile time (static binding);
• Run time (dynamic binding).
+aMethod()
Child1
Child2
+aMethod()
+aMethod()
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Example
Parent
+aMethod()
List<Parent> myObjects =
new ArrayList<Parent>();
Child1
Child2
+aMethod()
+aMethod()
myObjects.add(new Child1(arguments));
myObjects.add(new Child2(arguments));
// add more children of either type...
for (int i = 0; i < myObjects.size(); i++) {
myObjects.get(i).aMethod(arguments);
}
Now with abstract Employee class
ArrayList<Employee> employees = new ArrayList<Employee>();
employees.add(new HourlyEmployee(1, "Sneezy", "intern", 10.0, 40.0));
employees.add(new SalariedEmployee(2, "Doc", "physician", 100000.0));
employees.add(new SalariedEmployee(3, ”Snow White", ”CEO", 175000.0));
employees.add(new SalariedEmployee(4, "Dopey", "trainer", 65000));
employees.add(new HourlyEmployee(5, "Sleepy", "trainee", 15.00, 40.00));
for (int i = 0; i < employees.size(); i++) {
System.out.println(employees.get(i));
System.out.println("\tPay: $" + employees.get(i).computePay());
}
This now works because Java knows that every
employee will have a computePay method.
35
Fredrick P. Brooks (1931- )
The Mythical Man-Month
What’s the
Big Idea
Joys of programming
Woes of programming
We enjoy designing things
because we are created in the
image of God.
The “mindless” details can be
excessively tedious.
The computer is a powerful
and rewarding tool to use.
Products become obsolete too
quickly.
As the child delights in his mud pie, so the adult enjoys building things, especially things
of his own design. I think this delight must be an image of God's delight in making
things, a delight shown in the distinctness and newness of each leaf and each snowflake.
- F. P. Brooks, Jr. The Mythical Man-Month, 1975
images from: http://www.amazon.com/