Download Object-Oriented Design in Java William Marsh

Overview
• Java language
inheritance and composition
interfaces
• Case study: reusable list
Object-Oriented Design in Java
Java collections
• Case study
William Marsh
AWT Listeners
• Design guidelines
Composition and Inheritance
Account
Java Classes
Transaction
Deposit
• ‘Account’ has-a ‘Transaction’
• ‘Deposit’ is-a ‘Transaction’
• ‘Withdraw’ is-a ‘Transaction’
Withdraw
Overriding Methods
Widening
Transaction
• Alternative implementation
of method of the superclass
•
•
•
•
toString()
same name
same number of parameters
same parameter types and
same return types
• If parameters are different,
the method is overloaded
rather than overridden
Deposit
Withdraw
toString()
toString()
• Every subclass object
is also a superclass
object
Deposit d = ... ;
Transaction t ;
t = d ;
Transaction
toString()
Deposit
Withdraw
toString()
toString()
Every ‘Deposit’ is also
a ‘Transaction’
Narrowing
Quiz 1
• What about:
• How to prevent a method being over-ridden?
• How to require a method to be over-ridden?
• How to call method from the super class?
Deposit d ;
Transaction t = ... ;
d = t;
compilation error
Not all transactions are deposits!
Deposit d ;
Transaction t = ... ;
d = (Deposit) t;
down cast
Quiz 2 - Result of this Programme?
class A {
int x ;
A(int z) {
x = z;
}
int m(int y){
return x + y ;
}
}
class B extends A {
B(int z) {
super(z)
}
int m(int y){
return x - y ;
}
}
A o = new A(1);
System.out.println(o.m(1));
Quiz 4 - Result of this Programme?
class A {
int x ;
A(int z) {
x = z;
}
int m(int y){
return x + y ;
}
}
class B extends A {
B(int z) {
super(z)
}
int m(int y){
return x - y ;
}
}
A o = new B(1);
System.out.println(o.m(1));
Quiz 3 - Result of this Programme?
class A {
int x ;
A(int z) {
x = z;
}
int m(int y){
return x + y ;
}
}
class B extends A {
B(int z) {
super(z)
}
int m(int y){
return x - y ;
}
}
B o = new B(1);
System.out.println(o.m(1));
Interfaces
• An ‘interface’ is similar to a
class
methods but no attributes
signature but no
implementation
• A ‘contract’
• Class ‘C’ implements
interface ‘I’
class must have both methods
class C implements I {
void m1() { ... }
void m2() { ... }
}
«interface»
I
m1()
m2()
C
m1()
m2()
Interfaces as Types - ‘Known As’
Interface v. Abstract Class
• C can be ‘known as’ I
• Class can
implement
multiple interfaces
class C implements I {
void m1() { ... }
void m2() { ... }
}
«interface»
I
m1()
m2()
C
C cVar = new C() ;
I iVar = cVar ;
«interface»
I
m1()
m2()
• Abstract class
may contain
implementation
«interface»
J
m3()
m4()
C
m1()
m2()
m1()
m2()
m3()
m4()
List of Transactions?
Account
addTransaction()
Case Study - Reusable Lists
first
Transaction
getNext()
setNext()
next
• List code integrated with application code
• No reuse
• No abstraction
Reusable List
List Interface
• Abstraction
‘list’ is an abstraction:
head, tail, append
more than one list
implementation
need an interface
• Methods to provide the ‘list abstraction’
List
public interface List {
public int size() ;
public boolean isEmpty() ;
public Object head() ;
public void append(Object item) ;
public Object removeHead() ;
}
LinkedList
• Reuse
ArrayList
class for each
implementation
List Implementation
Polymorphism
• ‘Many forms’
• List should be polymorphic
public class LinkedList implements List {
public
public
public
public
public
element of any type
• Java solution: elements have type Object
polymorphism by inheritance
int size() { ... }
boolean isEmpty() { ... }
Object head() { ... }
void append(Object item) { ... }
Object removeHead() { ... }
}
Quiz 5
• How to hold an integer in a list?
basic types are not objects
Quiz 6
• Implement ‘size’ and ‘isEmpty’
Linked List Implementation
Linked List Implementation
public class LinkedList implements List {
protected int count ;
protected Node first ;
public class LinkedList implements List {
protected int count ;
protected Node first ;
public int size() { return count ; }
public boolean isEmpty() {
return Node == null ;
}
/* what is Node ? */
}
}
Linked List Implementation
Linked List Implementation
public class LinkedList implements List {
protected int count ;
protected Node first ;
...
static protected class Node {
Object element ;
Node next ;
}
public class LinkedList implements List {
public Object head() {
ObjectElem head ;
if (first != null) {
head = first.element ;
}
return head ;
}
public void append(Object item) { ... }
public Object removeHead() { ... }
Quiz 7
public Object head() { ... }
public void append(Object item) { ... }
public Object removeHead() { ... }
}
public int size() { return count ; }
public boolean isEmpty() {
return Node == null ;
}
static protected class Node {
Object element ;
Node next ;
}
nested class
}
Using the List – A Price to Pay
class Account {
List transactions ;
/* what is in the list? */
printTransactionsAmounts() {
...
Quiz 8
}
}
Using the List
List trans ; /* list of transactions */
printTransactionsAmounts() {
while (! trans.isEmpty() ) {
Transaction t =
(Transaction) trans.removeHead() ;
System.out.println(t.getAmount()) ;
}
}
down cast
• List contains ‘Object’ - only the variable name
suggests content
• Program to the ‘List’ interface not
implementation
• WHOOPS - list has been destroyed
Iterating Through the List
Iteration Methods
List trans ; /* list of transactions */
printTransactionsAmounts() {
Node n = trans.first ;
while ( node != null ) {
Transaction t =
(Transaction) node.element ;
System.out.println(t.getAmount()) ;
}
}
• Problem - node class must be visible
• How to add iteration to the interface?
• Current position in the list
‘cursor’
void reset()
Move to the start of the list
Object next()
Return the element in the current
position and advance the position
boolean hasNext()
Return true if not at the end
Quiz 9 – Using ‘Iteration Methods’
Using ‘Iteration Methods’
• Rewrite printTransactionAmounts
• Rewrite printTransactionAmounts
List trans ; /* list of transactions */
List trans ; /* list of transactions */
printTransactionsAmounts() {
printTransactionsAmounts() {
trans.reset() ;
while ( trans.hasNext() ) {
Transaction t =
(Transaction) trans.next() ;
System.out.println(t.getAmount()) ;
}
}
...
}
Idea 1 – Add ‘Iteration Methods’ to List Interface
Idea 2 – Separate ‘Iterator Interface’
protected Node cursor ;
void reset() { cursor = first ; }
boolean hasNext() { return (cursor != null) ; }
Object next() {
Node n ;
if (cursor != null) {
n = cursor.element ;
cursor = cursor.next ;
}
return n ;
}
public interface Iterator {
void reset() ;
boolean hasNext() ;
Object next() ;
}
Iterator
• ‘LinkedList’ implements
‘List’ and ‘Iterator’
LinkedList
List
Ideas 1 & 2 – Problem
Idea 3 – Separate the Iterator
Iterator
• Problem
List
List
Iterator
LinkedList
LinkedListIterator
only one iterator
may need more
LinkedList
• List can have many iterators
• ‘List’ interface includes method to create an
iterator
public interface List {
...
public Iterator iterator() ;
}
Java Collection Classes
• Good news!
difficult algorithms implemented for you
• Interfaces
Java Collections
Collection
Map
Java provides a set of ‘collection’ classes;
what are they for and how are they used?
SortedSet
Set
List
SortedMap
Java Collection Classes
More Topics
• Good news!
• ‘Map’ interface and implementations
difficult algorithms implemented for you
• Implementations
finite functions
• How sorted sets and map are handled
• Lab sheet 3 – ‘Using Java Collections’
Collection
Set
TreeSet
List
HashSet
LinkedList
ArrayList
Listener Interfaces
• How to a user action (e.g. ‘button press’) to
code?
‘Listener’ Interfaces in
AWT/Swing
public interface ActionListener {
public void ActionPerformed (
ActionEvent e) ;
}
• Create an implementation of the listener
• Add it to the component
void
addActionListener(ActionListener l)
Adds the specified action listener to receive
action events from this button.
Listener Interfaces
• Different listeners for each interaction
actions
mouse clicks and movements
....
• Listeners often implemented as inner-classes
• See Lab sheet 2
Summary
Design Guidelines
Summary
• Separate interface from implementation
• Java language
interfaces
inner classes
• ... used to design reusable software
Java library
YOUR SOFTWARE
Reading
• General
Jia chapters 5 & 6
Bennet et al. chapters 11 & 13
• Collections
Jia section 8.2
• AWT/Swing
Jia section 8.3
• On the web
Collections
http://java.sun.com/docs/books/tutorial/collections/index.html
Article on interfaces and abstract classes
http://www.javaworld.com/javaworld/jw-09-2001/jw-0921-interface_p.html
Article on interfaces and composition
http://www.javaworld.com/javaworld/jw-12-1998/jw-12-techniques.html