1. A Gentle Introduction to (Object-Oriented) Programming
Ugur Dogrusoz
Associate Professor, Computer Eng Dept, Bilkent University, Ankara, Turkey
December 2011
Computer Eng Dept, Bilkent Univ

2. Computer Eng Dept, Bilkent Univ
Computer hardware
Memory
Output Devices
Input Devices
Hard disk
ALU
Control Unit
Registers
Cache
CPU
Ugur Dogrusoz
Computer Eng Dept, Bilkent Univ

3. Computer Eng Dept, Bilkent Univ
Computer software
Source file
ADDF3 R1,R2, R3
noStudents++;
Executable file
Compiler / Linker
Operating System
Hardware
Application Programs
User
Specific computer
Specific JVM
Java Bytecode
Ugur Dogrusoz
Computer Eng Dept, Bilkent Univ

4. Computer Eng Dept, Bilkent Univ
How does a program work
A program tells a computer what to do, step by step, to get it to do something useful
Computers’ power comes from (nothing more than)
Accuracy
Speed
Ugur Dogrusoz
Computer Eng Dept, Bilkent Univ

5. Programming languages
Machine Code …
Lowest level: most efficient but (almost) humanly impossible to directly deal with
Assembly Language ADDF3 R1,R2, R3
Compiler / Interpreter Language noStudents++
Highest level: easier for humans to express their instructions in
Ugur Dogrusoz
Computer Eng Dept, Bilkent Univ

6. Object-oriented (OO) prog. languages
As opposed to procedural / functional / imperative approach
High level general purpose prog. languages
Each object (modeled after its real-life equivalent) has certain properties and responsibilities
Abstraction, encapsulation, polymorphism, reuse, maintenance
OO software = interacting collection of objects
Ugur Dogrusoz
Computer Eng Dept, Bilkent Univ

7. Computer Eng Dept, Bilkent Univ
OOP basics: memory
When a program is loaded into memory for execution:
Heap segment
Stack segment
Text (code) segment
Memory
Heap
Stack
Static data
Code
myObj = new Object();
int i = 9;
Ugur Dogrusoz
Computer Eng Dept, Bilkent Univ

8. Computer Eng Dept, Bilkent Univ
OOP basics: memory
class Node {
Object data;
Node next; }
… main(…) {
List myList = new List();
Integer i = new Integer(5);
myList.insertFront(i);
myList.insertRear(new Integer(8); }
class List {
private Node first;
public Object getFirst() {return(first.data);}
public void insertFront(Object newElement) { ... }
public void insertRear(Object newElement) { ... }
public void delete(Object existingElement) { ... } }
Ugur Dogrusoz
Computer Eng Dept, Bilkent Univ

9. Computer Eng Dept, Bilkent Univ
OOP basics: memory
class Node {
Object data;
Node next; }
… main(…) {
List myList = new List(); }
myList
first: null
List 1 3 2
class List {
private Node first;
public Object getFirst() {return(first.data);}
public void insertFront(Object newElement) { ... }
public void insertRear(Object newElement) { ... }
public void delete(Object existingElement) { ... } }
null
123
List
456
myList …
Heap
Stack
Ugur Dogrusoz
Computer Eng Dept, Bilkent Univ

10. Computer Eng Dept, Bilkent Univ
OOP basics: memory
class Node {
Object data;
Node next; }
… main(…) {
List myList = new List();
Integer i = new Integer(5); }
myList
first: null
List i
value: 5
Integer
class List {
private Node first;
public Object getFirst() {return(first.data);}
public void insertFront(Object newElement) { ... }
public void insertRear(Object newElement) { ... }
public void delete(Object existingElement) { ... } }
Ugur Dogrusoz
Computer Eng Dept, Bilkent Univ

11. Computer Eng Dept, Bilkent Univ
OOP basics: memory
class Node {
Object data;
Node next; }
… main(…) {
List myList = new List();
Integer i = new Integer(5);
myList.insertFront(i); }
myList
first
List i
value: 5
Integer
data
next: null
Node
class List {
private Node first;
public Object getFirst() {return(first.data);}
public void insertFront(Object newElement) { ... }
public void insertRear(Object newElement) { ... }
public void delete(Object existingElement) { ... } }
Ugur Dogrusoz
Computer Eng Dept, Bilkent Univ

12. Computer Eng Dept, Bilkent Univ
OOP basics: memory
class Node {
Object data;
Node next; }
… main(…) {
List myList = new List();
Integer i = new Integer(5);
myList.insertFront(i);
myList.insertRear(new Integer(8); }
myList
first
List i
value: 5
Integer
data
next
Node
next: null
value: 8
class List {
private Node first;
public Object getFirst() {return(first.data);}
public void insertFront(Object newElement) { ... }
public void insertRear(Object newElement) { ... }
public void delete(Object existingElement) { ... } }
Ugur Dogrusoz
Computer Eng Dept, Bilkent Univ

13. OOP basics: encapsulation/info hiding
Expose only what’s necessary
Hide implementation details
Unnecessary information should not be visible or modifiable
Ugur Dogrusoz
Computer Eng Dept, Bilkent Univ

14. OOP basics: encapsulation/info hiding
class MyRectangle {
private double width;
private double height;
private Point leftTop;
public double getWidth(){ return width; }
public void setWidth(double width){this.width=width;}
public void translate(double deltaX, double deltaY){…} }
Implementation details hidden
Proper class interface
Implementation changed
class MyRectangle {
private Rectangle rect;
public double getWidth(){ return rect.getWidth(); }
public void setWidth(double width){
rect.setSize(width, rect.getHeight()); }
public void translate(double deltaX, double deltaY){…}
Class interface unchanged
Ugur Dogrusoz
Computer Eng Dept, Bilkent Univ

15. OOP basics: reuse (inherit. & compo.)
Two main forms of re-use in software eng.:
Inheritance: ask whether instance of sub-class is-a(n) instance of super-class
Composition: ask whether whole has-a(n) part makes sense
Ugur Dogrusoz
Computer Eng Dept, Bilkent Univ

16. OOP basics: reuse (inherit. & compo.)
List
Stack
StringStack
+top()
+push(String)
+pop()
+getFirst()
+insertFront(Object)
+insertRear(Object)
+delete()
+top()
+push(Object)
+pop()
elements
elements
void push(Object newElement) {
elements.insertFront(newElement); }
void push(String newString) {
elements.push(newString); }
IncrStringStack
String pop() {
(String)(elements.pop()); }
Reuse through Delegation
+push(String)
void push(String newString) {
String top = getTop();
if (top==null || newString.compareTo(top)>0)
super.push(newString); }
Ugur Dogrusoz
Computer Eng Dept, Bilkent Univ

17. OOP basics: polymorphism
Entries in this file system are either files or folders or links to other entries
Files contain text
Single root
Ugur Dogrusoz
Computer Eng Dept, Bilkent Univ

18. OOP basics: polymorphism
Composite Design Pattern
Abstract methods needed for polymorphic use
Overriden in sub-classes for specific behavior
public File search(String aString) {
if (!this.isBroken())
return this.linkTo.search(aString); return null; }
public File search(String aString) {
if (this.contents.indexOf(aString) >= 0)
return this;
return null; }
public File search(String aString) {
for (Entry anEntry : this.entries) {
File aFile = anEntry.search(aString);
if (aFile != null)
return aFile; }
return null;
Example use of polymorphism
Ugur Dogrusoz
Computer Eng Dept, Bilkent Univ

19. Computer Eng Dept, Bilkent Univ
References
OOSE, Using UML, Patterns, and Java, 3rd Edition by Bernd Bruegge and Allen H. Dutoit, Prentice-Hall, 2010
Java Software Solutions, Foundations of Program Design by John Lewis and William Loftus, Addison-Wesley, 1998
Wikipedia,
Ugur Dogrusoz
Computer Eng Dept, Bilkent Univ