1. Data Structures and Algorithms revision

2. Objects An Object is “some data/attributes”,
and a set of ways (methods) of changing/accessing the data.

3. Classes vs Objects You have a single class e.g. Person.
You can make many objects of the type Person.
Person p1 = Person();
// the default object
Person p2 = Person(“John”, 99);
// makes a person called p2 with the values John with age 99
Person p3 = p2;
//makes a person p3 ….
All 3 Person objects belong to the Person class
CSCU9A3 - Autumn 2015

4. Analogy: In an Office (before computers)
Imagine you work in an office (bank).
You have a paper form with blanks in
(e.g. name, address, salary)
When somebody opens a bank account – you photocopy the form and fill in their individual details.
The blank form is like a Class – there is only one blank form (THE MASTER COPY).
Each photocopied form is like an Object.
CSCU9A3 - Autumn 2015

5. One class, many objects A class is a template for making objects. E.g.
PERSON CLASS
Int
Age
String
Name P2 P1
PERSON
OBJECT
Int
Age 55
String
Name
dave
PERSON
OBJECT
Int
Age 98
String
Name
james

6. DANGER OF PUBLIC
If your data is public – it can be changed to anything
PERSON
CLASS
public
int
age
string
Name
Person p1 = new Person(“dave”,55);
PERSON
OBJECT
int
age
-100
string
Name
“dave”
P1.age = -100; //SHOULD NOT HAPPEN

7. Private data
If your data is private– you are forces to use the set methods to change it.
PERSON
CLASS
private
Int
age
string
Name
Person p1 = new Person(“dave”,55);
p1.age = -100; //SHOULD CANNOT HAPPEN
We use a setAge method
PERSON
OBJECT
int
Age 55
String
Name
“dave”

8. setAge method public void setAge(int a) { if (a > 0) {
this.age = a;
} else {
System.out.println("age not set, cannot be negative"); }

9. Overloaded Methods
A class can have a number of methods with the same name, but different parameters
Example: println methods, one for each different type of thing you might want to print:
public void println(int output)
public void println(double output)
public void println(String output)
CSCU9A3 - Autumn 2015

10. Overloaded Methods
This solve the problem of having to explicitly tell the program which method:
public void printlnINT(int output)
public void printlnDOUBLE(double output)
public void printlnSTRING(String output)
CSCU9A3 - Autumn 2015

11. Encapsulation
Encapsulation means we do not need to know the details of how the data is stored.
You just need to know how to use it.
This is sometimes called “data hiding”
We do not want to expose the internal workings of out class – but have an agreed set of methods by which we communicate with other classes.

12. e.g. counter class A counter class with method
Increment (adds one to the count)
Reset (sets the count to zero)
getCount (give the current number)
(DO AN EXAMPLE WITH THE CLASS)
This is the interface of the class – it is just the methods you can see (and the constructor)

13. Encapsulation - example
A counter class with methods
Increment and getCount.
Could store an integer, which is incremented by one each time the counter is pressed.
Could store a string “ ”, and append “1” each time. The length represents the count.
Could store a number 1000, and multiply by 10 each time we press counter. The count would just be log of the number.

14. Syntax 3.1 Instance Variable Declaration
Big Java by Cay Horstmann
Copyright © 2009 by John Wiley & Sons. All rights reserved.

15. Accessing Instance Variables
The count method advances the counter value by 1:
public void count() {
value = value + 1; }
The getValue method returns the current value:
public int getValue()
return value;
Private instance variables can only be accessed by methods of the same class
Big Java by Cay Horstmann
Copyright © 2009 by John Wiley & Sons. All rights reserved.

16. Instance Variables
Encapsulation is the process of hiding object data and providing methods for data access
To encapsulate data, declare instance variables as private and declare public methods that access the variables
Encapsulation allows a programmer to use a class without having to know its implementation
Information hiding makes it simpler for the implementor of a class to locate errors and change implementations
Protects your data (get/set methods accessor/mutator methods) from others and from yourself!!!
How else could the class counter be implemented instead of using int ???? (float, string…)
Big Java by Cay Horstmann
Copyright © 2009 by John Wiley & Sons. All rights reserved.

17. Equals (==) for primitive data types
When comparing primitive data types
int
boolean
float
double
char, …
USE x == y
NOT x=y (means x becomes the value y)

18. Equals with objects Person p1 = new person(“John”, 55);
== compares memory address!!!
Need to use .equals
String name1 = “John”;
String name2 = “John”;
name1==name2;// give False
name1.equals(name2); // gives True

19. Steps to check equality
1/ is the object null
2/ is the object the same type as you want e.g. Person, or Percent or Thermometer..
3/ now check each of the relevant fields for equality. (usually all fields)

20. Example Percent public boolean equals(Object object) {
if (object == null) {
return false;
} else if (!(object instanceof Percent )) {
} else {
Percent percent = (Percent ) object;
return this.percent == percent.getPercent(); }

21. Static vs non-Static
Static variables are the same for every instance of a class
Non-Static variables are different for each instance of a class
E.g. we all have an individual blood pressure (e.g. 140 over 100 mmHg) non-static
But the normal for all of us is 120 over 80 mmHg.
What about height, weight and indicators of obesity as body mass index.
CSCU9A3 - Autumn 2015

22. Object References Rectangle box = new Rectangle(5, 10, 20, 30);
Object reference: describes the location of an object
The new operator returns a reference to a new object:
Rectangle box = new Rectangle();
Multiple object variables can refer to the same object:
Rectangle box = new Rectangle(5, 10, 20, 30);
Rectangle box2 = box;
box2.translate(15, 25);
Primitive type variables ≠ object variables
Big Java by Cay Horstmann
Copyright © 2009 by John Wiley & Sons. All rights reserved.

23. Object Variables and Number Variables
Big Java by Cay Horstmann
Copyright © 2009 by John Wiley & Sons. All rights reserved.

24. Object Variables and Number Variables
Big Java by Cay Horstmann
Copyright © 2009 by John Wiley & Sons. All rights reserved.

25. Inheritance Means you can use the methods of the super class.
This saves you time writing more java which is common to a set of classes.
The common methods and data attributes can abstracted out as a super class.

26. Inheritance e.g. 1 E.g. in the lecture/tutorial we looked at vehicles.
All vehicles (car, coach, train) have a weight/color/passengerCapacity/fuelType.
Things specific to the subclass can be stored only in that class e.g. for coach
doubleDecker = True
hasToilet = False

27. Inheritance e.g. 2
If you are storing information about people, there are commonalities that all people have
E.g. name, nationality, height, date of birth.
For different people you store different information
E.g. Professor class extends Person
String areaOfExpertize = “blackholes”;
String Chair = “Lucasian ”;
String university = “Cambridge”;
Int numberBooksPublished = 23;

28. Syntax 9.1 Inheritance Big Java by Cay Horstmann
Copyright © 2009 by John Wiley & Sons. All rights reserved.

29. Overriding Methods public class BankAccount { . . .
Example: deposit and withdraw methods of the CheckingAccount class override the deposit and withdraw methods of the BankAccount class to handle transaction fees:
public class BankAccount {
. . .
public void deposit(double amount) { }
public void withdraw(double amount) { }
public double getBalance() { } }
public class CheckingAccount extends BankAccount
public void deductFees() { }
Big Java by Cay Horstmann
Copyright © 2009 by John Wiley & Sons. All rights reserved.

30. Overriding Methods
A subclass method overrides a superclass method if it has the same name and parameter types as a superclass method
When such a method is applied to a subclass object, the overriding method is executed
Big Java by Cay Horstmann
Copyright © 2009 by John Wiley & Sons. All rights reserved.

31. Overriding Methods
Use the super reserved word to call a method of the superclass:
public class CheckingAccount extends BankAccount {
public void deposit(double amount)
transactionCount++;
// Now add amount to balance
super.deposit }
Big Java by Cay Horstmann
Copyright © 2009 by John Wiley & Sons. All rights reserved.

32. Syntax 9.2 Calling a Superclass Method
Big Java by Cay Horstmann
Copyright © 2009 by John Wiley & Sons. All rights reserved.

33. Overriding methods. The subclass may override superclass methods.
We do not want this to happen in some cases
E.g. getAge
To stop overriding say
final getAge
To force overriding say
abstact getAge