The document discusses key concepts of object-oriented programming including classes, fields, constructors, methods, parameters, and conditional statements. It explains that classes contain fields to store an object's state, constructors to initialize an object, and methods to define an object's behavior. Methods can take parameters, return values, and use conditional statements to make choices. The document also provides examples of how these concepts are implemented in a TicketMachine class.

1. Understanding class
definitions
Looking inside classes
5.0

2. 2
Main concepts to be covered
• fields
• constructors
• methods
• parameters
• assignment statements
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

3. 3
Ticket machines – an external
view
• Exploring the behavior of a typical
ticket machine.
– Use the naive-ticket-machine project.
– Machines supply tickets of a fixed price.
• How is that price determined?
– How is ‘money’ entered into a machine?
– How does a machine keep track of the
money that is entered?
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

4. 4
Ticket machines
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
Demo

5. 5
Ticket machines – an internal
view
• Interacting with an object gives us
clues about its behavior.
• Looking inside allows us to determine
how that behavior is provided or
implemented.
• All Java classes have a similar-looking
internal view.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

6. 6
Fields, constructor, method
• Fields  store data for each object
to use
• Constructor  allow each object to
be set up
• Methods  implement the behavior
of the object
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

7. 7Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

8. 8
Basic class structure
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
public class TicketMachine
{
Inner part omitted.
}
public class ClassName
{
Fields
Constructors
Methods
}
The outer wrapper
of TicketMachine
The inner
contents of a
class

9. 9
Keywords
• Words with a special meaning in the
language:
– public
– class
– private
– int
• Also known as reserved words.

10. 10
Fields/medan/keadaan
• Fields menyimpan nilai
dari suatu objek.
• Dikenal juga sebagai
instance variables
karena nilai yang
disimpan fields
beragam tiap waktu.
• Pd BlueJ, gunakan
pilihan Inspect utk
menampilkan field dr
suatu objek.
• Fields mendefinisikan
keadaan suatu objek.
public class TicketMachine
{
private int price;
private int balance;
private int total;
Constructor and methods omitted.
}
private int price;
visibility modifier type variable name

11. 11Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

12. 12
Constructors
• Method Constructors
menginisialisasi suatu
objek.
• Namanya haruslah sama
dengan nama class.
• Menyimpan nilai awal dari
field.
• Bisa saja memiliki
eksternal parameter untuk
memberi nilai awal pada
field.
• Constructor have a special
role to fulfill, its
responsibility to put each
object of class
public TicketMachine(int ticketCost)
{
price = ticketCost;
balance = 0;
total = 0;
}

13. 13
Passing data via parameters
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
Parameters are another
sort of variable.

14. 14Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

15. 15
Assignment
• Suatu nilai disimpan dalam field (dan
variabel lainnya) via assignment
statements:
– variable = expression;
– price = ticketCost;
• Satu variabel menyimpan satu nilai,
jadi nilai sebelumnya akan hilang.

16. 16
Choosing variable names
• There is a lot of freedom over choice
of names. Use it wisely!
• Choose expressive names to make
code easier to understand:
– price, amount, name, age, etc.
• Avoid single-letter or cryptic names:
– w, t5, xyz123

17. 17
Methods
• Suatu Method merupakan implementasi perilaku
dari objek.
• Secara umum ada 3:
– Method constructor: digunakan untuk mencipta
objek dan memberi nilai awal pada objek yang
dicipta
– Method accessor
– Method mutator
• Struktur method terdiri dari bagian header dan
body.
• TicketMachine has four methods:
– getPrice, getBalance, insertMoney, printTicket

18. 18
Method structure
• The header provides the method’s
signature:
– public int getPrice()
• The header tells us:
– the name of the method
– what parameters it takes
– whether it returns a result
– its visibility to objects of other classes
• The body encloses the method’s
statements.

19. 19
Accessor methods
• Method Accessors: mengembalikan informasi
mengenai objek.
• Bagian header mendefinisikan method’s
signature. public int getPrice()
• Bagian body melingkupi semua method’s
statements.
• Terdapat statement return pada body.

20. 20
Accessor (get) methods
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
public int getPrice()
{
return price;
}
return type
method name
parameter list
(empty)
start and end of method body (block)
return statement
visibility modifier

21. 22
Test
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
public class CokeMachine
{
private price;
public CokeMachine()
{
price = 300
}
public int getPrice
{
return Price;
}
}
;
()
int
-
• What is
wrong
here?
(there are five
errors!)

22. 23
Mutator methods
• Struktur method yang sama: header dan
body.
• Digunakan untuk mengubah keadaan suatu
objek.
• Dengan cara mengubah nilai satu atau
lebih field.
– Umumnya mengandung assignment statements.
– Umumnya menerima parameters.

23. 24
Mutator methods
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
public void insertMoney(int amount)
{
balance = balance + amount;
}
return type
method name parameter
visibility modifier
assignment statementfield being mutated

24. 25
set mutator methods
• Fields often have dedicated set
mutator methods.
• These have a simple, distinctive
form:
– void return type
– method name related to the field name
– single parameter, with the same type as
the type of the field
– a single assignment statement

25. 26
A typical set method
public void setDiscount(int amount)
{
discount = amount;
}
We can infer that discount
is a field of type int, i.e:
private int discount;

26. 27Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

27. 28
Protective mutators
• A set method does not have to assign
the parameter to the field.
• The parameter may be checked for
validity and rejected if
inappropriate.
• Mutators thereby protect fields.
• Mutators support encapsulation.

28. 29
Printing from methods
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
public void printTicket()
{
// Simulate the printing of a ticket.
System.out.println("##################");
System.out.println("# The BlueJ Line");
System.out.println("# Ticket");
System.out.println("# " + price + " cents.");
System.out.println("##################");
System.out.println();
// Update the total collected with the balance.
total = total + balance;
// Clear the balance.
balance = 0;
}

29. 30Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

30. 31
String concatenation
• 4 + 5
9
• "wind" + "ow"
"window"
• "Result: " + 6
"Result: 6"
• "# " + price + " cents"
"# 500 cents"
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
overloading

31. 32
Quiz
• System.out.println(5 + 6 + "hello");
• System.out.println("hello" + 5 + 6);
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
11hello
hello56

32. 33
Method summary
• Methods implement all object behavior.
• A method has a name and a return type.
– The return-type may be void.
– A non-void return type means the method will
return a value to its caller.
• A method might take parameters.
– Parameters bring values in from outside for the
method to use.

33. 34
Lihat kembali mesin tiket yang
dibuat
• Beberapa perilaku masih kurang sesuai
seperti:
– Tiada pengecekan nilai uang yang masuk.
– Tiada kembalian.
– Tiada pengecekan initialization yg masuk
akal.
• Cara memperbaiki?
– Perlu perilaku yang lebih bagus dan pintar.

34. 35
Making choices in everyday life
• If I have enough money left, then I
will go out for a meal
• otherwise I will stay home and watch
a movie.

35. 36
Making a choice in everyday life
if(I have enough money left) {
go out for a meal;
}
else {
stay home and watch a movie;
}

36. 37
Making choices in Java
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
if(perform some test) {
Do these statements if the test gave a true result
}
else {
Do these statements if the test gave a false result
}
‘if’ keyword
boolean condition to be tested
actions if condition is true
actions if condition is false
‘else’ keyword

37. 38
Making a choice in the
ticket machine
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
public void insertMoney(int amount)
{
if(amount > 0) {
balance = balance + amount;
}
else {
System.out.println(
"Use a positive amount: " +
amount);
}
}

38. 39
Making for loops in Java
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
for(initialize;perform some test;update) {
Do these statements if the test gave a true result
}
‘for’ keyword
test variable
actions if condition is true
update variableInitialize variable

39. 40
Writing a for loop
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
int i;
int sum=0;
for(i=0;i<10;i++) {
sum = sum + i;
}

40. 41
Review
• Class terdiri dari fields, constructors
dan methods.
• Fields menyimpan nilai yang
menentukan keadaan suatu objek.
• Constructors mencipta objek.
• Methods memberikan implementasi
perilaku dari objek.

41. 42
Review
• Fields, parameters dan variabel lokal,
semuanya merupakan variabel.
• Fields ‘hidup’ selama objek tersebut
hidup.
• Parameters digunakan untuk menerima
nilai pada constructor atau method.
• Variabel lokal digunakan sebagai tempat
penyimpanan sementara.

42. 43
Review
• Objects dpt memiliki pilihan via
conditional (if) statements.
• A true or false test memungkinkan
hanya satu aksi yang dipilih dari
beberapa alternatif pilihan.

43. 44
Review (2)
• Fields, parameters and local variables
are all variables.
• Fields persist for the lifetime of an
object.
• Parameters are used to receive values
into a constructor or method.
• Local variables are used for short-lived
temporary storage.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

44. 45
Review (3)
• Methods have a return type.
• void methods do not return anything.
• non-void methods return a value.
• non-void methods have a return
statement.