3 class definition

Understanding class definitions

Looking inside classes

5.0

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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

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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?


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Ticket machines


Demo

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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.


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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


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Basic class structure


public class TicketMachine { Inner part omitted. }

public class ClassName { Fields Constructors Methods }

The outer wrapper of TicketMachine

The inner contents of a

class

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Keywords

• Words with a special meaning in the language: – public – class – private – int

• Also known as reserved words.

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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

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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;

}

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Passing data via parameters


Parameters are another sort of variable.

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

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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.

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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

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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

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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.

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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.

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Accessor (get) methods


public int getPrice() { return price; }

return type method name

parameter list (empty)

start and end of method body (block)

return statement

visibility modifier

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Test


public class CokeMachine { private price; public CokeMachine() { price = 300 } public int getPrice { return Price; } }

;

()

int

-

• What is wrong here?

(there are five errors!)

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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.

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Mutator methods


public void insertMoney(int amount) { balance = balance + amount; }

return type method name parameter

visibility modifier

assignment statement field being mutated

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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

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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;


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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.

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Printing from methods


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; }


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String concatenation

• 4 + 5 9

• "wind" + "ow" "window"

• "Result: " + 6 "Result: 6"

• "# " + price + " cents" "# 500 cents"


overloading

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Quiz

• System.out.println(5 + 6 + "hello");

• System.out.println("hello" + 5 + 6);


11hello

hello56

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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.

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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.

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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.

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Making a choice in everyday life

if(I have enough money left) { go out for a meal; } else { stay home and watch a movie; }

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Making choices in Java


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

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Making a choice in the ticket machine


public void insertMoney(int amount) { if(amount > 0) { balance = balance + amount; } else { System.out.println( "Use a positive amount: " + amount); } }

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Making for loops in Java


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 variable Initialize variable

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Writing a for loop


int i; int sum=0; for(i=0;i<10;i++) { sum = sum + i; }

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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.

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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.

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Review

• Objects dpt memiliki pilihan via conditional (if) statements.

• A true or false test memungkinkan hanya satu aksi yang dipilih dari beberapa alternatif pilihan.

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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.


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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.

Date post:	01-Nov-2014
Category:	Education
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3 class definition

Education