Understanding class definitions: Exploring source code

Object-oriented programming with Java
Ahmed Al-Ajeli
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Understanding class definitions
Exploring source code
Ahmed Al-Ajeli
Lecture 2
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Main concepts to be covered
•fields
•constructors
•methods
•parameters
•assignment statements
Object-oriented programming with Java
Ahmed Al-Ajeli
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3
Ticket machines – an external view
•Exploring the behaviour of a typical ticket machine.
–Use the naive-ticket-machine project
(We will assume the customer is honest)
–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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public class TicketMachine
{
// The price of a ticket from this machine.
private int price;
// The amount of money entered by a customer so far.
private int balance;
// The total amount of money collected by this machine.
private int total;
public TicketMachine(int cost)
{
price = cost;
balance = 0;
total = 0;
}
Demo of naïve-ticket-machine
Object-oriented programming with Java
Ahmed Al-Ajeli
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5
public int getPrice()
{
return price;
}
public int getBalance()
{
return balance;
}
public void insertMoney(int amount)
{
balance = balance + amount;
}
public void printTicket()
{
// Simulate the printing of a ticket.
System.out.println("##################");
System.out.println("# The Eclipse Line");
System.out.println("# Ticket");
System.out.println("# " + price + " cents.");
System.out.println("##################");
System.out.println();
total = total + balance;
balance = 0;
}
}
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Ticket machines – an internal view
•Interacting with an object gives us clues about its behaviour.
•Looking inside allows us to determine how that behaviour is provided or implemented.
•All Java classes have a similar-looking internal view.
Object-oriented programming with Java
Ahmed Al-Ajeli
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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.
•Always entirely lower-case.
Object-oriented programming with Java
Ahmed Al-Ajeli
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Fields
•Fields store values for an object.
•They are also known as instance variables.
•Fields define the state of an object.
•Some values change often.
•Some change rarely (or not at all).
public class TicketMachine
{
private int price;
private int balance;
private int total;
Further details omitted.
}
private int price;
visibility modifier
type
variable name
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Constructors
•Initialize an object.
•Have the same name as their class.
•Public visibility (since called by the user)
•Close association with the fields:
–Initial values stored into the fields.
–Parameter values often used for these.
public TicketMachine(int cost)
{
price = cost;
balance = 0;
total = 0;
}
Object-oriented programming with Java
Ahmed Al-Ajeli
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Constructing objects
-Classname objectname = new Classname
(parameters list);
-TicketMachine ticketMachine1= new
TicketMachine (500);
•What will happen?
- The new operator makes a
TicketMachine object.
- It invokes (calls) the constructor
- It returns the object.
pattern
example
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Assignment
•Values are stored into fields (and other variables) via assignment statements:
–variable = expression;
–balance = balance + amount;
•A variable can store just one value, so any previous value is lost.
pattern
example
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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
•Methods implement the behaviour of objects.
•Methods have a consistent structure comprised of a header (signature) and a body.
•Accessor methods provide information about an object.
•Mutator methods alter the state of an object.
•Other sorts of methods accomplish a variety of tasks.
Object-oriented programming with Java
Ahmed Al-Ajeli
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Method structure
•The header:
–public int getPrice()
•The header tells us:
–the visibility to objects of other classes;
–whether the method returns a result;
–the name of the method;
–whether the method takes parameters.
•The body encloses the method’s statements.
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Calling (activating) methods
•Calling a method within an object is similar to sending a message to that object.
- Objectname.methodname (parameter list)
- ticketmachine1.getPrice()
pattern
example
Actual parameters
Object-oriented programming with Java
Ahmed Al-Ajeli
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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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Accessor methods
•An accessor method always has a return type that is not void.
•An accessor method returns a value (result) of the type given in the header.
•The method will contain a return statement to return the value.
•NB: Returning is not printing!
Object-oriented programming with Java
Ahmed Al-Ajeli
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Test
•What is wrong here?
public class CokeMachine
{
private price;
public CokeMachine()
{
price = 300
}
public int getPrice
{
return Price;
}
(there are five errors!)
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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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Ahmed Al-Ajeli
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Mutator methods
•Have a similar method structure: header and body.
•Used to mutate (i.e., change) an object’s state.
•Achieved through changing the value of one or more fields.
–They typically contain one or more assignment statements.
–Often receive parameters.
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Mutator methods
public void insertMoney(int amount)
{
balance = balance + amount;
}
return type
method name
formal parameter
visibility modifier
assignment statement
field being mutated
Object-oriented programming with Java
Ahmed Al-Ajeli
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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 formal 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 easily infer that discount is a field of type int, i.e: private int discount;
Object-oriented programming with Java
Ahmed Al-Ajeli
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Protective mutators
•A set method does not have to always assign unconditionally 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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String concatenation
•4 + 5
9
•"wind" + "ow"
"window"
•"Result: " + 6
"Result: 6"
•"# " + price + " cents"
"# 500 cents"
overloading
Object-oriented programming with Java
Ahmed Al-Ajeli
14
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Quiz
•System.out.println(5 + 6 + "hello");
•System.out.println("hello" + 5 + 6);
11hello
hello56