Transcript Objects and Methods

Objects & Methods
Defining Classes
Slide 2
Reference Variables Revisited

Remember: Object variables are
references (aka pointers)



Point to “null” by default
Need to call new to create
a new object
Different variables can point to same
object at the same time
Slide 3
Reference Variables Revisited

Think of Objects as balloons!


references are like strings (aka “handles”)
Idea from “Beginning Java Objects” by Jacquie
Barker (Wrox)
BankAccount x, y, z;
x = new BankAccount();
y = x;
z = new BankAccount();
y = z;
x = z;
x
(null)
y
(null)
z
(null)
Slide 4
Reference Variables Revisited

Think of Objects as balloons!


references are like strings (aka “handles”)
Idea from “Beginning Java Objects” by Jacquie
Barker (Wrox)
BankAccount x, y, z;
x = new BankAccount();
y = x;
z = new BankAccount();
y = z;
x = z;
x
y
(null)
z
(null)
Slide 5
Reference Variables Revisited

Think of Objects as balloons!


references are like strings (aka “handles”)
Idea from “Beginning Java Objects” by Jacquie
Barker (Wrox)
BankAccount x, y, z;
x = new BankAccount();
y = x;
z = new BankAccount();
y = z;
x = z;
x
y
z
(null)
Slide 6
Reference Variables Revisited

Think of Objects as balloons!


references are like strings (aka “handles”)
Idea from “Beginning Java Objects” by Jacquie
Barker (Wrox)
BankAccount x, y, z;
x = new BankAccount();
y = x;
z = new BankAccount();
y = z;
x = z;
x
y
z
Slide 7
Reference Variables Revisited

Think of Objects as balloons!


references are like strings (aka “handles”)
Idea from “Beginning Java Objects” by Jacquie
Barker (Wrox)
BankAccount x, y, z;
x = new BankAccount();
y = x;
z = new BankAccount();
y = z;
x = z;
x
y
z
Slide 8
Reference Variables Revisited

Think of Objects as balloons!


references are like strings (aka “handles”)
Idea from “Beginning Java Objects” by Jacquie
Barker (Wrox)
BankAccount x, y, z;
x = new BankAccount();
y = x;
z = new BankAccount();
y = z;
x = z;
x
y
z
Slide 9
Reference Variables Revisited

Think of Objects as balloons!


references are like strings (aka “handles”)
Idea from “Beginning Java Objects” by Jacquie
Barker (Wrox)
BankAccount x, y, z;
x = new BankAccount();
y = x;
z = new BankAccount();
y = z;
x = z;
x
y
z
Something to think about
Suppose


int a;
a = 150000;
int b = a;
a = a + 300000;
int c = b;
…
BankAccount x = new BankAccount();
x.deposit( 150000 );
BankAccount y = x;
x.deposit( 300000 );
int z = y.getBalance();
Slide 10
What is the final value
of c? z? Why?
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
Answer: c is 150000,
while z is 450000
Why? Because a
primitive-type variable
copies the actual value,
while object-type
variables copies the
reference
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b is independent from a
on the other hand, y and
x refer to the same
BankAccount instance.
(It’s a joint account!)
Objects (Part 2)
Defining Classes
Using BankAccount objects
BankAccount aliceAccount = new BankAccount();
BankAccount bobAccount = new BankAccount();
BankAccount chuckAccount = new BankAccount();
aliceAccount.deposit( 250 );
bobAccount.deposit( 100 );
int x = chuckAccount.getBalance();

Note: all BankAccount instances have the same structure
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int balance field
int getBalance(), deposit(int), and withdraw(int) methods
But each BankAccount instance has a distinct identity
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each instance has its own values for fields (e.g., balance)
methods work on an instance’s own balance
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aliceAccount.deposit( 250 ) changes only aliceAccount’s balance, not
bobAccount’s
Slide 12
Classes
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A Class describes the general structure of
objects belonging to that class
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fields/attributes (state)
methods (behavior)
e.g., The BankAccount class says that:
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
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Slide 13
all BankAccount objects have its own balance field of
type int
all BankAccount objects have a deposit method
which increments the object’s own balance field
A Class is like a “recipe” or “template”
Slide 14
Defining Classes
public class
{
BankAccount
private int balance;
Class name
(must be same as file name)
// current amount
public BankAccount()
{
// does nothing. defaults to balance=0
}
public int getBalance()
{
return balance;
}
public void deposit( int amount )
{
balance += amount;
}
}
Fields
Constructors
Methods
Applets are Objects too!
Class name
public class BankApplet1
{
BankAccount account;
extends
IOApplet
extends BankApplet1 “is a”
IOApplet, and “inherits”
code from it. (More later!)
Fields
public void setup()
{
(data members)
account = new BankAccount();
addInput( "Amount" );
addButton( "Deposit" );
addOutput();
println( "Balance is P" + account.getBalance() + "." );
}
public void onButtonPressed()
{
int amount = getInt( "Amount" );
account.deposit( amount );
clearOutput();
println( "Balance is P" + account.getBalance() + "." );
}
}
Slide 15
Methods
Slide 16
Method Declaration
<modifier>
<return type>
<method name>
( <parameters>
{
<statements>
}
Modifier
public
Return Type
void
Method Name
deposit
Parameter
(
int amount
{
balance += amount;
}
Statements
)
)
Value-Returning Method
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We call a method that returns a value a valuereturning method , or non-void method.
A value-returning method must include a return
statement in the following format:
return <expression> ;
public int getBalance( )
{
return balance;
}
Slide 17
Slide 18
How Methods Work
BankApplet.java
…
int aliceBalance =
aliceAccount.getBalance();
…
When the line above is run …
1) Find object pointed
to by aliceAccount
2) Find code for class
of that object
3) Find code for getBalance()
4) Run code
5) Return value
6) Use returned value
State of Memory
aliceBalance
aliceAccount
BankAccount
int balance
100
Slide 19
How Methods Work
BankApplet.java
…
int aliceBalance =
aliceAccount.getBalance();
…
When the line above is run …
1) Find object pointed
to by aliceAccount
2) Find code for class
of that object
3) Find code for getBalance()
4) Run code
5) Return value
6) Use returned value
State of Memory
aliceBalance
aliceAccount
BankAccount
getBalance()
int balance
100
Slide 20
How Methods Work
BankApplet.java
…
int aliceBalance =
aliceAccount.getBalance();
…
When the line above is run …
1) Find object pointed
to by aliceAccount
2) Find code for class
of that object
3) Find code for getBalance()
4) Run code
5) Return value
6) Use returned value
BankAccount.java
public class BankAccount
{ private int balance; // current amount
public BankAccount()
{ // does nothing. defaults to balance=0
}
public int getBalance()
{ return balance;
}
public void deposit( int amount )
{ balance += amount;
}
}
State of Memory
aliceBalance
aliceAccount
BankAccount
getBalance()
int balance
100
Slide 21
How Methods Work
BankApplet.java
…
int aliceBalance =
aliceAccount.getBalance();
…
When the line above is run …
1) Find object pointed
to by aliceAccount
2) Find code for class
of that object
3) Find code for getBalance()
4) Run code
5) Return value
6) Use returned value
BankAccount.java
public class BankAccount
{ private int balance; // current amount
public BankAccount()
{ // does nothing. defaults to balance=0
}
public int getBalance()
{ return balance;
}
public void deposit( int amount )
{ balance += amount;
}
}
State of Memory
aliceBalance
aliceAccount
BankAccount
getBalance()
int balance
100
Slide 22
How Methods Work
BankApplet.java
…
int aliceBalance =
aliceAccount.getBalance();
…
When the line above is run …
1) Find object pointed
to by aliceAccount
2) Find code for class
of that object
3) Find code for getBalance()
4) Run code
5) Return value
6) Use returned value
BankAccount.java
public class BankAccount
{ private int balance; // current amount
public BankAccount()
{ // does nothing. defaults to balance=0
}
public int getBalance()
{ return balance;
}
public void deposit( int amount )
{ balance += amount;
}
}
public int getBalance()
{
return balance;
}
State of Memory
aliceBalance
aliceAccount
BankAccount
getBalance()
int balance
100
Slide 23
How Methods Work
BankApplet.java
…
int aliceBalance =
aliceAccount.getBalance();
…
When the line above is run …
1) Find object pointed
to by aliceAccount
2) Find code for class
of that object
3) Find code for getBalance()
4) Run code
5) Return value
6) Use returned value
BankAccount.java
public int getBalance()
{
return balance;
}
State of Memory
aliceBalance
aliceAccount
BankAccount
getBalance()
100
int balance
100
Slide 24
How Methods Work
BankApplet.java
…
int aliceBalance =
aliceAccount.getBalance();
…
When the line above is run …
1) Find object pointed
to by aliceAccount
2) Find code for class
of that object
3) Find code for getBalance()
4) Run code
5) Return value
6) Use returned value
BankAccount.java
public int getBalance()
{
return balance;
}
State of Memory
aliceBalance
100
aliceAccount
BankAccount
getBalance()
100
int balance
100
Three Kinds of Variables

Field (aka Attribute or Instance Variable)
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Variables declared inside a class’ code, but outside any methods
Part of object’s “permanent” state
Use for state that is retained between method calls
Local Variable
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

Slide 25
Variables declared inside a method definition
Only exists while we’re inside the method
Use as a “scratchpad” (temporary storage) during a
computation
Parameter
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Variables declared in the parentheses of a method definition
Holds a copy of the value or reference passed as an argument
to the method call
Is also a local variable – i.e., only exists inside the method
Slide 26
Sample Method
From Wu’s CurrencyConverter class …
public double fromDollar(
double
dollar
)
{
double
amount, fee;
Parameter
Local
Variables
fee
= exchangeRate - feeRate;
amount
= dollar * fee;
return amount;
Fields *
}
* Although not shown here, exchangeRate and feeRate were declared
inside the CurrencyConverter class, but outside any methods
Local Variables Example
Code
A
amt
= yenConverter.fromDollar( 200 );
Slide 27
public double fromDollar( double
dollar )
{
double amount, fee;
fee
= exchangeRate - feeRate;
amount = dollar * fee;
return amount;
}
At A before fromDollar
yenConverter
Currency
Converter
amt
0.0
A. fromDollar’s local
variables (amount
and fee) do not exist
before the method
call
exchangeRate
129.2315
feeRate
5.0
State of Memory
Local Variables Example
Code
public double fromDollar( double
dollar )
{
double amount, fee;
fee
= exchangeRate - feeRate;
amount = dollar * fee;
return amount;
}
amt
= yenConverter.fromDollar( 200 );
After B
yenConverter
Currency
Converter
exchangeRate
129.2315
feeRate
5.0
Slide 28
is executed
amt
dollar
0.0
200.0
amount
fee
State of Memory
B
B. Memory space is
allocated for the local
variables and
parameter.
Parameter’s value is
copied from the
argument.
Slide 29
Local Variables Example
Code
public double fromDollar( double
dollar )
{
double amount, fee;
fee
= exchangeRate - feeRate;
amount = dollar * fee;
return amount;
}
amt
= yenConverter.fromDollar( 200 );
C
After C
yenConverter
Currency
Converter
exchangeRate
129.2315
feeRate
5.0
is executed
amt
dollar
amount
0.0
C. Computed
200.0
24846.3
124.2315
fee
State of Memory
values are
assigned to the
local variables.
Local Variables Example
Code
amt
= yenConverter.fromDollar( 200 );
D
Slide 30
public double fromDollar( double
dollar )
{
double amount, fee;
fee
= exchangeRate - feeRate;
amount = dollar * fee;
return amount;
}
At D after fromDollar
yenConverter
Currency
Converter
amt
24846.3
exchangeRate
129.2315
feeRate
5.0
State of Memory
D. Memory space
for local variables
and parameters is
deallocated upon
exiting the
fromDollar method.
Three Kinds of Variables

Field (aka Attribute or Instance Variable)




Variables declared inside a class’ code, but outside any methods
Part of object’s “permanent” state
Use for state that is retained between method calls
Local Variable




Slide 31
Variables declared inside a method definition
Only exists while we’re inside the method
Use as a “scratchpad” (temporary storage) during a
computation
Parameter



Variables declared in the parentheses of a method definition
Holds a copy of the value or reference passed as an argument
to the method call
Is also a local variable – i.e., only exists inside the method
Passing Parameters



Slide 32
Arguments are matched to parameters from left to
right. Types must match
The number of arguments in the method call must
match the number of parameters in the method
definition
Arguments are passed to a method using the pass-byvalue scheme



Parameters and arguments do not have to have the same name
Whether or not they have the same name, parameters are
separate copies of the arguments
Parameters are local to the method, i.e., they only exist while
inside the method. Changes made to the parameters will not
affect the value of corresponding arguments
Pass-By-Value Scheme
Code
A
x = 10;
y = 20;
tester.myMethod( x, y );
Slide 33
public void myMethod( int one, float
two )
{
one = 25;
two = 35.4f;
}
At A before myMethod
x
State of
Memory
y
10
A. Local variables do
20
10
not exist before the
method execution
Slide 34
Pass-By-Value Scheme
Code
x = 10;
y = 20;
tester.myMethod( x, y );
public void myMethod( int one, float
two )
{
one = 25;
two = 35.4f;
}
Values are copied at
x
State of
Memory
y
10
20
10
one
two
B
B
10
B. The values of
20.0f
10
arguments are copied
to the parameters.
Slide 35
Pass-By-Value Scheme
Code
x = 10;
y = 20;
tester.myMethod( x, y );
public void myMethod( int one, float
two )
{
one = 25;
two = 35.4f;
}
C
After C
x
State of
Memory
y
10
20
10
is executed
one
two
25
10
C. The values of
35.4f
10
parameters are
changed.
Pass-By-Value Scheme
Slide 36
Code
x = 10;
y = 20;
tester.myMethod( x, y );
D
public void myMethod( int one, float
two )
{
one = 25;
two = 35.4f;
}
At D after myMethod
x
State of
Memory
y
10
D. Parameters are
20
10
erased. Arguments
remain unchanged.
Constructors





Slide 37
A constructor is a special method that is called
with the new command
Used for initializing an object to a valid state
Name of a constructor must be the same as the
name of the class
No return type
If no constructor is defined, the Java compiler
will include a default constructor with no
arguments and no body
Slide 38
Defining Constructors

A constructor will have the following form:
public <class name> ( <parameters>
{
<statements>
}
Modifier
public
Class Name
BankAccount (
Parameter
)
)
Currently,
BankAccount’s
constructor has no
arguments and does
nothing.
{
Statements
}
Multiple Constructors

A class can include multiple constructors
without any problem, as long as the
constructors defined for the class have either



Slide 39
A different number of parameters
Different data types for the parameters if the
number of parameters is the same
This is known as “overloading” and can also be
done with ordinary methods
public MyClass( int
public MyClass(
value ) { … }
) { … }
public MyClass( float value ) { … }
These constructors will
not conflict with each
other, and therefore, valid.
public MyClass( String name, float value ) { … }
A Common Misconception
Slide 40
public class StudentRecord
{
private String name; // current amount
public StudentRecord( String name )
{
}
… methods (not shown) …
}
Some people think that you can
set a field by simply giving the
parameter same name as the
field. THIS DOES NOT WORK.
The parameter and the field are
two different and independent
variables, even if they have the
same name.
Slide 41
The Correct Way (for now)
public class StudentRecord
{
private String name; // current amount
public StudentRecord( String initialName )
{
name = initialName;
}
Give the
parameter a
different name in
order to be clear.
(We’ll discuss
another way
later.)
… methods (not shown) …
}
Don’t forget to set the
field through an
assignment statement.
Access Modifiers




Slide 42
public and private designate the accessibility of
fields and methods
private means code in other classes cannot
access it
public means anybody can access it
no modifier means public within the same
directory (more later when we get to “packages”)
in another class …
class Test
{
public int memberOne;
private int memberTwo;
}
Test myTest = new MyTest();
myTest.memberOne = 10;
myTest.memberTwo = 20;
Keeping fields private


Slide 43
In general, fields should be private so we can
have the flexibility of changing the
implementation details of the class
e.g., Suppose we want to keep a log of deposits


if balance is public, we cannot guarantee that
deposits will be logged because anyone can increment
balance directly
if balance is private, we can simply modify the
deposit method


Since users can only increment balance by calling deposit, all
deposits will be logged.
Users don’t even have to know that logging is taking place