Objects and Encapsulation

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Objects and Encapsulation
(incomplete) James Brucker
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Why Encapsulation?

• Encapsulation means that an object contains both
  data and methods to operate on the data.
• An object encapsulates its data (state).
• Responsibility an object is responsible for its
  own data
• Benefit of Encapsulation
• Enables you to change implementation without
  affecting other objects.
• To enforce this, an object usually declares its
  data members (attributes) to be protected or
  private.
• This practice is called data hiding.
• You can hide behavior, too -- methods can be
  private.

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Why Encapsulation?

• Encapsulation means that an object contains both
  data and methods to operate on the data.
• An object encapsulates its data (state).
• Responsibility an object is responsible for its
  own data
• Benefit of Encapsulation
• You to change the implementation without
  affecting other classes.
• Enforce data integrity a class's mutator method
  can check for valid data before saving it.
• Makes programs easier to test and maintain.

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Person class
public class Person private String
name private Date birthday / create a new
person / public Person( String name, Date
birthday ) this.name name this.birthday
birthday / accessor but no mutator
methods / public String getName() return
name public Date getBirthday() return
birthday
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Person class modified
Public methods don't change, only the internal
format of data changes.
public class Person private String
name private Calendar birthday / create a
new person / public Person( String name, Date
date ) this.name name this.birthday
new Calendar() this.birthday.setTime( date
) / accessor but no mutator methods /
public String getName() return name public
Date getBirthday() return
birthday.getTime() // creates a Date
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Mutability and Encapsulation
public class Person private String
name private Date birthday / create a new
person / public Person( String name, Date
birthday ) this.name name this.birthday
birthday / accessor but no mutator
methods / public String getName() return
name public Date getBirthday() return
birthday
Can you change a peron's name or birthday?.
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Mutability and Encapsulation
public class Person private String
name private Date birthday / create a new
person / public Person( String name, Date
birthday ) this.name name this.birthday
birthday / accessor but no mutator
methods / public String getName() return
name public Date getBirthday() return
birthday
Can you change a peron's name or birthday?.
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Breaking Encapsulation

• Can you change a Person's name (after creating
  object)?
• Can you change the Person's birthday (after
  creating object)?

Date bday new Date( 98, 1, 15 ) // Feb
15,1998 Person pee new Person( "Joe", bday )
pee.getName().toUpperCase()
pee.getBirthday().setMonth( 11 )
System.out.println( pee.getName() )
System.out.println( pee.getBirthday() )
bday.setYear( 0 ) System.out.println(
pee.getBirthday() )
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Mutable and Immutable Objects

• Mutable attributes of object can be changed
  after object is created.
• Immutable attributes of object cannot be changed
  after object is created.

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Mutable and Immutable Objects

• Which types are mutable? Which are immutable?
• String s new String( "Hello" ) s.toUpperCase()
  
• Integer n new Integer( 10 ) n
• Date d new Date( ) d.setMonth( 0 )
• StringBuffer sb new StringBuffer()
• sb.append("more")

Use BlueJ codepad to try this. Create 2
references to same object.
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Immutable Objects
private String name / accessor method for
name / public String getName( ) return
name

• this does not break encapsulation of Person even
  though it gives the caller a reference to the
  name of Person.
• Why?
• Because String objects are immutable.

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Test Yourself (1)
public class Person private StringBuffer
address // use Stringbuffer / accessor
method for address. / public StringBuffer
getAddress( ) return address

• does this break the encapsulation of the
  (private) address ?

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Fixing Encapsulation

• Accessor methods for mutable objects should
  always return a copy of the object, or an
  immutable form of the object.
• Return a copy

public StringBuffer getAddress( ) // return
a copy of the address return new
StringBuffer(address)

• convert to a String (immutable and also a copy)

public String getAddress( ) // return String
form of address return address.toString()
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Constructor can break encapsulation

• Does this constructor violate encapsulation of
  the private birthday attribute?

public class Person private String name //
person's name private Date birthday // person's
birthday / constructor for a new Person
/ public Person( String name, Date date )
this.name name this.birthday date
public Date getBirthday( ) return
birthday
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Constructor can break encapsulation

• Does this constructor violate encapsulation of
  the private birthday attribute?

public class Person private String name //
person's name private Date birthday // person's
birthday / constructor for a new Person
/ public Person( String name, Date date )
this.name name this.birthday
date.clone() public Date getBirthday( )
return birthday.clone()
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Constructor (2)
name "Ae Naresuan" date new Date(60, 2,
1) ae new Person(name,date)
name
reference
date
reference
public class Person Person(String aname,
Date date) this.name aname this.birthday
date
String "Ae Naresuan"
Date year 60 month 2 day 1 seconds 0
What does the constructor do?
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Constructor (3)
name "Ae Naresuan" date new Date(60, 2,
1) ae new Person(name,date)
name
reference
date
reference
public class Person Person(String aname,
Date date) this.name aname this.birthday
date
aePerson name String birthday Date
String "Ae Naresuan"
Date year 60 month 2 day 1 seconds 0
Constructor sets the attributes to refer to the
same objects as the parameters.
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Constructor (4)
// change the date date.setYear(80) date.setMonth
(11)
name
reference
date
reference
The "ae" Person object refers to the same Date
object as the caller. Any changes in date also
change ae's birthday.
aePerson name String birthday Date
String "Ae Naresuan"
Date year 80 month 11 day 1 seconds 0
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Constructor (5)

• Let's create a couple of people

public void dateTest() // Ae is born in March
1, 1960 Date date new Date(60,Calendar.MARCH,
1) Person ae new Person("Ae Naresuan",
date) // Be is born on April 20, 1960 // I'm
too lazy to make a new date object... date.setMon
th(Calendar.APRIL) date.setDate(20) Person be
new Person("Be Careful", date) // when was
Ae born? System.out.println("Ae's birthday is
" ae.getBirthday().toString() )
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Constructor how to fix

• Constructor should create a copy of any mutable
  objects.

public class Person private String name //
person's name private Date birthday // person's
birthday / constructor for a new Person
/ public Person( String aname, Date date )
this.name aname // the Date class
implements Cloneable this.birthday
date.clone( )
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Sometimes We Want Mutable Components
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Bank Account

• We want the Person data in Bank Account to change
  when Person updates his address, telephone, etc.

owner
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Breaking Encapsulation by Mutable Components
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Example Transaction

• KUCoupon has a Transaction class that saves an
  array of coupons

public class Transaction private Integer
coupons public Integer getCoupons( )
return coupons // return array //
constructor public Transaction( Integer
coupons ) this.coupons coupons
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Example Transaction (2)

• KUCoupon has a Transaction class that saves an
  array of coupons

// use autoboxing to convert int to
Integer Integer x new Integer 1, 2, 3
Transaction transaction new Transaction( x
) x1 15 Integer y
trans.getCoupons( ) y2 20
1. Does this change the coupons1 in transaction
?
2. Does this change the coupons2 in transaction
?
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The Cause

• An array is a mutable object.

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The Solution

• Make a copy of mutable objects.

public class Transaction private int
coupons public int getCoupons( ) return
java.util.Arrays.copyOf( coupons,
coupons.length )// return copy //
constructor - copy the parameter public
Transaction( int c ) this.coupons
Arrays.copyOf( c, c.length )
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This May Not Be Efficient

• Making a copy of a large array uses memory and
  time.
• In some application, you may not want to do this.