Chapter 14' Assigning and comparing objects

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• Chapter 14. Assigning and comparing objects

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Introduction

• Objects are represented by a handle, that
  reference the storage of the object
• The operation of assignment and comparison for
  equality may be performed in two distinct ways

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Assignment deep and shallow copy
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Comparison for equality
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Assignment of object (1/2)

• In java the normal assignment operator
  implements a shallow copy of an object
• Remember in Java all objects are represented by a
  handle or pointer to the object storage

public static void main(String args)
Account mike new Account(50.0) Account
shallow_copy_mike mike mike.deposit(100.00)
System.out.println(mike.account_balance())
System.out.println(shallow_copy_mike.account_bal
ance())
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Assignment of object (2/2)

• A shallow copy the storage for the object is not
  duplicated whereas in a deep copy the storage is
• If a shallow copy of an object is made any
  changes to the object will be reflected in all
  the shallow copies

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Making an object cloneable Deep copy

• To make an instance of a class cloneable a class
  must
• Implement the interface Cloneable
• Override the method clone in the class Object

public interface Cloneable
protected Object clone() throws
CloneNotSupportedException

• Is required to provide a method clone
• Not be able to see the method clone in the
  superclass Object if it were not overridden with
  a public version of the method

public Object clone()
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An example (1/2)
class Account implement Cloneable private
double the_balance 0.0d private double
the_min_balance 0.0d // code for
constructors public Object clone()
//Object o null try
return super.clone() // o super.clone()
catch (CloneNotSupportedException
e) System.exit(-1)
return new Object() // return o
//code for the method // account_balance,
withdraw, deposit, set_min_balance
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An example (2/2)
class Main public static void main(String
args) Account mike new
Account(50.00) Account deep_copy_mike
(Account)mike.clone()
mike.deposit(100.00) System.out.println(
Balance account mike mike.account_balance())
System.out.println(Balance account
deep copy mike
deep_copy_mike.account_balance())

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mike
50.00
instance
Account
public Object clone()
Deep_copy_mike
clone
50.00
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Duplicating the instance variables of a class
directly

• The body of the method clone can be used to copy
  the individual instance variables of the class
  directly
• If any of the instance variables are themselves
  objects then they are deep copied using their
  clone method

class Account implements Cloneable private
double the_balance 0.0d private double
the_min_balance 0.0d public Object clone()
Account copy new Account()
copy.the_balance the_balance
copy.the_min_balance the_min_balance
return copy //code for the methods
constructors, account_balance, withdraw,
deposit
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Deep and shallow equality

• If the objects share the same storage, they are
  deeply equal
• If the objects do not share the same storage, but
  they do contain the same state information, then
  they are shallow equal

Deep copy
Shallow copy
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Use of for equality of objects Deep equality
(1/2)

• The normal operator implements deep equality
  of objects

//true //false //false
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Use of for equality of objects Deep equality
(2/2)
mike
50.00
Account
corinna
instance
50.00
miranda
mike mike //true mike corinna
//false mike miranda //false
100.00
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Use of equal for equality of objects Shallow
equality (1/2)

• By overriding the method equal(in the class
  Object) the user can implement appropriate code
  to test the equality of the two objects

//true //true //false
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Use of equal for equality of objects Shallow
equality(2/2)
mike
50.00
Account
corinna
instance
50.00
miranda
mike.equals(mike) //true mike.equals(corinna)
//true mike.equals(miranda) //false
100.00