CS 102
Object Oriented Programming
Access Modifiers
Bank Account – version 9
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Ozyegin University - CS 102 - Object Oriented Programming
Bank Account – version 9
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Ozyegin University - CS 102 - Object Oriented Programming
Definition of deposit
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¨ We should not allow depositing negative amount of
money.
¨ How?
Source: http://www.merriam-webster.com/dictionary/deposit
Ozyegin University - CS 102 - Object Oriented Programming
deposit function
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Ozyegin University - CS 102 - Object Oriented Programming
deposit function
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Ozyegin University - CS 102 - Object Oriented Programming
deposit function
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Ozyegin University - CS 102 - Object Oriented Programming
Bank Account
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¨ Can you think of any other controls that we should
have?
Ozyegin University - CS 102 - Object Oriented Programming
Bank Account
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¨ Can you think of any other controls that we should
have?
¨ A bank account should get a number during
initialization.
¨ A bank account should not have negative initial
balance.
Ozyegin University - CS 102 - Object Oriented Programming
Constructors
10
¨ Assume that we don’t have the interest rate
¨ We have the following constructors:
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Constructors
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¨ A bank account should get a number during
initialization.
Ozyegin University - CS 102 - Object Oriented Programming
Constructors
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¨ A bank account should get a number during
initialization.
¨ Remove the following constructor.
Ozyegin University - CS 102 - Object Oriented Programming
Constructors
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¨ A bank account should get a number during
initialization.
Ozyegin University - CS 102 - Object Oriented Programming
Constructors
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¨ A bank account should not have negative initial
balance.
Ozyegin University - CS 102 - Object Oriented Programming
Negative Initial Balance
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Constructors
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¨ A bank account should not have negative initial
balance.
¨ We should have check the initial balance.
Ozyegin University - CS 102 - Object Oriented Programming
Constructors
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¨ A bank account should not have negative initial
balance.
¨ We should have check the initial balance.
¨ If it is negative, the balance should be 0.
Ozyegin University - CS 102 - Object Oriented Programming
What is the output?
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Ozyegin University - CS 102 - Object Oriented Programming
What is the output?
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Ozyegin University - CS 102 - Object Oriented Programming
So, are we done?
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¨ With changing the constructor and the deposit
function, are we sure that balance will not be a
negative amount?
Ozyegin University - CS 102 - Object Oriented Programming
What is the output?
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Ozyegin University - CS 102 - Object Oriented Programming
What is the output?
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Ozyegin University - CS 102 - Object Oriented Programming
Class instances
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¨ The class instances need to be protected.
¨ We need to keep the control of how these instances
are accessed.
¨ How?
Ozyegin University - CS 102 - Object Oriented Programming
Class instances
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¨ The class instances need to be protected.
¨ We need to keep the control of how these instances
are accessed.
¨ How?
¤ Encapsualtion: A mechanism that provides protection of
some features of the class from the users.
Ozyegin University - CS 102 - Object Oriented Programming
Class instances
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¨ The class instances need to be protected.
¨ We need to keep the control of how these instances
are accessed.
¨ How?
¤ Encapsualtion: A mechanism that provides protection of
some features of the class from the users. à Through
using access modifiers.
Ozyegin University - CS 102 - Object Oriented Programming
Access Modifier
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¨ They are used to set access levels for classes,
variables, and other entries.
Ozyegin University - CS 102 - Object Oriented Programming
Access Specification
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¨ Access modifier
¤ For the top level classes it can be either
n public or : visible to the earth
n default (no keyword) : visible only within the same package
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Access Specification
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¨ These variables don’t have any particular access
modifier, therefore they are visible …?
Ozyegin University - CS 102 - Object Oriented Programming
Access Specification
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¨ These variables don’t have any particular access
modifier, therefore they are visible and accessible
from only within the same package (package-
private).
Ozyegin University - CS 102 - Object Oriented Programming
Access Specification
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¨ Let’s try to use them from outside the package
Ozyegin University - CS 102 - Object Oriented Programming
Access Specification
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Ozyegin University - CS 102 - Object Oriented Programming
Access Specification
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Ozyegin University - CS 102 - Object Oriented Programming
Access Specification
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Ozyegin University - CS 102 - Object Oriented Programming
Access Specification
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number, balance and currency
are visible in everywhere!
No access
related errors!
Ozyegin University - CS 102 - Object Oriented Programming
Important!!!
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¨ However, making everything public is not the solution.
¤ When something is public, it can be accessed and also can
be modified from everywhere!
¨ It is also not a good idea to leave it package-private.
¤ In default case (without any access modifier) that
information can be accessed and modified everywhere
within the package.
¨ These are not optimum solutions.
¨ You should encapsulate that information and limit its
access and make sure that it can be modified only
within your control.
Ozyegin University - CS 102 - Object Oriented Programming
Controlling Access to Entries
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¨ Each entry (class, class instance, member function) in
a Java class is marked with one of the following
keywords to control which classes have access to
that entry:
¤ public
¤ private
¤ no keyword (default)
¤ protected
Ozyegin University - CS 102 - Object Oriented Programming
Controlling Access to Entries
37
¨ Each entry (class, class instance, member function) in
a Java class is marked with one of the following
keywords to control which classes have access to
that entry:
¤ public: the entry is accessible from everywhere
¤ private
¤ no keyword (default)
¤ protected
Ozyegin University - CS 102 - Object Oriented Programming
Controlling Access to Entries
38
¨ Each entry (class, class instance, member function) in
a Java class is marked with one of the following
keywords to control which classes have access to
that entry:
¤ public
¤ private: the entry is accessible only within the class,
invisible everywhere outside the class
¤ no keyword (default)
¤ protected
Ozyegin University - CS 102 - Object Oriented Programming
Controlling Access to Entries
39
¨ Each entry (class, class instance, member function) in
a Java class is marked with one of the following
keywords to control which classes have access to
that entry:
¤ public
¤ private
¤ no keyword (default): entry is accessible to classes
inside the same package, invisible to all the others.
package private.
¤ protected
Ozyegin University - CS 102 - Object Oriented Programming
Controlling Access to Entries
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¨ Each entry (class, class instance, member function) in
a Java class is marked with one of the following
keywords to control which classes have access to
that entry:
¤ public
¤ private
¤ no keyword (default)
¤ protected: entry is accessible to the class itself, other
classes inside the same package and all subclasses.
Ozyegin University - CS 102 - Object Oriented Programming
Access Modifiers
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¨ Which one is the most restrictive one?
¤ public
¤ private
¤ no keyword (default)
¤ protected
Ozyegin University - CS 102 - Object Oriented Programming
Access Modifiers
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¨ Which one is the most restrictive one?
¤ public
¤ private
¤ no keyword (default)
¤ protected
Ozyegin University - CS 102 - Object Oriented Programming
Access Modifiers
43
¨ Which one is the least restrictive one?
¤ public
¤ private
¤ no keyword (default)
¤ protected
Ozyegin University - CS 102 - Object Oriented Programming
Access Modifiers
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¨ Which one is the least restrictive one?
¤ public
¤ private
¤ no keyword (default)
¤ protected
Ozyegin University - CS 102 - Object Oriented Programming
Access Modifiers
45
¨ Rank them in increasing order of restrictiveness?
¤ public
¤ private
¤ no keyword (default)
¤ protected
Ozyegin University - CS 102 - Object Oriented Programming
Access Modifiers
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¨ Rank them in increasing order of restrictiveness?
¤ public
¤ private
¤ no keyword (default)
¤ protected
¨ Answer:
¤ public, protected, default, private
n protected entities can be accessed by subclasses in other
packages
Ozyegin University - CS 102 - Object Oriented Programming
Access Modifiers: Access levels
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¨ private: the class itself
¨ default: private + classes inside the same package
¨ protected: default + all subclasses
¨ public: all classes
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Access Modifiers: Access levels
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Class Package Subclass World
public
protected
default
private
Source: http://docs.oracle.com/javase/tutorial/java/javaOO/accesscontrol.html
Ozyegin University - CS 102 - Object Oriented Programming
Access Modifiers: Access levels
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Class Package Subclass World
public Y Y Y Y
protected Y Y Y N
default Y Y N N
private Y N N N
Source: http://docs.oracle.com/javase/tutorial/java/javaOO/accesscontrol.html
Ozyegin University - CS 102 - Object Oriented Programming
For most of the cases…
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¨ Class instances should be private
¤ Only the class itself can access these variables
¤ They are visible only inside the class definition
n Only member functions of the class can access them
¤ They are invisible outside the class
¤ Therefore, the control is on the class itself only.
¨ There may be times for exceptions.
¤ Example: during inheritance
Ozyegin University - CS 102 - Object Oriented Programming
For most of the cases…
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¨ Class methods should be public or private
¤ public if they will be used publicly
¤ private if they are useful for another class function but
not to be used by other classes directly
¨ There can be exceptions to these.
Ozyegin University - CS 102 - Object Oriented Programming
Bank Account – version 10
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¨ Class instances
¨ Member functions were public already
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No write access
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No read access
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Ozyegin University - CS 102 - Object Oriented Programming
Accessing Class Instances
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¨ Since class instances are private, we won’t have
direct access to those instances
¤ no read or write access
¨ How can we access them?
Ozyegin University - CS 102 - Object Oriented Programming
Accessing Class Instances
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¨ Since class instances are private, we won’t have
direct access to those instances
¤ no read or write access
¨ How can we access them?
¤ by using getters and setters
Ozyegin University - CS 102 - Object Oriented Programming
Getters and Setters
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¨ Get and set methods allow customized access to
class instances
¤ getter for read access
n returns the class instance without modifying
¤ setter for write access
n modifies the class instance
n mostly assigns the function argument’s value to the class
instance
Ozyegin University - CS 102 - Object Oriented Programming
An example getter function
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¨ getter for read access
¤ returns the class instance without modifying
Ozyegin University - CS 102 - Object Oriented Programming
An example getter function
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¨ getter for read access
¤ returns the class instance without modifying
¤ What other getter functions do we need?
Ozyegin University - CS 102 - Object Oriented Programming
Getter Function
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Ozyegin University - CS 102 - Object Oriented Programming
Setters
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¨ Using private for class instances give more control to
the class.
¨ The class can enforce legal value assignments
through setters.
Ozyegin University - CS 102 - Object Oriented Programming
An example setter function
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¨ setter for write access
¤ modifies the class instance
¤ mostly assigns the function argument’s value to the class
instance
Ozyegin University - CS 102 - Object Oriented Programming
An example setter function
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¨ setter for write access
¤ modifies the class instance
¤ mostly assigns the function argument’s value to the class
instance
¤ Do we need other setter functions?
Ozyegin University - CS 102 - Object Oriented Programming
Setter Functions
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¨ Do we need other setter functions?
¤ account number
n Initialized when an account is created
n Cannot be changed afterwards
¤ account balance
n We don’t use a set function but instead
n Deposit: to put money in a bank account
n Withdraw: to remove money from a bank account
Source:http://www.merriam-webster.com/dictionary
Ozyegin University - CS 102 - Object Oriented Programming
deposit and withdraw functions
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¨ We already have the deposit function
Ozyegin University - CS 102 - Object Oriented Programming
deposit and withdraw functions
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¨ Can you write down the withdraw function?
Ozyegin University - CS 102 - Object Oriented Programming
deposit and withdraw functions
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¨ Can you write down the withdraw function?
¤ Do not let withdraw if
n withdraw amount is negative
n withdraw amount is larger than the balance
¤ Otherwise
n withdraw the money and update the balance
Ozyegin University - CS 102 - Object Oriented Programming
deposit and withdraw functions
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Ozyegin University - CS 102 - Object Oriented Programming
deposit and withdraw functions
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Ozyegin University - CS 102 - Object Oriented Programming
deposit and withdraw functions
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setCurrency function
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¨ Lets review setCurrency function
¨ 1 USD = 18. 22TL
¨ How should we modify the above function?
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setCurrency function
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What is the output?
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Unknown currency?
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¨ What happens in the following case?
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Unknown currency?
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¨ How can we fix this setCurrency function?
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Fixing setCurrency Function
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Unknown currency?
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¨ The same thing can happen in constructor as well.
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Unknown currency?
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¨ The same thing can happen in constructor as well.
¨ In default we should set it to “TL”
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Fixing Constructors (Account V12)
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Code Repetition
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Code Repetition
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How can we write a
function for this check?
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Private Function
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Private Function
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Private Function
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Private Function
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¨ Functions which are helper functions to other
member functions should be kept private.
¤ Private function can be accessed from within the class.
¤ Private function can not be accessed from outside the
class.
Ozyegin University - CS 102 - Object Oriented Programming
Get and Set Functions
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¨ Setter methods usually begins with ‘set’ prefix.
¤ setCurrency
¨ Getter methods usually begins with ‘get’ prefix.
¤ getCurrency
¤ But there is an exception for Boolean values
n For Boolean values the prefix ‘is’ usually used.
Ozyegin University - CS 102 - Object Oriented Programming
Boolean Get Functions (Account V13)
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¨ Getter methods usually begins with ‘get’ prefix.
¤ But there is an exception for Boolean values
n For Boolean values the prefix ‘is’ usually used.
¤ Assume that some accounts can be active while some of
them are not. They can be on hold.
n Keep active information within a boolean
Ozyegin University - CS 102 - Object Oriented Programming
Boolean Get Functions (Account V13)
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¨ Getter methods usually begins with ‘get’ prefix.
¤ But there is an exception for Boolean values
n For Boolean values the prefix ‘is’ usually used.
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Get Functions (Account V13)
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Get Functions (Account V13)
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¨ For set functions you can still use ‘set’ prefix
¤ setActive
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Ways of printing out the object - 1
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¨ get methods for accessing class instances one by one
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Ways of printing out the object - 2
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¨ report method for printing report of the account
Ways of printing out the object
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¨ Similar to other primitive types, can we just use the
object inside System.out.println() function?
¨ What do you think the output will look like?
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Ways of printing out the object
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¨ Similar to other primitive types, can we just use the
object inside System.out.println() function?
Ozyegin University - CS 102 - Object Oriented Programming
Ways of printing out the object
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¨ Similar to other primitive types, can we just use the
object inside System.out.println() function?
¨ In order to get smt meaningful, we need to override
toString method of the class.
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toString method
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¨ toString method tells Java how to display an object
of the class.
¨ It returns a String representation of the object.
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toString method
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103 Any Questions ?
Ozyegin University - CS 102 - Object Oriented Programming
