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Objectives

On completion of this period, you would be able to learn• Access protection• Access modifiers• Applying access protection

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Recap

Packages • Name provide to a group of classes • A collection of related classes and interfaces • A name space management tool provided by

Java

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Access Protection

• What is access protection• In objects, certain members have to be

protected from unauthorized access• In C++, we learnt how to do such

protection• We used private, public, and protected

keywords for this purpose• Java also has such features

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Access Protection

• Access Protection• To protect the access of members of an

object• Done with the help of access modifiers• Keywords are – private, public and

protected• Access modifiers are also known as

Visibility Modifiers

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Access Protection

• Purpose of Access Protection• Enforces

• information hiding • encapsulation

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Access Modifiers

• Types of Access Modifiers• public • private• protected• default (no special keyword)

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Access Modifiers Contd..• public access

• Member is visible or accessible to the world• Any method of any object can use a public member• keyword : public • eg. : public int number;• public void sum(){ • …• }• Generally access methods are public

Put before return type

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Access Modifiers Contd..

• private access• Member is visible or accessible only within its

own class• Members cannot be inherited by subclasses• Members cannot be accessible in subclasses• Keyword : private

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Access Modifiers Contd..

• private access• eg. : private double average;• private void debug (String msg) {• …• }• Provides the highest degree of protection

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Access Modifiers Contd..• protected access

• Member is visible or accessible to• The current class• Subclasses of the current class• All classes that are in the same package as that

of the class• Keyword : protected• eg. : protected String message Str;• The level of protection is between public access

and default access

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Access Modifiers Contd..

• default access or package access• Members are accessible to

• All the classes in the same package• No special keyword is used

eg. : int length;

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Access Modifiers Contd..

Table 27.1: Class Member Accessibility

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Rules of thumb• public : if the member to be visible

everywhere• protected : if the member is visible in the

current package and also subclasses in other packages

• default : if the member to be visible in the current package

• private : if the member is to be used only within the class

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Example• Consider the access modifier for a member x in class A. If it is:• private – it can only be

used inside A.• default – it can be used

anywhere in package1, that is, A,B and C.

• protected – it can be used anywhere in package1, and in subclasses of A in other packages, here S.

• public – it can be used everywhere in the system

Fig. 27.1. Packages with its classes

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Example Contd..

• Consider the access modifier for the class B. If it is:• default – the class

can only be used in package1.

• public – the class can be used from anywhere.

Fig. 27.2. Packages with its classes

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Using Access Modifiers

• Guidelines :• Make all data members private• Create public getter and setter methods for data

members as necessary• If possible, make a method private, else make it

default, or make it public

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Example Program• With the help of the following program, we

will try to analyze the access protection features

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Example Program• Consider the Person class

• What access modifiers should be used, and which access should define?

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Example Programpublic class Person {

int SSNumber;

String name;

String address;

}

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Example Program Contd..public class Person {

private final int SSNumber;

private String name;

private String address;

public Person(int ssn, String name) {

SSNumber = ssn;

this. name = name;

}

public String getName() {

return name;

}

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Example Program Contd..

public String getAddress () {

return address;

}

public void setAddress (String address) { this. address = address;

}

}

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Example Program Contd..• The analysis is as follows• The SSNumber* must be given when the person object

is created, and cannot be changed later• The Name must be given when the object is created

Normally it will not change later• The address need not be present, but it can be changed

along the way

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Summary

• In this class we have discussed• Access protection• Access modifiers

• public, private, protected• How to use access modifiers

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Frequently Asked Questions

1. List the access modifiers in Java ?

2. What is the difference between default and protected modifiers ?

3. Explain how to use access modifiers

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1. We would like to make a member of a class visible In all subclasses regardless of what package they are in. Which keyword is useful for that ?1.private2.public3.protected4.No keyword (default)

Quiz

CM604.27 26

2. Which keyword can make a class visible within a package?

• private• public• protected• No keyword (default)

Quiz Contd..

CM604.27 27

3. Which keyword is useful for implementing information hiding principle?1.private2.public3.protected4.No keyword (default)

Quiz Contd..

CM604.27 28

4. Which one of the following is NOT a keyword related to Access protection?

1.private2.public3.protected4.default

Quiz Contd..