Access Protection
- 1. Objectives On completion of this period, you would be able to learn • Access protection • Access modifiers • Applying access protection 1
- 2. Recap Packages • Name provide to a group of classes • A collection of related classes and interfaces • A name space management tool provided by Java 2
- 3. 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 3
- 4. 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 4
- 5. Access Protection • Purpose of Access Protection • Enforces • information hiding • encapsulation 5
- 6. Access Modifiers • Types of Access Modifiers • public • private • protected • default (no special keyword) 6
- 7. 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(){ • … Put before return type • } • Generally access methods are public 7
- 8. 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 8
- 9. Access Modifiers Contd.. • private access • eg. : private double average; • private void debug (String msg) { • … • } • Provides the highest degree of protection 9
- 10. 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 10
- 11. 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; 11
- 12. Access Modifiers Contd.. Table 27.1: Class Member Accessibility 12
- 13. 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 13
- 14. 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 14 Fig. 27.1. Packages with its classes
- 15. 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 15
- 16. 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 16
- 17. Example Program • With the help of the following program, we will try to analyze the access protection features 17
- 18. Example Program • Consider the Person class • What access modifiers should be used, and which access should define? 18
- 19. Example Program public class Person { int SSNumber; String name; String address; } 19
- 20. 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; } 20
- 21. Example Program Contd.. public String getAddress () { return address; } public void setAddress (String address) { this. address = address; } } 21
- 22. • The analysis is as follows Example Program Contd.. • 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 22
- 23. Summary • In this class we have discussed • Access protection • Access modifiers • public, private, protected • How to use access modifiers 23
- 24. 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 24
- 25. Quiz 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. private 2. public 3. protected 4. No keyword (default) CM604.27 25
- 26. Quiz Contd.. 2. Which keyword can make a class visible within a package? • private • public • protected • No keyword (default) CM604.27 26
- 27. Quiz Contd.. 3. Which keyword is useful for implementing information hiding principle? 1. private 2. public 3. protected 4. No keyword (default) CM604.27 27
- 28. Quiz Contd.. 4. Which one of the following is NOT a keyword related to Access protection? 1. private 2. public 3. protected 4. default CM604.27 28