CS111: PROGRAMMING LANGUAGE II

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1 1 CS111: PROGRAMMING LANGUAGE II Computer Science Department Lecture 4(b): Subclasses and Superclasses

2 OOP

3 OOP - Inheritance Inheritance represents the is a relationship between data types (e.g. student/person) Existing class is the superclass (more general) New class is the subclass (more specific) Java supports only single inheritance each class is derived from exactly one direct superclass. Possible to Create hierarchy of classes Dr. Amal Khalifa, 2014

4 Lecture Contents 4 Creating subclasses Protected members Dr. Amal Khalifa, 2014

5 Example: CommissionEmployee Class 5 Inheritance hierarchy containing types of employees in a company s payroll application Commission employees are paid a percentage of their sales Dr. Amal Khalifa, 2014

6 Class CommissionEmployee 6 Extends object not required; this will be done implicitly Constructors are not inherited

7 Class CommissionEmployee 7 Set/get methods

8 Class CommissionEmployee 8 Set/get methods

9 Class CommissionEmployee 9 Set/get methods Dr. Amal Khalifa, 2014

10 Class CommissionEmployee ensures overridded method has the same signature as the base class Dr. Amal Khalifa, 2014

11 Test 11 New employee object

12 Test 12 Implicit call to the method tostring

13 Creating a new class!! 13 Base-salaried commission employees receive a base salary plus a percentage of their sales. Class BasePlusCommissionEmployee: Data: first name, last name, social security number, gross sales amount, commission rate and base salary. (( All but the base salary are in common with class CommissionEmployee)). services: a constructor, and methods earnings, tostring and get and set for each instance variable ((Most are in common with class CommissionEmployee )) Dr. Amal Khalifa, 2014

14 Creating a new class 14 Copy & paste Inheritance copy CommissionEmployee code, pasted it into BasePlusCommissionEm ployee modify the new class to include a base salary and methods that manipulate the base salary. error prone time consuming. Too many copies of the same code bad maintenance Extend an existing class and add only the needed data/functionality Reusability Don t reinvent the wheel!! Dr. Amal Khalifa, 2014

15 Goal : code reuse 15 Inheritance can save time during program development: rather than declaring completely new members, you can designate that the new class should inherit the members of an existing class. by basing new classes on existing proven and debugged high-quality software. Increases the likelihood that a system will be implemented and maintained effectively. Dr. Amal Khalifa, 2014

16 Constructors in Subclasses Constructors are not inherited. The first task of a subclass constructor is to call its direct superclass s constructor explicitly or implicitly Ensures that the instance variables inherited from the superclass are initialized properly. If the code does not include an explicit call to the superclass constructor, Java implicitly calls the superclass s default or no-argument constructor. A class s default constructor calls the superclass s default or no-argument constructor. Dr. Amal Khalifa, 2014

17 BasePlusCommissionEmployee Class 17 New class defined based on an existing one Subclass constructor must call the superclass one to handle initialization

18 BasePlus-CommissionEmployee Class 18 public members are accessible from anywhere private members are accessible only within the class itself.

19 Errors!! 19 A superclass s private members are hidden in its subclasses Compilation errors occur when the subclass attempts to access the superclass s private instance variables.

20 Protected Members 20 To enable a subclass to directly access superclass instance variables, we can declare those members as protected in the superclass. an intermediate level of access between public and private. can be accessed by members of that superclass, by members of its subclasses and by members of other classes in the same package (package access). All public and protected superclass members retain their original access modifier when they become members of the subclass. Dr. Amal Khalifa, 2014

21 21 Programming Pitfalls To enable a subclass to directly access superclass instance variables, we can declare those members as protected in the superclass

22 That s all for today.. Book Ref. working on Chap. 9 Dr. Amal Khalifa, 2014

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