CS100A, 15 Sept Lecture 5 1 CS100A, 5 Sept This lecture continues the discussion of classes. The important new concept of this lecture is inheritance. This will require to introduce constructors and to use the Java keywords extends, this, and super. Inheritance is motivated by extending class Employee of the previous lecture to distin- guish between kinds of employees: VIPs (the CEO and other major officers), salaried em- ployees, and regular employees. Each kind of employee requires different information in their record, and there is different processing for each. Placing the information for all three kinds of employees in class Employee wastes space and causes complications in the code; using classes that are extensions of class Employee eliminates these problems.

CS100A, 15 Sept Lecture 5 2 CS100A, 15 September 1998 Concepts for this lecture: Inheritance: a subclass B of a class A inherits fields and methods from class A. A is a super-class of B. In Java, keyword extends is used to define a subclass. Using the constructor of a super class. Access modifier protected Overriding methods Method toString() for every class Reading: Holmes --look in index and try to find appropriate pages to read

CS100A, 15 Sept Lecture 5 3 Method toString // Yield a String containing the data for // the person public String toString() {return name + " " + salary + " " + hireDate; } Convention: Every class implements method toString! In + B (catenation), if B is not a String, method B.toString() is automatically used to convert it to a String.

CS100A, 15 Sept Lecture 5 4 // An instance of Employee contains a person's name, // salary, and year hired. It has a constructor and // methods for raising the salary, printing the data, and // retrieving the person's name and the year hired. public class Employee {public String name; // The person's name public double pay;// The person's yearly salary public int hireDate;// The year hired // Constructor: a person with name n, salary s, and // year d hired public Employee(String n, double s, int d) {name= n; pay= s; hireDate= d; } Our old class Employee

CS100A, 15 Sept Lecture 5 5 // Raise the salary by p percent public void raiseSalary(double p) {pay= pay * (1 + p/100.0);} // Yield the year the person was hired public int hireYear() {return hireDate;} // Yield the person's name public String getName() {return name;} // Yield a String containing the data for the person public String toString() {return name + " " + pay + " " + hireDate; } (old class Employee)

CS100A, 15 Sept Lecture 5 6 Modify class Employees to take into account three different kinds: VIPS (e.g. CEO, president, vice president): Need a field bonus, since VIPS get (big) bonuses. Get a yearly salary. Salaried: Expected to work overtime whenever necessary, without extra pay. Regular: Time cards! Have an hourly wage instead of a yearly salary. (Need also to record the number of hours worked on each day, but we’ll forego that here and assume 40 hours per week. Our task today

CS100A, 15 Sept Lecture 5 7 How to implement? Add fields: int employeeKind; // 1 = VIP, 2 = salaried, // 3 = regular double bonus; // bonus for VIP (only) double hourlyWage; // (for regular // employees only // Set bonus to b (if employee is a VIP) public setBonus(double b) { if (employeeKind = 1) bonus= b; } A lot of other changes are required.

CS100A, 15 Sept Lecture 5 8 Problems with this approach Each employee’s record has a field for a bonus and for an hourly wage, even though it may not need it. Within the methods of class Employee that deal with salary and wages and hours, tests will have to be made to determine which kind of employee is being processed. This makes the code longer, less well-structured, and confusing. If a new kind of employee were to be added that required a different processing for pay, all the code that deals with pay would have to be changed. Instead, use a different approach: inheritance.

CS100A, 15 Sept Lecture 5 9 // An instance of VIP contains a VIP's data public class VIP extends Employee {private double bonus; // The VIP's bonus // Constructor: a person with name n, year d hired, // yearly pay s, and bonus b public VIP(String n, int d, double s, double b) {super (n,d); pay= s; bonus= b;} // Yield a String containing the data for the person public String toString() {return “VIP + " name + " " + pay + " " + " " + bonus + " " + hireDate; } // Change the bonus to p public void changeBonus(double p) {bonues= p;} }

CS100A, 15 Sept Lecture 5 10 An instance of class VIP has every field and method that an instance of class Employee does, plus the ones VIP declares. Employee x; x= new Employee(“Gries”, 1969); Employee y= new VIP(“Cardie”,1994, 90000,1000); Employee Name Gries pay 0 hireDate 1969 Employee, hireYear, getName, toString x VIP Name Cardie pay hireDate 1994 Employee, hireYear, getName, toString bonus 1000 VIP changeBonus y

CS100A, 15 Sept Lecture 5 11 Use of super For an instance of VIP. A constructor of super class Employee should be called to initialize the fields declared within Employee. This is as the first statement in a constructor for VIP: super (n,d); Use of protected If fields of Employee are public, they can be referenced from anywhere. If they are private, they can be referenced only from instances of Employee. If they are protected, they can be referenced only in same package --concept discussed much later!!!

CS100A, 15 Sept Lecture 5 12 Referencing methods VIP y; y= new VIP(“Perkins”, 1983, 90000, 1000); y.getName() refers to method getName of its superclass, Employee. This is because getName is not defined in VIP. y.getBonus() refers to method getBonus of VIP. This is because getBonus is defined in VIP. y.toString() refers to method toString of VIP. This is because toString is defined in VIP. Method toString of superclass Employee has been overridden in class VIP.