1. CS100A, Fall 1997. Lecture 4 1 CS100A, Fall 1997 Lecture, Thursday, 12 September. This lecture continues the discussion of classes. In addition, the following will be introduced: Class String, Printing output using System.out.print, System,out.println, and System.out.flush(). Accessing modifiers public and private Functions versus procedures, and the modifier void. The concepts are demonstrated using a class Employee, an instance of which records information about an employee. This class appears later in these slides. With each concept, CodeWarrior Java on the Macintosh is used to demonstrate the concept and its application. This is important. For example, after making a field private, we syntax-check the program and see that references to it (outside the class) are now illegal.

2. CS100A, Fall 1997. Lecture 4 2 CS100A, Fall 1997, 11 September Concepts for this lecture: Sequences of characters:class String. L/L 125-126. Output, using System.out Methods of a class. L/L 134-140. Functions versus procedures Some security using access modifiers public and private. L/L 146-147.

3. CS100A, Fall 1997. Lecture 4 3 // 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 salary;// The person's 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; salary= s; hireDate= d; }

4. CS100A, Fall 1997. Lecture 4 4 // Raise the salary by p percent public void raiseSalary(double p) {salary= salary * (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;} // Print the data for the person public void print() {System.out.println(name + “ “ + salary + “ “ + hireDate); System.out.flush(); }

5. CS100A, Fall 1997. Lecture 4 5 Class String. An instance is a “string” of characters. Declaration String d; Assignment d= new String(“David Gries”); or d= “David Gries”; Catenation of strings: infix operator + d + “ ” + “xyz” evaluates to “David Gries xyz” d= “David” + “ ” + “Gries” L/L 125-127, 800-803

6. CS100A, Fall 1997. Lecture 4 6 Checking string equality s1 s2 if (s1==s2) --condition if false! Use instead if (s1.equals(s2)) equals:a method of class String. gries

7. CS100A, Fall 1997. Lecture 4 7 To print string s in the output widnow, use System.out.print(s); or System.out.println(s); The latter one prints an “end of line” after printing s. Thus, the following are equivalent: (1) System.out.print(s1); System.out.println(s2); (2) System.out.println(s1 + s2); // “Flush” the output buffer, making // sure that everything is written out. System.out.flush();

8. CS100A, Fall 1997. Lecture 4 8 Accessing a field of an Employee object. Employee c; c= new Employee(“G”, 20000, 1969); c.salary= 220000; (Demonstrate using CodeWarrior) Allowing access to field salary allows any program that can reference c to change salary. This may not be desired! Changing a field like salary should be limited to methods of the class.

9. CS100A, Fall 1997. Lecture 4 9 Alternative: // Anyone can reference field salary! public int salary; Alternative: // Only methods within Employee can // reference salary! private int salary; So, the following is illegal: Employee c; c= new Employee(“G”,3000,1969); c.salary= 250000; Now, to change c1.salary, you have to call method raiseSalary.

10. CS100A, Fall 1997. Lecture 4 10 Procedures versus functions Method raiseSalary has prefix void: public void raiseSalary(double p); void indicates that the method performs a task but does not “return” a result. Method getName has prefix String: public String getName (); The term String indicates that the method “returns” a result.

11. CS100A, Fall 1997. Lecture 4 11 // Yield the person’s name public String getName (); {return name;} Execution of statement return terminates execution of the method (function) in which it appears and “returns” the value of to the place of call. Two examples of calls: String s= e1.getName(); System.out.println(“name ” + e1.getName());