CS100A, 10 Sept. 1998 Lecture 4: Continue discussion of classes and introduces 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. Constructors Concepts demonstrated using 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. CS100A, 10 Sept. 1998. Lecture 4

Concepts for this lecture: CS100A, Fall 1998, 10 September Concepts for this lecture: Sequences of characters:class String. Holmes 1.8 (p. 30-34). Output, using System.out Method of a class. Holmes, Sec. 6.2. Functions versus procedures. Some security using access modifiers public and private. Holmes p 229. Constructors: Holmes, Sec. 205 (not worth much) CS100A, 10 Sept. 1998. Lecture 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 // Set the name to n public void setName(String n) {name= n;} // Set the salary to s public void setSalary(double s) {pay= s;} // Set the year to y public void setHireDate(int y) {hireDate= y;} CS100A, 10 Sept. 1998. Lecture 4

// Yield the year the person was hired public int hireYear() {return hireDate;} // Raise the pay by p percent public void raiseSalary(double p) {pay= pay * (1 + p/100.0);} // Yield the person's name public String getName() {return name;} // Yield a String containing person’s data public String toString() { String s= name; s= s + " " + pay + " " + hireDate; return s; } CS100A, 10 Sept. 1998. Lecture 4

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” Holmes: Sect. 1.8, p 30 CS100A, 10 Sept. 1998. Lecture 4

Checking string equality s1 s2 if (s1==s2) --condition if false! Use instead if (s1.equals(s2)) equals:a method of class String. gries gries CS100A, 10 Sept. 1998. Lecture 4

// “Flush” the output buffer, making To print string s in the output window, 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(); CS100A, 10 Sept. 1998. Lecture 4

Changing a field like pay should be limited to methods of the class. Accessing a field of an Employee object. Employee c; c= new Employee(); c.name= “G”; c.pay= 1000000; c.hireDate= 1998; c.pay= 220000; Allowing access to field pay allows any program that can reference c to change pay. This may not be desired! Changing a field like pay should be limited to methods of the class. CS100A, 10 Sept. 1998. Lecture 4

// Anyone can reference field salary! public int salary; Alternative: // Anyone can reference field salary! public int salary; // Only methods within Employee can // reference pay! private int pay; So, the following is illegal: Employee c; c= new Employee( ); c.pay= 1000000; Now, to change c1.pay, you have to call method setSalary or raiseSalary. CS100A, 10 Sept. 1998. Lecture 4

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. CS100A, 10 Sept. 1998. Lecture 4

// Yield the person’s name public String getName (); {return name;} Execution of statement return <expression> terminates execution of the method (function) in which it appears and “returns” the value of <expression> to the place of call. Two examples of calls: String s= e1.getName(); System.out.println(e1.toString()); CS100A, 10 Sept. 1998. Lecture 4

A constructor: a normal method except Constructors A constructor: a normal method except (0) Its name is the same as the name of the class (1) It has no type-class prefix or void //Constructor: name n, year d hired, //salary s public Employee(String n, double s, int d) { name= n; pay= s; hireDate= d; } When the following is executed, after the instance of Employee is created, the constructor is executed, with “Gries” for n, 50000 for s, and 1969 for d: c= new Employee(“Gries”, 50000, 1969); Constructor is used to provide initialization of fields of the instance. CS100A, 10 Sept. 1998. Lecture 4