1. 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 Lecture 4

2. Concepts for this lecture:
CS100A, Fall 1998, 10 September
Concepts for this lecture:
Sequences of characters:class String. Holmes 1.8 (p ).
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 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 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 Lecture 4

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

7. // “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 Lecture 4

8. 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= ;
c.hireDate= 1998;
c.pay= ;
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 Lecture 4

9. // 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= ;
Now, to change c1.pay, you have to call method setSalary or raiseSalary.
CS100A, 10 Sept 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.
CS100A, 10 Sept Lecture 4

11. // 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 Lecture 4

12. 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, 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 Lecture 4