CS100A, Fall 1997

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, 11 September

• Concepts for this lecture
• Sequences of charactersclass 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

• // 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
// 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

• 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

• Checking string equality
• s1
• s2
• if (s1s2) --condition if false!
• Use instead
• if (s1.equals(s2))
• equalsa method of class String.

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

• 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
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

• 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

• // Yield the persons name
• public String getName ()
• return name
• Execution of statement
• return ltexpressiongt
• terminates execution of the method (function) in
  which it appears and returns the value of
  ltexpressiongt to the place of call.
• Two examples of calls
• String s e1.getName()
• System.out.println(name e1.getName())