CS110- Lecture 9
Mar 7, 2005
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Agenda
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Conditional Operator
Encapsulation
Constructors
Practice Session
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CS110-Spring 2005, Lecture 9
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Conditional Operator
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Java conditional operator is similar to
an if-else statement in some ways.
It is a ternary operator because it
requires three operands.
A conditional operator expression starts
with a boolean expression followed by
a ? and then two expressions
separated by a colon (:).
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Conditional Operator
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If the boolean expression is true then
the first of the two expressions is
returned; otherwise the second of the
two expressions is returned.
Example 1
1.if( n1 > n2)
max = n1;
else
max = n2;
2. max = (n1 > n2)? n1 : n2;
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Conditional
operator
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Conditional Operator
Example 2
if( hoursWorked <= 40)
pay = hoursWorked * payRate;
else
pay = 40 * payRate + 1.5 * (hoursWorked – 40) * payRate;
OR
pay = ( hoursWorked <= 40)? hoursWorked * payRate: 40 *
payRate + 1.5 * (hoursWorked – 40) * payRate;
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Conditional Operator
Example 3
public boolean isEven(int num)
{
if(num % 2 == 0)
return true;
else
return false;
}
OR
public boolean isEven(int num)
{
return (num % 2 == 0)? true :false;
}
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Conditional Operator
Example 4
String msg;
if(count1 == count2)
msg = “A Tie!”;
else if ( count1 > count2)
msg = name1 + “ wins!”;
else
msg = name2 + “ wins!”;
OR
String msg = (count1 == count2) ? "A Tie!"
:(count1 > count2) ? name1 + " wins!" :
name2 + " wins!";
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A Class as an Outline
Class name: Account
Fields (Instance data):
owner
account number
balance
Data determine the state. It is called
instance data because memory is
determined for each instance of the
class that is created. Scope of instance
data is the whole class
Methods (actions):
initialize data: (Constructor)
deposit money:
How: balance = balance + amount
withdraw money:
How: balance = balance – amount
get the balance:
How: return the balance
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Same name as of class
and is called when the
new operator is used
to create an instance
of class
CS110-Spring 2005, Lecture 9
Methods determine
the behavior
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Account Class
public class Account
{
private String owner;
private long acctNumber;
private double balance;
public Account(String o, long a, double b) {
owner = o; acctNumber = a; balance = b;
}
public double deposit(double amount) {
balance = balance + amount;
return balance;
}
public double withdraw(double amount) {
balance = balance – amount;
return balance;
}
public double getBalance() {
return balance;
}
}
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Encapsulation
Object
Client or
programmer
who uses
the class
Methods
Data
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Instance data
of an object
should be
modified only
by that object
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Encapsulation
A well encapsulated class definition
Implementation
private instance data
Interface
private constants
Comments
public constants
Headings of public methods
private methods
public defined constants
Programmer
who uses
the class
public methods
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Visibility Modifiers
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We accomplish object encapsulation using
modifiers.
Some modifiers are called visibility modifiers
because they control access to the members
of a class.
public , private and protected
public - could be directly referenced from
outside of the object.
private - can be used anywhere in the class
definition but cannot be referenced outside
the object.
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Visibility modifiers
public
data
methods
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private
Violate
Encapsulation
Enforce
Encapsulation
Provide services
to clients
Support other
methods in
the class
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Accessors and Mutators
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Because instance data is generally
declared private, a class usually
provides services to access and modify
data.
Accessors (getters) – read only access
to data. Usually named as getX where X
is the variable name to which it
provides access.
Mutators (setters) - changes the value
of data. Usually named setX.
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Constructors
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We often use a constructor to initialize
variables associated with each object.
A constructor differs from a regular
method in two ways:
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Name of constructor is the same name as
that of class.
Constructor cannot return a value and does
not have a return type specified on the
method header.
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Constructors
Example 1: Constructor of Die class
public Die()
{
faceValue = 1;
}
Example 2: Constructor of Account class
public Account(String o, long a, double b)
{
owner = o;
acctNumber = a;
balance = b;
}
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Practice Session (Extra Credit)
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Design and implement a class called
Circle that contains instance data that
represents the circle’s radius. Define
the Circle constructor to accept and
initialize the radius, and include getter
and setter methods for the radius.
Include method area that calculates
and return the area and another
method perimeter that calculates and
return the perimeter.
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Practice Session (Extra Credit)
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Design and implement a class called
Person that contains instance data that
represents the person’s name and age.
Define the Person constructor to
accept and initialize the name and age,
and include getter and setter methods
for the name and age.
Create a driver class called
PersonClient whose main method
instantiates and update several Person
objects.
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Practice Session (Extra Credit)
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Design and implement a class called
Book that contains instance data that
represents the book’s title, author,
publisher, edition and isbn number.
Define the Book constructor to accept
and initialize the instance data, and
include getter and setter methods for all
instance data.
Create a driver class called BookShelf
whose main method instantiates and
update several Book objects.
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Practice Session (Page 277)
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5.17: Write a for loop to print the odd numbers from
1 to 99.
5.18: Write a for loop to print the multiples of 3 from
300 down to 3.
5.19: Write a code fragment that reads 10 integer
values from the user and prints the highest value
entered.
5.20: Write a code fragment that determines and
prints the number of times the character ‘a’ appears
in a String object called name.
5.21: Write a code fragment that prints the
characters stored in a String object called str
backward.
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Practice Session (Page 277)
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5.22: Write a code fragment that prints
every other character in a String object
called word starting from the first
character.
5.23: Write a method called
powersOfTwo that prints the first 10
powers of 2 (starting with 2). The
method takes no parameter and doesn’t
return anything.
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