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Writing Classes (Chapter 4)
• We've been using predefined classes. Now we
will learn to write our own classes to define
objects
• Chapter 4 focuses on:
–
–
–
–
–
class definitions
instance data
constructors
encapsulation and Java modifiers
method declaration and parameter passing
4-1
Classes, Objects, & Methods
• Object-oriented programming uses
classes, objects, and methods as basic
programming components.
• These components help to
– organize a large program into small modules
– design and think about an intricate program
– find and remove errors (bugs)
4-2
predefined classes
• In Java, we have used predefined
classes defined in the Java standard
class library such as Scanner,
Random, and String.
4-3
Writing Classes
• Now we will begin to design programs that
rely on classes that we write ourselves
• True object-oriented programming is based
on defining classes that represent objects
with well-defined characteristics and
functionality
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Classes and Objects
4-5
Classes and Objects
• We can represent a bank account in
software by designing a class called
Account that models this state and
behavior
– The class serves as the blueprint for a Account
object
• We can then instantiate as many Account
objects as we need for any particular
program
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Classes
• A class can contain data declarations and
method declarations
double balance;
Data declarations
Method declarations
4-7
Classes
• An object has state and behavior
• The values of the data define the state of
an object created from the class
• The functionality of the methods define the
behaviors of the object
4-8
Bank Account Example
• We’ll represent a bank account by a class
named Account
• It’s state can include the account number,
the current balance, and the name of the
owner
• An account’s behaviors (or services)
include deposits and withdrawals, and
adding interest
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Driver Programs
• A driver program drives the use of other,
more interesting parts of a program
• Driver programs are often used to test
other parts of the software
• The Transactions class contains a
main method that drives the use of the
Account class, exercising its services
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Bank Account Example
• To compile and run
– Place Account.java and Transactions.java in
the same folder
– compile Account.java
– compile Transactions.java
– run Transactions
4-11
Constructor
• A constructor is a special method that is
used to set up an object when it is initially
created
• A constructor has the same name as the
class
• The programmer does not have to define a
constructor for a class: Each class has a
default constructor that accepts no
parameters
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Constructor
• Note that a constructor has no return type
specified in the method header, not even
void
• A common error is to put a return type on a
constructor, which makes it a “regular”
method that happens to have the same
name as the class
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Constructor
Transactions.java
main
Account.java
public Account(…)
Account acct1 = new Account
("Ted Murphy", 72354, 102.56);
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Instance Data: Bank Account Example
• The objects of a class share the method
definitions, but each object has its own
data space
acct1
acctNumber
72354
balance 102.56
“Ted Murphy”
name
acct2
acctNumber
69713
balance
40.00
name
Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
“Jane Smith”
4-15
Instance Data
• A class declares the type of the data, but it
does not reserve any memory space for it
• Every time a Account object is created, a
new name variable is created as well
• Instance variables, declared at the class
level, exists as long as the object exists
4-16
UML(Unified Modeling Language)
Diagrams
• UML(Unified Modeling Language) diagrams
show relationships among classes and objects
• A UML class diagram consists of one or more
classes, each with sections for the class name,
attributes (data), and operations (methods)
• A dotted arrow shows that one class uses the
other (calls its methods)
Transactions
main (args : String[]) : void
Account
name: String
…
deposit(double amount) : double
…
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Exercise
• Write a class called Dog. The Dog class
assuming a Dog object is described by
three pieces of instance data: name (a
String), breed (a String), age(an int).
– Write a constructor that has three
parameters—a String containing the name of
the dog, a String containing the breed of the
dog, and a int containing the age of the dog.
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Method
4-19
Method Declarations
• A method declaration specifies the code that will be
executed when the method is invoked (called)
• When a method is invoked, the flow of control jumps to
the method and executes its code
• When complete, the flow returns to the place where the
method was called and continues
• The invocation may or may not return a value,
depending on how the method is defined
4-20
Method Control Flow
• The called method is often part of another
class or object
Transactions.java
main
Account.java
deposit
acct1.deposit(25.85);
4-21
Method Header
• A method declaration begins with a
method header
int add(int num1, int num2)
method
name
return
type
parameter list
The parameter list specifies the type
and name of each parameter
The name of a parameter in the method
declaration is called a formal parameter
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Method Body
• The method header is followed by the
method body
int add(int num1, int num2)
{
int sum = num1 + num2;
return sum;
}
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The return Statement
• The return type of a method indicates the
type of value that the method sends back
to the calling location
• A method that does not return a value has
a void return type, for example,
public static void main(String[] args)
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The return Statement
• A return statement specifies the value that
will be returned
return expression;
• Its expression must conform to the return
type
int add(int num1, int num2)
{
int sum = num1 + num2;
return sum;
}
The return expression
must be consistent with
the return type
4-25
Parameters
• Formal Parameters: the names of the
parameters in the header of the method
declaration.
• Actual Parameters: In an invocation, the
values passed into a method.
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Parameters
• When a method is called, the values of actual
parameters in the invocation are copied into the formal
parameters in the method header
actual parameters
int num = obj.add (24, 192);
int add(int num1, int num2)
{
int sum = num1 + num2;
formal parameters
return sum;
}
Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
4-27
Exercise
• Dog program continued
• Add a method:
– A method ageInPersonYears that takes no
parameter. The method should compute and
return the age of the dog in “person years”
(seven times the dog’s age).
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Encapsulation
4-29
Encapsulation
• We can take one of two views of an object:
– internal - the details of the variables and
methods of the class that defines it
– external - the services that an object provides
and how the object interacts with the rest of the
system
4-30
Encapsulation
• From the external view, an object is an
encapsulated entity, providing a set of
specific services
• These services define the interface to the
object
• The client invokes the interface methods of
the object, which manages the instance
data
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Encapsulation
• An encapsulated object can be thought of
as a black box -- its inner workings are
hidden from the client
Client
Methods
Data
Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
4-32
Encapsulation: Why?
• Take Bank Account as an example,
• The client of an object may request its services (call
its methods) such as withdraw and deposit, but it
should not have to be aware of how those services
are accomplished
• Any changes to the object's state (its variables) such
as balance should be made by that object's methods
• We should make it difficult, if not impossible, for a
client to access an object’s variables directly
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Visibility Modifiers
• In Java, we accomplish encapsulation
through the appropriate use of visibility
modifiers
• A modifier is a Java reserved word that
specifies particular characteristics of a
method or data
Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
4-34
Visibility Modifiers
• We've used the final modifier to define
constants
• Java has three visibility modifiers: public,
protected, and private
• The protected modifier involves
inheritance, which we will discuss later.
Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
4-35
Visibility Modifiers
• Members of a class that are declared with
public visibility can be referenced
anywhere, for example,
public double deposit()
{ …
}
4-36
Visibility Modifiers
• Members of a class that are declared with
private visibility can be referenced only
within that class, for example,
private double balance;
• Members declared without a visibility
modifier have default visibility and can be
referenced by any class in the same
package (same directory)
4-37
Visibility Modifiers
Variables
Methods
public
private
Violate
encapsulation
Enforce
encapsulation
Provide services
to clients
Support other
methods in the
class
Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
4-38
Visibility Modifiers
• Methods that provide the object's services
are declared with public visibility so that
they can be invoked by clients
• A method created simply to assist a
service method is called a support method
• Since a support method is not intended to
be called by a client, it should not be
declared with public visibility
4-39
Exercise
• Dog program: Add visibility modifiers to the
instance data and method.
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Data Scope
4-41
Data Scope
• The scope of data is the area in a program
in which that data can be referenced (used)
• Data declared at the class level can be
referenced by all methods in that class, for
example,
private String name;
4-42
Local Data
• Data declared within a method is called
local data
• the variable sum is declared inside the add
method -- it is local to that method and
cannot be referenced anywhere else
int add(int num1, int num2)
{
int sum = num1 + num2;
return sum;
}
sum is local data
It is created each
time the method is
called, and is
destroyed when it
finishes executing
4-43
Local Data
• As we’ve seen, local variables can be
declared inside a method
• The formal parameters of a method create
automatic local variables when the method
is invoked
• When the method finishes, all local
variables are destroyed.
4-44
More on Methods: toString, getters and
setters
4-45
The toString Method
• All classes that represent objects should
define a toString method
• The toString method returns a character
string that represents the object in some
way
• It is called automatically when an object is
concatenated to a string or when it is
passed to the print/println method.
4-46
The toString Method
• For example, in Transactions.java
System.out.println (acct1);
toString method defined in Account.java
public String toString () {
NumberFormat fmt =
NumberFormat.getCurrencyInstance();
return (acctNumber + "\t" + name + "\t" +
fmt.format(balance));
}
4-47
Accessors and Mutators
• Because instance data is private, a class
usually provides services to access and
modify data values
• An accessor method returns the current
value of a variable
• A mutator method changes the value of a
variable
4-48
Accessors and Mutators
• The names of accessor and mutator
methods take the form getX and setX,
respectively, where X is the name of the
value
• They are sometimes called “getters” and
“setters”
4-49
Accessors and Mutators
• Examples:
//Returns the current balance of the account
public double getBalance () {
return balance;
}
// Sets name of a student
public void setName (String newName) {
name = newName;
}
4-50
Exercise
• Dog program: Add a toString method that
returns a string containing the name,
breed, the age of the dog, and the age in
person years in a format similar to the
following:
Name: yoyo
Breed: shih tzu
Age: 5
Age in person years: 35
4-51
Exercise
• Dog program:
• Complete the driver program that creates a
dog object and then implicitly calls the
toString() method.
4-52
Readings and Assignments
• Reading: Chapter 4.1-4.5
• Lab Assignment: Java Lab 7
• Exercises and Programming Projects:
– After Chapter Exercises
• EX 4.1, 4.5, 4.6, 4.7
– Programming Projects
• PP 4.1, 4.4, 4.5, 4.6, 4.7, 4.8
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