Eo gaddis java_chapter_03_5e

Copyright © 2015 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
C HA PT E R 3
A First Look
at Classes
and Objects

Topics
Classes
More about Passing Arguments
Instance Fields and Methods
Constructors
A BankAccount Class
Classes, Variables, and Scope
Packages and import Statements
Focus on Object Oriented Design: Finding the
Classes and their Responsibilities

Classes
In an object-oriented programming language,
like Java, you create programs that are made
of objects.
In software, an object has two capabilities:
An object can store data.
An object can perform operations.
The data stored in an object are commonly
called attributes or fields.
The operations that an object can perform
are called methods.

Strings as Objects
A primitive data type can only store
data, as it has no other built-in
capabilities.
An object can store data and perform
operations on that data.
In addition to storing strings, String objects
have numerous methods that perform
operations on the strings they hold.

Strings as Objects (cont’d)
• From chapter 2, we learned that a
reference variable contains the address
of an object.

Strings as Objects (cont’d)
The length() method of the String class
returns and integer value that is equal to the
length of the string.
int stringLength = cityName.length();
The variable stringLength will contain 10
after this statement since the string
"Charleston" has 10 characters.
Primitives can not have methods that can be
run whereas objects can.

Classes and Instances
Many objects can be created from a class.
Each object is independent of the others.

(cont’d)

(cont’d)
Each instance of the String class
contains different data.
The instances are all share the same
design.
Each instance has all of the attributes
and methods that were defined in the
String class.
Classes are defined to represent a
single concept or service.

Access Modifiers
An access modifier is a Java key word
that indicates how a field or method can
be accessed.
There are three Java access modifiers:
public
private
protected

Access Modifiers
public: This access modifier states that any
other class can access the resource.
private: This access modifier indicates that
only data within this class can access the
resource.
protected: This modifier indicates that only
classes in the current package or a class
lower in the class hierarchy can access this
resource.
These will be explained in greater detail later.

Access Modifiers
Classes that need to be used by other classes are
typically made public.
If there is more than one class in a file, only one may
be public and it must match the file name.
Class headers have a format:
AccessModifier class ClassName
{
Class Members
}

Encapsulation
Classes should be as limited in scope as
needed to accomplish the goal.
Each class should contain all that is needed
for it to operate.
Enclosing the proper attributes and methods
inside a single class is called encapsulation.
Encapsulation ensures that the class is self-
contained.

Designing a Class
When designing a class, decisions
about the following must be made.
what data must be accounted for
what actions need to be performed
what data can be modified
what data needs to be accessible
any rules as to how data should be modified
Class design typically is done with the
aid of a Unified Modeling Language
(UML) diagram.

UML Class Diagram
A UML class diagram is a graphical tool
that can aid in the design of a class.
The diagram has three main sections.
Class Name
Attributes
Methods
UML diagrams are easily converted
to Java class files. There will be more
about UML diagrams a little later.
The class name should concisely reflect what
the class represents.

Attributes
The data elements of a class define the
object to be instantiated from the class.
The attributes must be specific to the
class and define it completely.
Example: A rectangle is defined by
length
width
The attributes are then accessed by
methods within the class.

Data Hiding
Another aspect of encapsulation is the
concept of data hiding.
Classes should not only be self-contained
but they should be self-governing as well.
Classes use the private access modifier on
fields to hide them from other classes.
Classes need methods to allow access and
modification of the class’ data.

Methods
• The class’ methods define the actions that an
instance of the class can perform
• Methods headers have a format:
AccessModifier ReturnType
MethodName(Parameters)
{
//Method body.
}
• Methods that need to be used by other classes
should be made public.

Methods
• The attributes of a class might need to
be:
– changed
– accessed
– calculated
• The methods that change and access
attributes are called accessors and
mutators.

Accessors and Mutators
• Because of the concept of data hiding, fields
in a class are private.
• The methods that retrieve the data of fields
are called accessors.
• The methods that modify the data of fields
are called mutators.
• Each field that the programmer wishes to be
viewed by other classes needs an accessor.
• Each field that the programmer wishes to be
modified by other classes needs a mutator.

Accessors and Mutators
• For the Rectangle example, the accessors
and mutators are:
– setLength : Sets the value of the length field.
public void setLength(double len) …
– setWidth : Sets the value of the width field.
public void setLength(double w) …
– getLength : Returns the value of the length field.
public double getLength() …
– getWidth : Returns the value of the width field.
public double getWidth() …
• Other names for these methods are getters
and setters.

Stale Data
• Some data is the result of a calculation.
• Consider the area of a rectangle.
– length times width
• It would be impractical to use an area
variable here.
• Data that requires the calculation of various
factors has the potential to become stale.
• To avoid stale data, it is best to calculate the
value of that data within a method rather than
store it in a variable.

Stale Data
• Rather than use an area variable in a
rectangle class:
public double getArea()
{
return length * width;
}
• This dynamically calculates the value of the
rectangle’s area when the method is called.
• Now, any change to the length or width
variables will not leave the area of the
rectangle stale.

UML Data Type and
Parameter Notation
• UML diagrams are language independent.
• UML diagrams use an independent notation
to show return types, access modifiers, etc.
Rectangle
- width : double
+ setWidth(w : double) : void
Access modifiers
are denoted as:
+ public
- private
# protected

UML Data Type and
Parameter Notation
Rectangle
- width : double
Variable types are
placed after the variable
name, separated by a
colon.

UML Data Type and
Parameter Notation
Rectangle
- width : double
Method return types are
placed after the method
declaration name,
separated by a colon.

UML Data Type and
Parameter Notation
Rectangle
- width : double
Method parameters
are shown inside the
parentheses using the
same notation as
variables.

3-28
Converting the UML Diagram to Code
• Putting all of this information together, a Java
class file can be built easily using the UML
diagram.
• The UML diagram parts match the Java class
file structure.
ClassName
Attributes
Methods
class header
{
Attributes
Methods
}

Rectangle
- width : double
- length : double
+ setLength(len : double): void
+ getWidth() : double
+ getLength() : double
+ getArea() : double
The structure of the class can be
compiled and tested without having
bodies for the methods. Just be sure to
put in dummy return values for methods
that have a return type other than void.
public class Rectangle
{
private double width;
private double length;
public void setWidth(double w)
{
}
public void setLength(double len)
{
}
public double getWidth()
{ return 0.0;
}
public double getLength()
{ return 0.0;
}
{ return 0.0;
}
}

Rectangle
- width : double
- length : double
Once the class structure has been tested,
the method bodies can be written and
tested.
{
{ width = w;
}
{ length = len;
}
{ return width;
}
{ return length;
}
{ return length * width;
}
}

Class Layout Conventions
The layout of a source code file can
vary by employer or instructor.
Generally the layout is:
Attributes are typically listed first
Methods are typically listed second
The main method is sometimes first, sometimes
last.
Accessors and mutators are typically grouped.

A Driver Program
An application in Java is a collection of
classes that interact.
The class that starts the application
must have a main method.
This class can be used as a driver to
test the capabilities of other classes.
In the Rectangle class example, notice
that there was no main method.

A Driver Program
public class RectangleDemo
{
public static void main(String[] args)
{
Rectangle r = new Rectangle();
r.setWidth(10);
r.setLength(10);
System.out.println("Width = "
+ r.getWidth());
System.out.println("Length = "
+ r.getLength());
System.out.println("Area = "
+ r.getArea());
}
}
This RectangleDemo class is a
Java application that uses the
Rectangle class.
{
{ width = w;
}
{ length = len;
}
{ return width;
}
{ return length;
}
{ return length * width;
}
}

Multiple Arguments
Methods can have multiple parameters.
The format for a multiple parameter method
is:
AccessModifier ReturnType MethodName(ParamType ParamName,
ParamType ParamName,
etc)
{
}
Parameters in methods are treated as local
variables within the method.
Example: MultipleArgs.java

Arguments Passed By Value
In Java, all arguments to a method are
passed “by value”.
If the argument is a reference to an
object, it is the reference that is passed
to the method.
If the argument is a primitive, a copy of
the value is passed to the method.

Fields and methods that are declared
as previously shown are called
instance fields and instance methods.
Objects created from a class each have
their own copy of instance fields.
Instance methods are methods that are
not declared with a special keyword,
static.

Instance fields and instance methods require
an object to be created in order to be used.
Example: RoomAreas.java
Note that each room represented in this
example can have different dimensions.
Rectangle kitchen = new Rectangle();
Rectangle bedroom = new Rectangle();
Rectangle den = new Rectangle();

Constructors
Classes can have special methods
called constructors.
Constructors are used to perform
operations at the time an object is
created.
Constructors typically initialize
instance fields and perform other
object initialization tasks.

Constructors
Constructors have a few special properties
that set them apart from normal methods.
Constructors have the same name as the class.
Constructors have no return type (not even void).
Constructors may not return any values.
Constructors are typically public.
Example: ConstructorDemo.java
Example: RoomConstructor.java

The Default Constructor
If a constructor is not defined, Java provides
a default constructor.
It sets all of the class’ numeric fields to 0.
It sets all of the class’ boolean fields to false.
It sets all of the class’ reference variables, the default
constructor sets them to the special value null.
The default constructor is a constructor with
no parameters.
Default constructors are used to initialize an
object in a default configuration.

Constructors in UML
In UML, the most common way constructors
are defined is:
Rectangle
- width : double
- length : double
+Rectangle(len:double, w:double)
Notice there is no
return type listed
for constructors.

The String Class
Constructor
One of the String class constructors
accepts a string literal as an argument.
This string literal is used to initialize a
String object.
For instance:
String name = new String("Michael Long");

The String Class
Constructor
This creates a new reference variable name
that points to a String object that
represents the name “Michael Long”
Because they are used so often, Strings can
be created with a shorthand:
String name = "Michael Long";

The BankAccount Example
BankAccount
- balance : double
- interestRate : double
- interest : double
+BankAccount(startBalance:double,
intRate :double):
+ deposit(amount : double) : void
+ withdrawl(amount : double: void
+ addInterest() : void
+ getBalance() : double
+ getInterest() : double

Classes, Variables and Scope
The list below shows the scope of a variable
depending on where it is declared.
Inside a method:
Visible only within that method.
Called a local variable.
In a method parameter:
Called a parameter variable.
Same as a local variable
Visible only within that method.
Inside the class but not in a method:
Visible to all methods of the class.
Called an instance field.

Shadowing
A parameter variable is, in effect, a local variable.
Within a method, variable names must be unique.
A method may have a local variable with the same
name as an instance field.
This is called shadowing.
The local variable will hide the value of the instance
field.
Shadowing is discouraged and local variable names
should not be the same as instance field names.

Packages and import
Statements
A package is a group of related classes.
The classes in the Java API are organized into
packages.
For example, the Scanner class is in the java.util
package.
Many of the API classes must be imported before
they can be used. For example, the following
statement is required to import the Scanner class:
import java.util.Scanner; This statement appears at
the top of the program's
source code.

Packages and import
Statements
Explicit import Statements
An explicit import statement specifies a single
class:
import java.util.Scanner;
Wildcard import Statements
A wildcard import statement imports all of the
classes in a package:
import java.util.*;

Packages and import
Statements
The java.lang package
Automatically imported into every Java
program.
Contains general classes such as String
and System.
You do not have to write an import
statement for the java.lang package.

Packages and import
Statements
You will use other packages as you learn more about Java.
Table 3-2 lists a few examples.

Object Oriented Design
Finding Classes and Their Responsibilities
Finding the classes
Get written description of the problem
domain
Identify all nouns, each is a potential class
Refine list to include only classes relevant
to the problem
Identify the responsibilities
Things a class is responsible for knowing
Things a class is responsible for doing
to the problem

Object Oriented Design
Finding Classes and Their Responsibilities
Identify the responsibilities
Things a class is responsible for knowing
Things a class is responsible for doing
to the problem

Eo gaddis java_chapter_03_5e

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