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Writing Classes
Writing Classes 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
encapsulation and Java modifiers
method declaration and parameter passing
Constructors
Method overloading
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Outline
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Anatomy of a Class
Encapsulation
Anatomy of a Method
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Writing Classes The programs we’ve written in previous examples
have used classes defined in the Java standard class library
Now we will begin to design programs that rely on classes that we write ourselves
The class that contains the main method is just the starting point of a program
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 Recall from our overview of objects in Chapter 1 that
an object has state and behavior
Consider a six-sided die (singular of dice)
It’s state can be defined as which face is showing
It’s primary behavior is that it can be rolled
We can represent a die in software by designing a class called Die that models this state and behavior
The class serves as the blueprint for a die object
We can then instantiate as many die objects as we need for any particular program
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Classes A class can contain data declarations and method
declarations
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int size, weight;
char category; Data declarations
Method declarations
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Classes 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
For our Die class, we might declare an integer that represents the current value showing on the face
One of the methods would “roll” the die by setting that value to a random number between one and six
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Classes We’ll want to design the Die class with other data and
methods to make it a versatile and reusable resource
Any given program will not necessarily use all aspects of a given class
See RollingDice.java (page 157)
See Die.java (page 158)
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The Die Class The Die class contains two data values
a constant MAX that represents the maximum face value
an integer faceValue that represents the current face value
The roll method uses the random method of the Math class to determine a new face value
There are also methods to explicitly set and retrieve the current face value at any time
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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 println method
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Constructors As mentioned previously, 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 Die constructor is used to set the initial face value of each new die object to one
We examine constructors in more detail later in this chapter
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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
Data declared within a method can be used only in that method
Data declared within a method is called local data
In the Die class, the variable result is declared inside the toString method -- it is local to that method and cannot be referenced anywhere else
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Instance Data The faceValue variable in the Die class is called
instance data because each instance (object) that is created has its own version of it
A class declares the type of the data, but it does not reserve any memory space for it
Every time a Die object is created, a new faceValue variable is created as well
The objects of a class share the method definitions, but each object has its own data space
That's the only way two objects can have different states
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Instance Data We can depict the two Die objects from the RollingDice program as follows:
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die1 5 faceValue
die2 2 faceValue
Each object maintains its own faceValue
variable, and thus its own state
UML Diagrams UML stands for the Unified Modeling Language
UML 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)
Lines between classes represent associations
A dotted arrow shows that one class uses the other (calls its methods)
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UML Class Diagrams A UML class diagram for the RollingDice
program:
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RollingDice
main (args : String[]) : void
Die
faceValue : int
roll() : int
setFaceValue (int value) : void
getFaceValue() : int
toString() : String
Outline
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Anatomy of a Class
Encapsulation
Anatomy of a Method
Graphical Objects
Graphical User Interfaces
Buttons and Text Fields
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
From the external view, an object is an encapsulated entity, providing a set of specific services
These services define the interface to the object
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Encapsulation One object (called the client) may use another object
for the services it provides
The client of an object may request its services (call its methods), but it should not have to be aware of how those services are accomplished
Any changes to the object's state (its variables) 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
That is, an object should be self-governing
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Encapsulation An encapsulated object can be thought of as a black
box -- its inner workings are hidden from the client
The client invokes the interface methods of the object, which manages the instance data
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Methods
Data
Client
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
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
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Visibility Modifiers Members of a class that are declared with public
visibility can be referenced anywhere
Members of a class that are declared with private visibility can be referenced only within that class
Members declared without a visibility modifier have default visibility and can be referenced by any class in the same package
An overview of all Java modifiers is presented in Appendix E
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Visibility Modifiers Public variables violate encapsulation because they
allow the client to “reach in” and modify the values directly
Therefore instance variables should not be declared with public visibility
It is acceptable to give a constant public visibility, which allows it to be used outside of the class
Public constants do not violate encapsulation because, although the client can access it, its value cannot be changed
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Visibility Modifiers Methods that provide the object's services are declared
with public visibility so that they can be invoked by clients
Public methods are also called service methods
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
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Visibility Modifiers
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public private
Variables
Methods Provide services
to clients
Support other
methods in the
class
Enforce
encapsulation
Violate
encapsulation
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
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”
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Mutator Restrictions The use of mutators gives the class designer the ability
to restrict a client’s options to modify an object’s state
A mutator is often designed so that the values of variables can be set only within particular limits
For example, the setFaceValue mutator of the Die class should have restricted the value to the valid range (1 to MAX)
We’ll see in Chapter 5 how such restrictions can be implemented
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Outline
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Anatomy of a Class
Encapsulation
Anatomy of a Method
Graphical Objects
Graphical User Interfaces
Buttons and Text Fields
Method Declarations Let’s now examine method declarations in more detail
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
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Method Control Flow If the called method is in the same class, only the
method name is needed
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myMethod();
myMethod compute
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Method Control Flow The called method is often part of another class or
object
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doIt
helpMe
helpMe();
obj.doIt();
main
Method Header A method declaration begins with a method header
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char calc (int num1, int num2, String message)
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
Method Body The method header is followed by the method body
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char calc (int num1, int num2, String message)
{
int sum = num1 + num2;
char result = message.charAt (sum);
return result;
}
The return expression must be consistent with the return type
sum and result
are local data They are created each time the method is called, and are destroyed when it finishes executing
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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
A return statement specifies the value that will be returned
return expression;
Its expression must conform to the return type
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Parameters When a method is called, the actual parameters in
the invocation are copied into the formal parameters in the method header
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char calc (int num1, int num2, String message)
{
int sum = num1 + num2;
char result = message.charAt (sum);
return result;
}
ch = obj.calc (25, count, "Hello");
Objects as Parameters Another important issue related to method design
involves parameter passing
Parameters in a Java method are passed by value
A copy of the actual parameter (the value passed in) is stored into the formal parameter (in the method header)
Therefore passing parameters is similar to an assignment statement
When an object is passed to a method, the actual parameter and the formal parameter become aliases of each other
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Passing Objects to Methods What a method does with a parameter may or may not
have a permanent effect (outside the method)
See ParameterTester.java (page 327)
See ParameterModifier.java (page 329)
See Num.java (page 330)
Note the difference between changing the internal state of an object versus changing which object a reference points to
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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 (including the formal parameters)
Keep in mind that instance variables, declared at the class level, exists as long as the object exists
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Bank Account Example Let’s look at another example that demonstrates the
implementation details of classes and methods
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
See Transactions.java (page 172)
See Account.java (page 173)
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Bank Account Example
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acct1 72354 acctNumber
102.56 balance
name “Ted Murphy”
acct2 69713 acctNumber
40.00 balance
name “Jane Smith”
Bank Account Example There are some improvements that can be made to the Account class
Formal getters and setters could have been defined for all data
The design of some methods could also be more robust, such as verifying that the amount parameter to the withdraw method is positive
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Constructors Revisited 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
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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Method Overloading Method overloading is the process of giving a single
method name multiple definitions
If a method is overloaded, the method name is not sufficient to determine which method is being called
The signature of each overloaded method must be unique
The signature includes the number, type, and order of the parameters
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Method Overloading The compiler determines which method is being
invoked by analyzing the parameters
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float tryMe(int x)
{
return x + .375;
}
float tryMe(int x, float y)
{
return x*y;
}
result = tryMe(25, 4.32)
Invocation
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Method Overloading The println method is overloaded:
println (String s)
println (int i)
println (double d)
and so on...
The following lines invoke different versions of the println method:
System.out.println ("The total is:");
System.out.println (total);
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Overloading Methods The return type of the method is not part of the
signature
That is, overloaded methods cannot differ only by their return type
Constructors can be overloaded
Overloaded constructors provide multiple ways to initialize a new object
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Summary Chapter 4 focused on:
class definitions
instance data
encapsulation and Java modifiers
method declaration and parameter passing
constructors
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