122 ECE Engineering Problem Solving with Java
Lecture 6
Problem Definition and Implementation
ECE122 L6: Problem Definition and Implementation
February 14, 2008
Outline Problem: Create, read in and print out four sets of student grades Setting up the problem Breaking the problem into pieces Writing needed classes and methods Additional Java stuff
Encapsulation, private/public, conditionals
ECE122 L6: Problem Definition and Implementation February 14, 2008
Encapsulation 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
Provides a set of specific services
These services define the interface to the object
ECE122 L6: Problem Definition and Implementation
February 14, 2008
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)
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
ECE122 L6: Problem Definition and Implementation
February 14, 2008
Encapsulation An encapsulated object can be thought of as a black box
Inner workings are hidden from the client
The client invokes the interface methods of the object, which manages the instance data
Client
Methods
Data
ECE122 L6: Problem Definition and Implementation
February 14, 2008
Visibility Modifiers Accomplish encapsulation through the appropriate use of visibility modifiers A modifier is a Java reserved word
Specifies particular characteristics of a method or data
The final modifier to define constants Java has three visibility modifiers: public, protected, and private The protected modifier will be discussed later
ECE122 L6: Problem Definition and Implementation
February 14, 2008
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
Can be referenced by any class in the same package
ECE122 L6: Problem Definition and Implementation
February 14, 2008
Visibility Modifiers Methods that provide the object's services are declared with public visibility
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 Support method should be declared with private visibility
ECE122 L6: Problem Definition and Implementation
February 14, 2008
Visibility Modifiers
public
Variables
private Enforce encapsulation Support other methods in the class
Violate encapsulation Provide services to clients
Methods
ECE122 L6: Problem Definition and Implementation
February 14, 2008
Accessors and Mutators Instance data is usually 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 methods mutator often take the form getX and setX
X is the name of the value
They are sometimes called getters and setters
ECE122 L6: Problem Definition and Implementation
February 14, 2008
Mutator Restrictions The use of mutators restricts modifications to an objects state A mutator is often designed so that the values of variables can be set only within particular limits Example:
setFaceValue mutator of the Die class should have restricted the value to the valid range (1 to MAX)
ECE122 L6: Problem Definition and Implementation
February 14, 2008
Bank Account Example Example that demonstrates the implementation details of classes and methods Well represent a bank account by a class named Account Its state can include the account number, the current balance, and the name of the owner An accounts behaviors (or services) include deposits and withdrawals, and adding interest
ECE122 L6: Problem Definition and Implementation
February 14, 2008
Bank Account Example
acct1
acctNumber
72354
balance 102.56 name Ted Murphy
acct2
acctNumber balance name
69713 40.00 Jane Smith
ECE122 L6: Problem Definition and Implementation
February 14, 2008
Constructors 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
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
ECE122 L6: Problem Definition and Implementation
February 14, 2008
// Account.java Author: Lewis/Loftus // Represents a bank account with basic services such as deposit and withdraw. //******************************************************************** Instance data. EACH object of import java.text.NumberFormat; public class Account. type Account will get its own { copy of these variables. private final double RATE = 0.035; // interest rate of 3.5% Note: all private. (means only private long acctNumber...