to Intro OOP with Java, C. Thomas Wu
Chapter 4
Defining Your Own Classes Part 1
Animated Version
The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
4th Ed Chapter 4 - 1
Objectives
After you have read and studied this chapter, you should be able to
Define a class with multiple methods and data members Differentiate the local and instance variables Define and use value-returning methods Distinguish private and public methods Distinguish private and public data members Pass both primitive data and objects to a method
The McGraw-Hill Companies, Inc.
1
Intro to OOP with Java, C. Thomas Wu
Why Programmer-Defined Classes
Using just the String, Math, JFrame and other standard classes will not meet all of our needs. We need to be able to define our own classes customized for our applications. Learning how to define our own classes is the first step toward mastering the skills necessary in building large programs. Classes we define ourselves are called __________ _____________ _______________
The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
4th Ed Chapter 4 - 3
First Example: Using the Bicycle Class
class BicycleRegistration { public static void main(String[] args) { Bicycle bike1, bike2; String owner1, owner2; //Create and assign values to bike1
bike1 = new Bicycle( );
bike1.setOwnerName("Adam Smith"); //Create and assign values to bike2 //Output the information System.out.println(owner1 + " owns a bicycle."); System.out.println(owner2 + " also owns a bicycle."); } }
The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
4th Ed Chapter 4 - 4
The McGraw-Hill Companies, Inc.
2
Intro to OOP with Java, C. Thomas Wu
The Definition of the Bicycle Class
class Bicycle { // Data Member private String ownerName; //Constructor: Initialzes the data member public void Bicycle( ) { } //Returns the name of this bicycle's owner public String getOwnerName( ) { } //Assigns the name of this bicycle's owner public void setOwnerName(String name) { }
}
The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
4th Ed Chapter 4 - 5
Multiple Instances
Once the Bicycle class is defined, we can create multiple instances.
bike1
Bicycle bike1, bike2; bike1 = new Bicycle( ); bike1.setOwnerName("Adam Smith"); bike2 = new Bicycle( ); bike2.setOwnerName("Ben Jones"); : Bicycle ownerName "Adam Smith" : Bicycle ownerName "Ben Jones"
bike2
Sample Code
The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
4th Ed Chapter 4 - 6
The McGraw-Hill Companies, Inc.
3
Intro to OOP with Java, C. Thomas Wu
The Program Structure and Source Files
BicycleRegistration
Bicycle
BicycleRegistration.java
Bicycle.java
There are two source files. Each class definition is stored in a separate file.
To run the program: 1. javac Bicycle.java (compile) 2. javac BicycleRegistration.java (compile) 3. java BicycleRegistration (run)
The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
4th Ed Chapter 4 - 7
Class Diagram for Bicycle
Bicycle Bicycle( ) getOwnerName( ) setOwnerName(String)
Method Listing We list the name and the data type of an argument passed to the method.
The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
4th Ed Chapter 4 - 8
The McGraw-Hill Companies, Inc.
4
Intro to OOP with Java, C. Thomas Wu
Template for Class Definition
Import Statements
Class Comment
class
{
Class Name
Data Members
Methods
(incl. Constructor)
}
The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
4th Ed Chapter 4 - 9
Data Member Declaration
<modifiers> <data type> <name> ;
private
String
ownerName ;
Note: There's only one modifier in this example.
The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
4th Ed Chapter 4 - 10
The McGraw-Hill Companies, Inc.
5
Intro to OOP with Java, C. Thomas Wu
Method Declaration
<modifier> } <return type> <method name> ( <parameters> ){
<statements>
public
void
setOwnerName
(
String
name
) {
ownerName = name; }
The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
4th Ed Chapter 4 - 11
Constructor
A constructor is a special method that is executed when a new instance of the class is created.
public <class name> ( <parameters> ){ <statements> }
public }
The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Bicycle
(
) {
ownerName = "Unassigned";
4th Ed Chapter 4 - 12
The McGraw-Hill Companies, Inc.
6
Intro to OOP with Java, C. Thomas Wu
Second Example: Using Bicycle and Account
class SecondMain { //This sample program uses both the Bicycle and Account classes public static void main(String[] args) { Bicycle bike; Account acct; String myName = "Jon Java";
bike = new Bicycle( ); bike.setOwnerName(myName); acct = new Account( ); acct.setOwnerName(myName); acct.setInitialBalance(250.00); acct.add(25.00); acct.deduct(50); //Output some information System.out.println(bike.getOwnerName() + " owns a bicycle and"); System.out.println("has $ " + acct.getCurrentBalance() + " left in the bank"); } }
The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
4th Ed Chapter 4 - 13
The Account Class
class Account { private String ownerName; private double balance; public Account( ) { } public void add(double amt) { } public void deduct(double amt) { } public double getCurrentBalance( ) { } public String getOwnerName( ) { } } public void setInitialBalance (double bal) { } public void setOwnerName (String name) { }
Page 1
Page 2
The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
4th Ed Chapter 4 - 14
The McGraw-Hill Companies, Inc.
7
Intro to OOP with Java, C. Thomas Wu
The Program Structure for SecondMain
Bicycle SecondMain Account
SecondMain.java
Bicycle.java
Account.java
To run the program: 1. javac Bicycle.java 2. javac Account.java 2. javac SecondMain.java 3. java SecondMain
(compile) (compile) (compile) You (run)
Note: only need to compile the class once. Recompile only when you made changes in the code.
4th Ed Chapter 4 - 15
The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Actual and Virtual Parameters
class Sample { public static void main(String[] arg) { Account acct = new Account(); . . . acct.add(400); . . . class Account { . . . public void add(double amt) { } } balance = balance + amt;
virtual
} }
. . .
. . .
actual
Actual - the value we pass to a method Virtual - a placeholder in the called method to hold the value of the passed argument.
The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
4th Ed Chapter 4 - 16
The McGraw-Hill Companies, Inc.
8
Intro to OOP with Java, C. Thomas Wu
Matching Actual and Virtual Parameters
The _________ must be the same
3 actuals
Demo demo = new Demo( ); int i = 5; int k = 14; demo.compute(i, k, 20);
Passing Side
Paired _____ to _______
class Demo { public void compute(int i, int j, double x) { . . . }
3 virtuals
}
Receiving Side
The matched pair must be assignment______________ (e.g. you cannot pass a double actual to a int virtual parameter)
The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
4th Ed Chapter 4 - 17
Memory Allocation
Separate memory space is allocated for the receiving method. Values of arguments are passed into memory allocated for parameters.
i k
5 14 20
i j x
5 14 20.0
Passing Side
Literal constant has no name
Receiving Side
The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
4th Ed Chapter 4 - 18
The McGraw-Hill Companies, Inc.
9
Intro to OOP with Java, C. Thomas Wu
Passing Objects to a Method
As we can pass int and double values, we can also pass an object to a method. When we pass an object, we are actually passing the reference (name) of an object
it means a duplicate of an object is NOT created in the called method
The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
4th Ed Chapter 4 - 19
Information Hiding and Visibility Modifiers The modifiers ______ and _______ designate the accessibility of data members and methods. If a class component (data member or method) is declared ______, client classes cannot access it. If a class component is declared _______, client classes can access it. Internal details of a class are declared private and hidden from the clients. This is ______________ ___________.
The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
4th Ed Chapter 4 - 20
The McGraw-Hill Companies, Inc.
10
Intro to OOP with Java, C. Thomas Wu
Accessibility Example
... Service obj = new Service(); obj.memberOne = 10; obj.memberTwo = 20; obj.doOne(); obj.doTwo(); ...
class Service { public int memberOne; private int memberTwo; public void doOne() { ... } private void doTwo() { ... } }
Client
The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Service
4th Ed Chapter 4 - 21
Data Members Should Be private Data members are the implementation details of the class, so they should be invisible to the clients. Declare them __________ . Exception: _________ can (should) be declared public if they are meant to be used directly by the outside methods.
The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
4th Ed Chapter 4 - 22
The McGraw-Hill Companies, Inc.
11
Intro to OOP with Java, C. Thomas Wu
Guideline for Visibility Modifiers
Guidelines in determining the visibility of data members and methods:
Declare instance variables private. Declare methods private if they are used only by the other methods in the same class. Declare the class constants public ...