Chapter 2
An Introduction to Objects and Classes
Chapter Goals
- To understand the concepts of classes and objects
- To realize the difference between objects and object references
- To become familiar with the process of implementing classes
- To be able to implement simple methods
- To understand the purpose and use of constructors
- To understand how to access instance fields and local variables
- To appreciate the importance of documentation comments
Objects and Classes
- Object: entity that you can manipulate in your programs (by invoking
methods)
- Each object belongs to a class
- Class: Set of objects with the same behavior
- Class determines legal methods
"Hello".println() // Error
"Hello".length() // OK
Rectangle Class
- Construct a rectangle:
new Rectangle(5, 10, 20, 30)
new Rectangle()
- Use the constructed object
System.out.println(new Rectangle(5, 10, 20, 30));
prints
java.awt.Rectangle[x=5,y=10,width=20,height=30]
Rectangle Shapes
A Rectangle Object
Syntax 2.1: Object Construction
|
new ClassName(parameters)
|
Example:
|
new Rectangle(5, 10, 20, 30)
new Car("BMW 540ti", 2004)
|
Purpose:
To construct a new object, initialize it with the construction parameters,
and return a reference to the constructed object. |
Object Variables
- Declare and optionally initialize:
Rectangle cerealBox = new Rectangle(5, 10, 20, 30);
Rectangle crispyCrunchy;
- Apply methods:
cerealBox.translate(15, 25);
- Share objects:
r = cerealBox;
Uninitialized and Initialized Variables
Uninitialized:
Initialized:
Two Object Variables Referring to the Same Object
Syntax 2.2: Variable Definition
|
TypeName variableName;
TypeName variableName = expression;
|
Example:
|
Rectangle cerealBox;
String name ="Dave"; |
Purpose:
To define a new variable of a particular type and optionally supply an initial
value |
Writing a Test Program
- Invent a new class, say MoveTest
- Supply a main method
- Place instructions inside the main method
- Import library classes by specifying the package and class name:
import java.awt.Rectangle;
- You don't need to import classes in the java.lang package
such as String and System
Syntax 2.3 : Importing a Class from a Package
|
importpackageName.ClassName
;
|
Example:
|
import java.awt.Rectangle; |
Purpose:
To import a class from a package for use in a program.
|
File MoveRect.java
1 |
import java.awt.Rectangle; |
2 |
|
3 |
public class MoveTest |
4 |
{ |
5 |
public static void main(String[] args) |
6 |
{ |
7 |
Rectangle cerealBox = new Rectangle(5, 10, 20, 30); |
8 |
// move the rectangle |
9 |
cerealBox.translate(15, 25); |
10 |
// print the moved rectangle |
11 |
System.out.println(cerealBox); |
12 |
} |
A Simple Class
public class Greeter
{
public String sayHello()
{
String message ="Hello,World!";
return message;
}
}
Method Definition
- access specifier (such as public)
- return type (such as String or void)
- method name (such as sayHello)
- list of parameters (empty for sayHello)
- method body in { }
Method Parameters
public class Rectangle
{
. . .
public void translate(int x, int y)
{
method body
}
. . .
}
Syntax 2.4: Method Implementation
public class ClassName { ... accessSpecifier returnType methodName(parameterType parameterName,...) { method body } ... }
|
|
Example:
|
public class Greeter { public String sayHello() { String message ="Hello,World!"; return message; } }
|
Purpose:
To define the behavior of a method A method definition specifies the method
name, parameters, and the statements for carrying out the method's actions.
|
Syntax 2.5: The return Statement
|
return expression;
or
return; |
Example:
Purpose:
To specify the value that a method returns, and exit the method immediately.
The return value becomes the value of the method call expression.
|
Testing a Class
- Test class: a class with a main method that contains statements to
test another class.
- Typically carries out the following steps:
- Construct one or more objects of the class that is being tested.
- Invoke one or more methods.
- Print out one or more results
A Test Class for the Greeter Class
public class GreeterTest
{
public static void main(String [] args))
{
Greeter worldGreeter = new Greeter();
System.out.println(worldGreeter.sayHello());
}
}
Building a Test Program
- Make a new subfolder for your program.
- Make two files, one for each class.
- Compile both files.
- Run the test program.
Testing with the SDK Tools
mkdir greeter
cd greeter
edit Greeter.java
edit GreeterTest.java
javac Greeter.java
javac GreeterTest.java
java GreeterTest
Testing with BlueJ
Instance Fields
public class Greeter
{
...
private String name;
}
- access specifier (such as private)
- type of variable (such as String)
- name of variable (such as name)
Instance Fields
Accessing Instance Fields
- The sayHello method of the Greeter class can access
the private instance field:
public String sayHello()
{
String message = "Hello, " + name + "!";
return message;
}
- Other methods cannot:
public class GreeterTest
{
public static void main(String[] args)
{
. . .
System.out.println(daveGreeter.name); // ERROR
}
}
- Encapsulation = Hiding data and providing access through methods
Syntax 2.6 : Instance Field Declaration
|
accessSpecifier class ClassName { ... accessSpecifier fieldType fieldName; ... }
|
Example:
|
public class Greeter { ... private String name; ... }
|
Purpose:
To define a field that is present in every object of a class
|
Constructors
Syntax 2.7 : Constructor Implementation
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accessSpecifier class ClassName { ... accessSpecifier ClassName(parameterType parameterName ...) { constructor implementation } ... }
|
Example:
|
public class Greeter { ... public Greeter(String aName) { name = aName; } ... }
|
Purpose:
To define the behavior of a constructor, which is used to initialize the
instance fields of newly created objects
|
File Greeter.java
1 |
public class Greeter |
2 |
{ |
3 |
public Greeter(String aName) |
4 |
{ |
5 |
name = aName; |
6 |
} |
7 |
|
8 |
public String sayHello() |
9 |
{ |
10 |
String message = "Hello, " + name + "!"; |
11 |
return message; |
12 |
} |
13 |
|
14 |
private String name; |
15 |
} |
File GreeterTest.java
1 |
public class GreeterTest |
2 |
{ |
3 |
public static void main(String[] args) |
4 |
{ |
5 |
Greeter worldGreeter = new Greeter("World"); |
6 |
System.out.println(worldGreeter.sayHello()); |
7 |
|
8 |
Greeter daveGreeter = new Greeter("Dave"); |
9 |
System.out.println(daveGreeter.sayHello()); |
10 |
} |
11 |
} |
Designing the Public Interface
Behavior of bank account:
- deposit money
- withdraw money
- get balance
Methods of BankAccount class:
- deposit
- withdraw
- getBalance
BankAccount Public Interface
public BankAccount()
public BankAccount(double initialBalance)
public void deposit(double amount)
public void withdraw(double amount)
public double getBalance()
Using the Public Interface
Commenting the Public Interface
/**
Withdraws money from the bank account.
@param the amount to withdraw
*/
public void withdraw(double amount)
{
implementation filled in later
}
/**
Gets the current balance of the bank account.
@return the current balance
*/
public double getBalance()
{
implementation filled in later
}
Class Comment
/**
A bank account has a balance that can
be changed by deposits and withdrawals.
*/
public class BankAccount
{
...
}
Javadoc Method Summary
Javadoc Method Detail
BankAccount Class Implementation
- Determine instance variables to hold object state
private double balance
- Implement methods and constructors
File BankAccount.java
1 | /** |
2 | A bank account has a balance that can be changed by |
3 | deposits and withdrawals. |
4 | */ |
5 | public class BankAccount |
6 | { |
7 | /** |
8 | Constructs a bank account with a zero balance |
9 | */ |
10 | public BankAccount() |
11 | { |
12 | balance = 0; |
13 | } |
14 | |
15 | /** |
16 | Constructs a bank account with a given balance |
17 | @param initialBalance the initial balance |
18 | */ |
19 | public BankAccount(double initialBalance) |
20 | { |
21 | balance = initialBalance; |
22 | } |
23 | |
24 | /** |
25 | Deposits money into the bank account. |
26 | @param amount the amount to deposit |
27 | */ |
28 | public void deposit(double amount) |
29 | { |
30 | double newBalance = balance + amount; |
31 | balance = newBalance; |
32 | } |
33 | |
34 | /** |
35 | Withdraws money from the bank account. |
36 | @param amount the amount to withdraw |
37 | */ |
38 | public void withdraw(double amount) |
39 | { |
40 | double newBalance = balance - amount; |
41 | balance = newBalance; |
42 | } |
43 | |
44 | /** |
45 | Gets the current balance of the bank account. |
46 | @return the current balance |
47 | */ |
48 | public double getBalance() |
49 | { |
50 | return balance; |
51 | } |
52 | |
53 | private double balance; |
54 | } |
File BankAccountTest.java
1 | /** |
2 | A class to test the BankAccount class. |
3 | */ |
4 | public class BankAccountTest |
5 | { |
6 | /** |
7 | Tests the methods of the BankAccount class. |
8 | @param args not used |
9 | */ |
10 | public static void main(String[] args) |
11 | { |
12 | BankAccount harrysChecking = new BankAccount(); |
13 | harrysChecking.deposit(2000); |
14 | harrysChecking.withdraw(500); |
15 | System.out.println(harrysChecking.getBalance()); |
16 | } |
17 | } |
Calling a Method in BlueJ
The Method Return Value in BlueJ
Variable Types
- Instance fields (balance in BankAccount)
- Local variables (newBalance in deposit method)
- Parameter variables (amount in deposit method)
Explicit and Implicit Parameters
public void withdraw(double amount)
{
double newBalance = balance - amount;
balance = newBalance;
}
balance is the balance of the object to the left of the dot:
momsSavings.withdraw(500)
means
double newBalance = momsSavings.balance - amount;
momsSavings.balance = newBalance;