第二章

物件和類別簡介

2.1 物件和類別

物件(object): entity that you can manipulate in your programs (by invoking methods)
每一物件都屬於某一類別(class)
類別: 一組具有相同行為的物件集合
類別決定有那些方法可以使用, 如
"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.

2.2 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:

兩個物件變數指到同一物件

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

寫一測試程式

Java 應用程式(Application) 由一組類別(class)構成, 其中有一主類別。執行程式指執行在 main 類別中的方法(或稱函式) main。

界定一主類別, 例如 MoveTest
提供一 main 方法
在 main 方法內, 寫碼實現所要做的工作
引入(import) Java 的庫存類別, 如:
import java.awt.Rectangle;
java.awt 是package, 而 Rectangle是其中的一個類別
java.lang 中的 package, 如 String 和 System, 不需引入

Syntax 2.3 : Importing a Class from a Package

import packageName.ClassName;

Example:

import java.awt.Rectangle;

Purpose:

To import a class from a package for use in a program.

File MoveTest.java


import java.awt.Rectangle;

public class MoveTest
{
   public static void main(String[] args)
   {
      Rectangle cerealBox = new Rectangle(5, 10, 20, 30);
      // move the rectangle
      cerealBox.translate(15, 25);
      // print the moved rectangle
      System.out.println(cerealBox);
   }
}

界定類別

一個簡單類別

public class Greeter 
{ 
   public String sayHello() 
   { 
      String message ="Hello,World!"; 
      return message; 
   } 
}

方法的界定

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 { }

方法參數

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:

return message;

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.

2.4 測試一類別

測試用的類別(Test class): 一類別,其中的 main 方法含有測試另一類別的敘述。
通常進行下列步驟:

建構一個或更多屬於要測試類別的物件。
引用(invoke)一個或更多方法。
列出一個或更多的結果。

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()); 
   } 
}

說明:

字串類別用 String 。
System.out 為類別 printStream 的個例(instance), println 為其方法。

建造測試用的程式

造一新目錄以便容納所寫的程式。
造二個檔案, 一個儲存要測試的類別程式碼(如 Greeter.java), 另一個儲存測試用的類別(如 GreeterTest.java)。
編譯這兩個程式檔。
執行測試用的程式。

使用爪哇系統設計工具(Java System Design Kit)測試

用法

編譯:
- javac Greeter.java
- javac GreeterTest.java
執行: java GreeterTest

注意:

編譯時檔名要有 extension, 執行時檔名不要有 extension (預設為 .class)。
編譯時, 檔名和類別名稱必須相同, 大小寫要一致。

mkdir greeter 
cd greeter 
edit Greeter.java 
edit GreeterTest.java 
javac Greeter.java 
javac GreeterTest.java 
java GreeterTest

使用 BlueJ 測試

2.5 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

2.6 Constructors

A constructor initializes the instance variables

Constructor name = class name

public class Greeter
{
   public Greeter(String aName)
   {
      name = aName;
   }
   . . .
}

Invoked in new expression
new Greeter("Dave")

Syntax 2.7 : Constructor Implementation

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


public class Greeter
{
   public Greeter(String aName)
   {
      name = aName;
   }

   public String sayHello()
   {
      String message = "Hello, " + name + "!";
      return message;
   }

   private String name;
}

2.7 設計公用界面

Behavior of bank account:

存款(deposit)
提款(withdraw)
查詢餘額(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()

使用公用界面

轉移金額Transfer balance

double amt = 500;
momsSavings.withdraw(amt);
harrysChecking.deposit(amt);

添加利息(interest)

double rate = 5; // 5%
double amt = acct.getBalance() * rate / 100;
acct.deposit(amt);

2.8 註解公用界面

/**
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
}

註解類別

/**
A bank account has a balance that can
be changed by deposits and withdrawals.
*/public class BankAccount { ... }

Javadoc Method Summary

Javadoc Method Detail

2.9 實現類別

實現 `BankAccount` 類別

界定用以保存物件狀態的個例變數(instance variable)
private double balance
實現方法和建構式(constructor)

File BankAccount.java


/**
   A bank account has a balance that can be changed by
   deposits and withdrawals.
*/
public class BankAccount
{
   /**
      Constructs a bank account with a zero balance
   */
   public BankAccount()
   {
      balance = 0;
   }

   /**
      Constructs a bank account with a given balance
      @param initialBalance the initial balance
   */
   public BankAccount(double initialBalance)
   {
      balance = initialBalance;
   }

   /**
      Deposits money into the bank account.
      @param amount the amount to deposit
   */
   public void deposit(double amount)
   {
      double newBalance = balance + amount;
      balance = newBalance;
   }

   /**
      Withdraws money from the bank account.
      @param amount the amount to withdraw
   */
   public void withdraw(double amount)
   {
      double newBalance = balance - amount;
      balance = newBalance;
   }

   /**
      Gets the current balance of the bank account.
      @return the current balance
   */
   public double getBalance()
   {
      return balance;
   }

   private double balance;
}

File BankAccountTest.java


/**
   A class to test the BankAccount class.
*/
public class BankAccountTest
{
   /**
      Tests the methods of the BankAccount class.
      @param args not used
   */
   public static void main(String[] args)
   {
      BankAccount harrysChecking = new BankAccount();
      harrysChecking.deposit(2000);
      harrysChecking.withdraw(500);
      System.out.println(harrysChecking.getBalance());
   }
}

在 BlueJ 引用方法

The Method Return Value in BlueJ

2.10 變數型式

Instance fields (balance in BankAccount)
belong to an object
Local variables (newBalance in deposit method)
belong to a method
must be initialized
Parameter variables (amount in deposit method)
belong to a method
initialized in a method call


public class BankAccount
{
    ...

   public void deposit(double amount)      // parameter varialbe
   {
      double newBalance = balance + amount; // local varialbe
      balance = newBalance;
   }

   public void withdraw(double amount)        // ? varialbe
   {
      double newBalance = balance - amount;    // ? varialbe
      balance = newBalance;
   }

   public double getBalance()
   {
      return balance;              // ? varialbe
   }

   private double balance;          // instance varialbe
}

2.11 Explicit and Implicit Parameters

public void withdraw(double amount)
{  
   double newBalance = balance - amount;
   balance = newBalance;
}

explicit: amount
implicit: the bank account object

balance is the balance of the object momsSavings:

momsSavings.withdraw(500)

means

   double newBalance = momsSavings.balance - amount;
   momsSavings.balance = newBalance;

implicit 參數可用this表明, 如

public void deposit(double amount)
{
    double newBalance = this.balance + amount;
    this.balance = newBalance;
}

第二章

物件和類別簡介

2.1 物件和類別

Rectangle Class

Rectangle Shapes

A Rectangle Object

Syntax 2.1: Object Construction

Example:

Purpose:

2.2 Object Variables

Uninitialized and Initialized Variables

兩個物件變數指到同一物件

Syntax 2.2: Variable Definition

Example:

Purpose:

寫一測試程式

Syntax 2.3 : Importing a Class from a Package

Example:

Purpose:

File MoveTest.java

界定類別

一個簡單類別

方法的界定

方法參數

Syntax 2.4: Method Implementation

Example:

Purpose:

Syntax 2.5: The return Statement

Example:

Purpose:

2.4 測試一類別

A Test Class for the Greeter Class

建造測試用的程式

使用爪哇系統設計工具(Java System Design Kit)測試

使用 BlueJ 測試

2.5 Instance Fields

Instance Fields

Accessing Instance Fields

Syntax 2.6 : Instance Field Declaration

Example:

Purpose:

2.6 Constructors

Syntax 2.7 : Constructor Implementation

Example:

Purpose:

File Greeter.java

2.7 設計公用界面

BankAccount Public Interface

使用公用界面

2.8 註解公用界面

註解類別

Javadoc Method Summary

Javadoc Method Detail

2.9 實現類別

實現 BankAccount 類別

File BankAccount.java

File BankAccountTest.java

在 BlueJ 引用方法

The Method Return Value in BlueJ

2.10 變數型式

2.11 Explicit and Implicit Parameters

A Test Class for the `Greeter` Class

`BankAccount` Public Interface

實現 `BankAccount` 類別