Java Programming（Part 3） Object-Oriented Programming

1. Java Programming（Part 3）Object-Oriented Programming 鄧姚文 http://w3.im.knu.edu.tw/~joseph/ joseph@im.knu.edu.tw

2. 內容大綱 • 物件與類別 • 資料成員 • Methods • 繼承 • 介面 • 物件型態轉換 • 範例程式

3. 物件與類別 • 物件（Object） • 一個完整的個體 • 包含資料（Data Member，Property，Instance Variable） • 包含處理資料所需的方法（Method） • 封裝（Encapsulation） • 對外提供一致的介面 • 隱藏內部細節（Information Hiding） • 類別（Class） • 物件的定義 • 套件（Package） • 相關類別的集合

4. 物件與類別 Integer s = new Integer("321"); Integer n = new Integer(100); s 類別 物件 Integer 321 n m Integer 100 Integer m = n;

5. 物件與類別：資料成員 (Data Member) • 修飾詞 • 存取權限 • public：可供外部存取 • protected：只供內部存取，但衍生類別亦可存取 • private：只供內部存取 • final：不可更改內容（常數） • static：Class Variable，相同類別的所有物件共用一份資料 • 資料型態： • Primitive type • Class

6. public class BlackHouse { private static Blueprint bp = new Blueprint(1); // 設計圖 public NumberPlate np; // 門牌 protected Fireplace fp; // 壁爐 } public class GrayHouse extends BlackHouse { private static Blueprint bp = new Blueprint(2); // 設計圖 // 自動有一個門牌 // 自動有一個壁爐 } … BlackHouse bh1 = new BlackHouse(); BlackHouse bh2 = new BlackHouse(); // bh1 和 bh2 的設計圖是同一份 // bh1 和 bh2 各有一個門牌

7. 存取權限

8. 範例：存取權限、Package package com.shinewave.erp; import java.util.Hashtable; import java.awt.*; // wildcard but no recursive import java.awt.font.TextAttribute; import java.awt.color.*; import com.shinewave.bank.*; public class AccountTest { SavingsAccount momsSavings; // package access public TimeDepositAccount collegeFund; private CheckingAccount harrysChecking; ... }

9. 物件與類別：Method • 修飾詞 • 存取權限 public、protected、private • static：相同類別的所有物件共用一份 • abstract：只定義 signature 不提供實作，強迫衍生類別實作 • Return type: primitive type、Class、void • 參數傳遞： • Primitive types: call by value • Class: call by reference • Signature: method name + parameter types

10. Primitive Types: call by value; Objects: call by reference import java.util.Hashtable; public class Calls { public Calls(int data, String str, Hashtable ht) { data += 100; str += " modified ?"; int v = ((Integer)ht.get("key1")).intValue() + 200; ht.put("key1", new Integer(v)); } public static void main(String[] args) { String strObject = "String object"; int iData = 100; Hashtable ht = new Hashtable(); ht.put("key1", new Integer(iData)); Calls cc = new Calls(iData, strObject, ht); System.out.println("iData = " + iData); System.out.println("strObject = " + strObject); System.out.println("ht(key1) = "+ ((Integer)ht.get("key1")).toString()); } }

11. 範例：Method Signature、Overloading、Abstract method public class BasicAccountManager { public void endOfMonth(SavingsAccount savings) { savings.addInterest(); } public void endOfMonth(CheckingAccount checking) { checking.deductFees(); } // must override this one public abstract void printBalance(Account account); }

12. 物件與類別：建構子（Constructor） • A constructor is a special method • Constructor 的 method name 必須與 class name 相同（包括大小寫） • Constructor 沒有傳回值，也不必標示傳回值資料型態（連 void 都不必寫） • Constructor 必須是 public • 一個 class 可以有多個 constructor，但必須每一個 constructor 的 method signature 都不相同（Overloading）

13. public class Sum1 { private int n; public Sum1() { this.n = 10; } public Sum1(String N) { this.n = Integer.parseInt(N); } public void print() { ... } public static void main(String[] args) { Sum1 sum = (args.length == 1) ? new Sum1(args[0]) : new Sum1(); sum.print(); } }

14. 繼承（Inheritance） • The “is-a” relationship between a more general superclass and a more specialized subclass • A savings account is a bank account • A checking account is a bank account • A time deposit account is a savings account • super：the superclass • this：this class

15. BankAccount getBalance deposit withdraw SavingsAccount CheckingAccount addInterest deductFees deposit withdraw TimeDepositAccount addInterestwithdraw

16. public class BankAccount { public BankAccount() { balance = 0; } public BankAccount(double initialBalance) { balance = initialBalance; } public void deposit(double amount) { balance = balance + amount; } public void withdraw(double amount) { balance = balance - amount; } public double getBalance() { return balance; } public void transfer(BankAccount other, double amount) { withdraw(amount); other.deposit(amount); } private double balance; }

17. public class SavingsAccount extends BankAccount { public SavingsAccount(double rate) { interestRate = rate; } public void addInterest() { double interest = getBalance() * interestRate / 100; deposit(interest); } private double interestRate; }

18. public class CheckingAccount extends BankAccount { public CheckingAccount(int initialBalance) { // construct superclass super(initialBalance); // initialize transaction count transactionCount = 0; } public void deposit(double amount) { transactionCount++; // now add amount to balance super.deposit(amount); } public void withdraw(double amount) { transactionCount++; // now subtract amount from balance super.withdraw(amount); } public void deductFees() { if (transactionCount > FREE_TRANSACTIONS) { double fees = TRANSACTION_FEE * (transactionCount - FREE_TRANSACTIONS); super.withdraw(fees); } transactionCount = 0; } private int transactionCount; private static final int FREE_TRANSACTIONS = 3; private static final double TRANSACTION_FEE = 2.0; }

19. public class TimeDepositAccount extends SavingsAccount { public TimeDepositAccount(double rate, int maturity) { super(rate); periodsToMaturity = maturity; } public void addInterest() { periodsToMaturity--; super.addInterest(); } public void withdraw(double amount) { if (periodsToMaturity > 0) super.withdraw(EARLY_WITHDRAWAL_PENALTY); super.withdraw(amount); } private int periodsToMaturity; private static double EARLY_WITHDRAWAL_PENALTY = 20; }

20. public class AccountTest { public static void main(String[] args) { SavingsAccount momsSavings = new SavingsAccount(0.5); TimeDepositAccount collegeFund = new TimeDepositAccount(1, 3); CheckingAccount harrysChecking = new CheckingAccount(0); momsSavings.deposit(10000); collegeFund.deposit(10000); momsSavings.transfer(harrysChecking, 2000); collegeFund.transfer(harrysChecking, 980); harrysChecking.withdraw(500); harrysChecking.withdraw(80); harrysChecking.withdraw(400); endOfMonth(momsSavings); endOfMonth(collegeFund); endOfMonth(harrysChecking); printBalance("mom's savings", momsSavings); printBalance("the college fund", collegeFund); printBalance("Harry's checking", harrysChecking); } public static void endOfMonth(SavingsAccount savings) { savings.addInterest(); } public static void endOfMonth(CheckingAccount checking) { checking.deductFees(); } public static void printBalance(String name, BankAccount account) { System.out.println("The balance of "+name+ " account is $"+account.getBalance()); } }

21. 介面（Interface） • A specialized view of an object • An interface does not have instance variables • Only constants • Automatic public static final • All methods in an interface are abstract • All methods in an interface are automatically public

22. 範例：interface package com.shinewave.bank; public interface Account { String DOLLAR_SIGN = "$"; String printBalance(String accountName); }

23. package com.shinewave.bank; public class SavingsAccount extends BankAccount implements Account { public SavingsAccount(double rate) { interestRate = rate; } public void addInterest() { double interest = getBalance() * interestRate / 100; deposit(interest); } public String printBalance(String name) { String s = "The balance of " + name + " saving account is " + DOLLAR_SIGN + this.getBalance(); return s; } private double interestRate; }

24. package com.shinewave.bank; public class CheckingAccount extends BankAccount implements Account { public CheckingAccount(int initialBalance) { // construct superclass super(initialBalance); // initialize transaction count transactionCount = 0; } … public String printBalance(String name) { String s = "The balance of " + name + " checking account is " + DOLLAR_SIGN + this.getBalance(); return s; } … }

25. package com.shinewave.bank; public class TimeDepositAccount extends SavingsAccount implements Account { public TimeDepositAccount(double rate, int maturity) { super(rate); periodsToMaturity = maturity; } … public String printBalance(String name) { String s = "The balance of " + name + " time deposit account is " + DOLLAR_SIGN + this.getBalance(); return s; } … }

26. SavingsAccount sa = new SavingsAccount(); CheckingAccount ca = new CheckingAccount(); TimeDepositAccount tda = new TimeDepositAccount(); ... Account a1 = (Account)sa; Account a2 = (Account)ca; Account a3 = (Account)tda; ... a1.printBalance(); a2.printBalance(); a3.printBalance(); .... printBalance() printBalance() getBalance() printBalance() BankAccount SavingsAccount CheckingAccount TimeDepositAccount

27. 物件型態轉換 • 可以將 subclass 物件轉換成 superclass 物件使用 • 將 superclass 物件轉換成 subclass 物件使用時要小心！

28. 範例：轉型 int iData = 100; Integer iObj = new Integer(iData); Hashtable ht = new Hashtable(); ht.put("key1", iObj); // convert Integer to Object ... Integer iObj2 = (Integer)ht.get("key1"); int iData2 = iObj2.intValue() + 200; ...

29. Polymorphism • Selecting a method among several methods that have the same name on the basis of the actual types of the implicit parameters • 實際上被執行的 method 是該物件真實類別所實作的 method

30. SavingsAccount sa = new SavingsAccount(); CheckingAccount ca = new CheckingAccount(); TimeDepositAccount tda = new TimeDepositAccount(); ... AccountReport ba1 = (AccountReport)sa; AccountReport ba2 = (AccountReport)ca; AccountReport ba3 = (AccountReport)tda; ... ba1.printBalance(); ba2.printBalance(); ba3.printBalance(); .... printBalance() printBalance() getBalance() printBalance() BankAccount SavingsAccount CheckingAccount TimeDepositAccount