# 處理對象的多種狀態及其相互轉換——狀態模式（三） 3 完整解決方案 Sunny軟件公司開發人員使用狀態模式來解決賬戶狀態的轉換問題，客戶端只需要執行簡單的存款和取款操作，系統根據余額將自動轉換到相應的狀態，其基本結構如圖4所示：  圖4 銀行賬戶結構圖 在圖4中，Account充當環境類角色，AccountState充當抽象狀態角色，NormalState、OverdraftState和RestrictedState充當具體狀態角色。完整代碼如下所示： 溫馨提示：代碼有點長，需要有耐心！ ``` //銀行賬戶：環境類 class Account { private AccountState state; //維持一個對抽象狀態對象的引用 private String owner; //開戶名 private double balance = 0; //賬戶余額 public Account(String owner,double init) { this.owner = owner; this.balance = balance; this.state = new NormalState(this); //設置初始狀態 System.out.println(this.owner + "開戶，初始金額為" + init); System.out.println("---------------------------------------------"); } public double getBalance() { return this.balance; } public void setBalance(double balance) { this.balance = balance; } public void setState(AccountState state) { this.state = state; } public void deposit(double amount) { System.out.println(this.owner + "存款" + amount); state.deposit(amount); //調用狀態對象的deposit()方法 System.out.println("現在余額為"+ this.balance); System.out.println("現在帳戶狀態為"+ this.state.getClass().getName()); System.out.println("---------------------------------------------"); } public void withdraw(double amount) { System.out.println(this.owner + "取款" + amount); state.withdraw(amount); //調用狀態對象的withdraw()方法 System.out.println("現在余額為"+ this.balance); System.out.println("現在帳戶狀態為"+ this. state.getClass().getName()); System.out.println("---------------------------------------------"); } public void computeInterest() { state.computeInterest(); //調用狀態對象的computeInterest()方法 } } //抽象狀態類 abstract class AccountState { protected Account acc; public abstract void deposit(double amount); public abstract void withdraw(double amount); public abstract void computeInterest(); public abstract void stateCheck(); } //正常狀態：具體狀態類 class NormalState extends AccountState { public NormalState(Account acc) { this.acc = acc; } public NormalState(AccountState state) { this.acc = state.acc; } public void deposit(double amount) { acc.setBalance(acc.getBalance() + amount); stateCheck(); } public void withdraw(double amount) { acc.setBalance(acc.getBalance() - amount); stateCheck(); } public void computeInterest() { System.out.println("正常狀態，無須支付利息！"); } //狀態轉換 public void stateCheck() { if (acc.getBalance() > -2000 && acc.getBalance() <= 0) { acc.setState(new OverdraftState(this)); } else if (acc.getBalance() == -2000) { acc.setState(new RestrictedState(this)); } else if (acc.getBalance() < -2000) { System.out.println("操作受限！"); } } } //透支狀態：具體狀態類 class OverdraftState extends AccountState { public OverdraftState(AccountState state) { this.acc = state.acc; } public void deposit(double amount) { acc.setBalance(acc.getBalance() + amount); stateCheck(); } public void withdraw(double amount) { acc.setBalance(acc.getBalance() - amount); stateCheck(); } public void computeInterest() { System.out.println("計算利息！"); } //狀態轉換 public void stateCheck() { if (acc.getBalance() > 0) { acc.setState(new NormalState(this)); } else if (acc.getBalance() == -2000) { acc.setState(new RestrictedState(this)); } else if (acc.getBalance() < -2000) { System.out.println("操作受限！"); } } } //受限狀態：具體狀態類 class RestrictedState extends AccountState { public RestrictedState(AccountState state) { this.acc = state.acc; } public void deposit(double amount) { acc.setBalance(acc.getBalance() + amount); stateCheck(); } public void withdraw(double amount) { System.out.println("帳號受限，取款失敗"); } public void computeInterest() { System.out.println("計算利息！"); } //狀態轉換 public void stateCheck() { if(acc.getBalance() > 0) { acc.setState(new NormalState(this)); } else if(acc.getBalance() > -2000) { acc.setState(new OverdraftState(this)); } } } ``` 編寫如下客戶端測試代碼： ``` class Client { public static void main(String args[]) { Account acc = new Account("段譽",0.0); acc.deposit(1000); acc.withdraw(2000); acc.deposit(3000); acc.withdraw(4000); acc.withdraw(1000); acc.computeInterest(); } } ``` 編譯并運行程序，輸出結果如下： ``` 段譽開戶，初始金額為0.0 --------------------------------------------- 段譽存款1000.0 現在余額為1000.0 現在帳戶狀態為NormalState --------------------------------------------- 段譽取款2000.0 現在余額為-1000.0 現在帳戶狀態為OverdraftState --------------------------------------------- 段譽存款3000.0 現在余額為2000.0 現在帳戶狀態為NormalState --------------------------------------------- 段譽取款4000.0 現在余額為-2000.0 現在帳戶狀態為RestrictedState --------------------------------------------- 段譽取款1000.0 帳號受限，取款失敗 現在余額為-2000.0 現在帳戶狀態為RestrictedState --------------------------------------------- 計算利息！ ```