銀行取錢肯定不能透支這個是眾所周知的，實現此功能就需要藉助多執行緒的知識！

先來看一下采取同步措施前產生的結果：

好的，我們先來定義一個賬戶類：

/**
 * 賬戶類，含卡號，還有餘額 通過同步方法防止出現餘額為負的情況
 * 
 * @author Mr.Gao
 */
public class Account {
	private String accountNo;
	private double balance;

	public Account(String accountNo, double balance) {
		super();
		this.accountNo = accountNo;
		this.balance = balance;
	}

	public String getAccountNo() {
		return accountNo;
	}

	public void setAccountNo(String accountNo) {
		this.accountNo = accountNo;
	}

	public double getBalance() {
		return balance;
	}

	public void setBalance(double balance) {
		this.balance = balance;
	}

	public int hashCode() {
		final int prime = 31;
		int result = 1;
		result = prime * result
				+ ((accountNo == null) ? 0 : accountNo.hashCode());
		long temp;
		temp = Double.doubleToLongBits(balance);
		result = prime * result + (int) (temp ^ (temp >>> 32));
		return result;
	}

	public boolean equals(Object obj) {
		if (this == obj)
			return true;
		if (obj == null)
			return false;
		if (getClass() != obj.getClass())
			return false;
		Account other = (Account) obj;
		if (accountNo == null) {
			if (other.accountNo != null)
				return false;
		} else if (!accountNo.equals(other.accountNo))
			return false;
		if (Double.doubleToLongBits(balance) != Double
				.doubleToLongBits(other.balance))
			return false;
		return true;
	}

	public String toString() {
		return "Account [accountNo=" + accountNo + ", balance=" + balance + "]";
	}

	/**
	 * 同步取錢方法，防止餘額為負
	 * @param accountAmount
	 */
	public synchronized void draw(double accountAmount) {
		if (balance > accountAmount) {
			System.out.println(Thread.currentThread().getName() + "  取錢    "
					+ accountAmount + "  成功");
			try {
				Thread.sleep(100);
			} catch (InterruptedException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			}
			balance -= accountAmount;
			System.out.println("餘額為\t" + balance);
		} else {
			System.err.println("取錢失敗");
		}
	}
}
 

/**
 * 取錢執行緒採用的是繼承的方式
 * 無須自己實現取錢的操作，直接呼叫account的draw()方法實現，符合"加鎖--修改--釋放鎖"的邏輯
 * 符合 領域驅動設計（每個類都是完整的領域物件，例如使用者賬戶提供相應的方法去處理賬戶所對應的需求） 
 * @author Mr.Gao
 * 
 */
public class DrawThread extends Thread {
	private Account account;
	private double drawAmount;

	public DrawThread(String name, Account account, double drawAmount) {
		super(name);
		this.account = account;
		this.drawAmount = drawAmount;
	}

	public void run() {
		// synchronized (account) {
		// if (account.getBalance() > drawAmount) {
		// System.out.println(this.getName() + "  取錢     " + drawAmount
		// + "   成功");
		//
		// try {
		// Thread.sleep(100);
		// } catch (InterruptedException e) {
		// // TODO Auto-generated catch block
		// e.printStackTrace();
		// }
		// account.setBalance(account.getBalance() - drawAmount);
		// System.out.println("餘額為\t" + account.getBalance());
		//
		// } else {
		// System.out.println("餘額不足");
		// }
		// }
		/*
		 *  直接呼叫account物件的draw()方法，同步方法的同步監視器是this，
		 *  this代表呼叫draw()方法的物件，也就是說，再進入draw()方法前先對account物件進行加鎖
		 */
		account.draw(drawAmount);
	}

}
 

/**
 * 結果測試
 * 
 * @author Mr.Gao
 * 
 */
public class AccountTest {

	public static void main(String[] args) {
		Account acc = new Account("123", 1000);
		new DrawThread("甲", acc, 800).start();
		new DrawThread("乙", acc, 800).start();
	}
}