異常處理（005）_如何自定義異常

1、如何定義自己的異常

Java支持自己創建的異常。瞭解異常看這裏：什麼是java中的異常

方法如下：

1、所有的異常必須是Throwable的子類。
2、如果想寫一個檢查異常，需要擴展Exception類。
3、如果想編寫一個運行時異常，則需要擴展RuntimeException類。
4、異常類與任何其他類一樣，可以包含字段和方法。
我們可以定義如下自己的異常處理類：

class MyException extends Exception{
}

例子：

import java.io.*;

public class InsufficientFundsException extends Exception
{
private double amount;
public InsufficientFundsException(double amount)
{
this.amount = amount;
}
public double getAmount()
{
return amount;
}
}

爲了證明我們的使用用戶定義的異常，下面的CheckingAccount類包含一個withdraw()方法拋出一個InsufficientFundsException。

import java.io.*;

public class CheckingAccount
{
private double balance;
private int number;
public CheckingAccount(int number)
{
this.number = number;
}
public void deposit(double amount)
{
balance += amount;
}
public void withdraw(double amount) throws
InsufficientFundsException
{
if(amount <= balance)
{
balance -= amount;
}
else
{
double needs = amount - balance;
throw new InsufficientFundsException(needs);
}
}
public double getBalance()
{
return balance;
}
public int getNumber()
{
return number;
}
}

下面BankDemo程序演示調用deposit()和withdraw() 方法。

public class BankDemo
{
public static void main(String [] args)
{
CheckingAccount c = new CheckingAccount(101);
System.out.println("Depositing $500...");
c.deposit(500.00);
try
{
System.out.println("
Withdrawing $100...");
c.withdraw(100.00);
System.out.println("
Withdrawing $600...");
c.withdraw(600.00);
}catch(InsufficientFundsException e)
{
System.out.println("Sorry, but you are short $"
+ e.getAmount());
e.printStackTrace();
}
}
}

編譯所有上述三個文件並運行BankDemo，這將產生以下結果：
Depositing $500...

Withdrawing $100...

Withdrawing $600...
Sorry, but you are short $200.0
InsufficientFundsException
at CheckingAccount.withdraw(CheckingAccount.java:25)
at BankDemo.main(BankDemo.java:13)