异常

异常是封装错误信息的对象，一般包含类型名称，提示信息和代码的错误行号。继承结构如下所示。
      Throwable
        |- Error        系统级错误
        |- Exception    可修复的异常
            |- 其他Exception
            |- RuntimeExceptin
                 |- NullPointerException
                 |- ArrayIndexOutOfBoundsException
                 |- ArithmeticException
                 |- NumerFormatException
                 |- InputMismatchException
                 |- ClassCastException

异常一般使用try-catch-finally来捕捉。使用throws可抛出存在默认抛出管道的异常。throw可手动抛出异常。

public class Test1 {
    public static void main(String[] args) {
        System.out.println(
         "输入两个浮点数：");
        double a = new Scanner(System.in).nextDouble();
        double b = new Scanner(System.in).nextDouble();
        
        try {
            double c = divide(a, b);
            System.out.println(
                    a+"/"+b+"="+c);
        } catch(ArithmeticException e) {
            System.out.println(
             "除数不能为0");
            
            System.out.println(e.getMessage());
            //打印完整异常信息
            e.printStackTrace();
        }
        
    }

    private static double divide(
            double a, 
            double b) {
        if(b == 0) {
            ArithmeticException e = 
             new ArithmeticException(
             "/ by zero");
            throw e;
        }
        double r = a / b;
        return r;
    }
}

不能抛出的异常要包装成可以抛出的类型进行抛出。

public class Test1 {
    public static void main(String[] args) {
        List<String> list = new ArrayList<>();
        Collections.addAll(list, 
         "2015-11-20",
         "2015-9-30",
         "2015-9-8",
         "2015-9-19",
         "2015-12-1",
         "2015-5-3",
         "kurgteurhtoehrotiheklrhtkeurh",
         "2015-5-19",
         "2015-5-8");
        System.out.println(list);
        Collections.sort(list);
        System.out.println(list);
        System.out.println("---------------");
        
        Collections.sort(list, new Comparator<String>() {
            @Override
            public int compare(
                    String o1,
                    String o2) {
                SimpleDateFormat fmt = 
                 new SimpleDateFormat(
                 "yyyy-MM-dd");
                try {
                    Date d1 = fmt.parse(o1);
                    Date d2 = fmt.parse(o2);
                    return d1.compareTo(d2);
                } catch (Exception e) {
                    //不是要修复异常
                    //而是包装成能抛出的异常，再抛出
                    throw new RuntimeException(e);
                }
            }
        });
        System.out.println(list);
        
    }
}

一般常见的类型有，NumberFormatException，ArrayIndexOutOfBoundsException，ParseException，IOException，ArithmeticException等等。用户除了系统定义的异常外也可以使用自定义异常。

public class Test1 {
    public static void main(String[] args) {
        System.out.println("用户名：");
        String n = new Scanner(System.in).nextLine();
        System.out.println("密码：");
        String p = new Scanner(System.in).nextLine();
        try {
            login(n, p);
            System.out.println("登陆成功");
        } catch (UsernameNotFoundException e) {
            System.out.println("用户名错误");
        } catch (WrongPasswordException e) {
            System.out.println("密码错误");
        }
    }

    private static void login(
            String n, 
            String p) throws UsernameNotFoundException, WrongPasswordException {
        
        if(! "abc".equals(n)) {
            throw new UsernameNotFoundException();
        }
        
        if(! "123".equals(p)) {
            throw new WrongPasswordException();
        }
    }
}

public class UsernameNotFoundException 
                extends Exception{

    public UsernameNotFoundException() {
        super();
    }

    public UsernameNotFoundException(String message, Throwable cause,
            boolean enableSuppression, boolean writableStackTrace) {
        super(message, cause, enableSuppression, writableStackTrace);
    }

    public UsernameNotFoundException(String message, Throwable cause) {
        super(message, cause);
    }

    public UsernameNotFoundException(String message) {
        super(message);
    }

    public UsernameNotFoundException(Throwable cause) {
        super(cause);
    }
    
}

public class WrongPasswordException 
                extends Exception{

    public WrongPasswordException() {
        super();
    }

    public WrongPasswordException(String message, Throwable cause,
            boolean enableSuppression, boolean writableStackTrace) {
        super(message, cause, enableSuppression, writableStackTrace);
    }

    public WrongPasswordException(String message, Throwable cause) {
        super(message, cause);
    }

    public WrongPasswordException(String message) {
        super(message);
    }

    public WrongPasswordException(Throwable cause) {
        super(cause);
    }
    
}