1、异常处理概述
从一个读取两个整数并显示商的例子：
public static void main(String args[]) { Scanner input = new Scanner(System.in); System.out.print("Enter two integers: "); int number1 = input.nextInt(); int number2 = input.nextInt(); System.out.println(number1 + " / " + number2 + " is " + (number1 / number2)); }
Enter two integers: 3 0
Exception in thread "main" java.lang.ArithmeticException: / by zero
at Main.main(Main.java:18)
解决的一个简单的办法是添加一个if语句来测试第二个数字：
public class Main { public static void main(String args[]) { Scanner input = new Scanner(System.in); System.out.print("Enter two integers: "); int number1 = input.nextInt(); int number2 = input.nextInt(); if(number2 != 0) System.out.println(number1 + " / " + number2 + " is " + (number1 / number2)); else System.out.println("Divisor cannot be zero "); } }
为了演示异常处理的概念，包括如何创建、抛出、捕获以及处理异常，继续改写上面的程序如下：
public class Main { public static void main(String args[]) { Scanner input = new Scanner(System.in); System.out.print("Enter two integers: "); int number1 = input.nextInt(); int number2 = input.nextInt(); try { if(number2 == 0) throw new ArithmeticException("Divisor cannot be zero"); System.out.println(number1 + " / " + number2 + " is " + (number1 / number2)); } catch(ArithmeticException ex) { System.out.println("Exception: an integer " + "cannot be divided by zero "); } System.out.println("Execution continues ..."); } }
2、异常处理的优势
改用方法来计算商：
public class Main { public static int quotient(int number1, int number2) { if(number2 == 0) throw new ArithmeticException("Divisor cannot be zero"); return number1 / number2; } public static void main(String args[]) { Scanner input = new Scanner(System.in); System.out.print("Enter two integers: "); int number1 = input.nextInt(); int number2 = input.nextInt(); try { int result = quotient(number1, number2); System.out.println(number1 + " / " + number2 + " is " + result); } catch(ArithmeticException ex) { System.out.println("Exception: an integer " + "cannot be divided by zero "); } System.out.println("Execution continues ..."); } }
异常处理的优势就是将检测错误从处理错误中分离出来。
3、异常类型
4、关于异常处理的更多知识
java的异常处理模型基于三种操作：声明一个异常、抛出一个异常、捕获一个异常
声明异常
在方法中声明异常，就是在方法头中使用关键字throws，如下所示：
public void myMethod throws Exception1,Exception2,……,ExceptionN
抛出异常
检测一个错误的程序可以创建一个正确的异常类型的实例并抛出它
实例：
IllegalArgumentException ex = new IllegalArgumentException("Worng Argument"); throw ex;
或者直接：
throw new IllegalArgumentException("Worng Argument");
捕获异常
当抛出一个异常时，可以在try-catch中捕获和处理它：
try { statements; } catch (exception1 ex1){ handler for exception1; } catch (exception1 ex2){ handler for exception2; } …… catch (exception1 exN){ handler for exceptionN; }
从异常中获取信息
可以采用Throwable类中的方法获取异常的信息
public class test { public static void main(String[] args) { try { System.out.println(sum(new int[]{1,2,3,4,5})); } catch(Exception ex) { ex.printStackTrace(); System.out.println(ex.getMessage()); System.out.println(ex.toString()); System.out.println("Trace Info Obtained from getBackTrace"); StackTraceElement[] traceElements = ex.getStackTrace(); for(int i = 0; i < traceElements.length; i++) { System.out.print("monthod " + traceElements[i].getMethodName()); System.out.println("(" + traceElements[i].getClassName()); System.out.println(traceElements[i].getLineNumber() + ")"); } } } private static int sum(int[] list) { int sum = 0; for(int i = 0; i <= list.length; i++) { sum += list[i]; } return sum; } }
finally语句
无论异常是否出现，都希望执行某些代码，这时可以采取finally子句：
public class test { public static void main(String[] args) { PrintWriter output = null; try { output = new PrintWriter("wu.txt"); output.println("wlecome tio java"); } catch(IOException ex) { ex.printStackTrace(); } finally { if(output != null) output.close(); } System.out.println("End of the program"); } }
