java学习笔记系列:
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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");
}
}