1. 如何捕获异常
try
{
可能会出现异常的代码段；
}
catch(异常类型名 处理该异常对象)
{
异常处理代码段；
}
1 import java.io.*; 2 3 public class TryCatchTest { 4 5 public static void main(String[] args) { 6 File file = new File("abc.txt"); 7 int a[] = {1, 2}; 8 9 try10 {11 System.out.println(3/0);12 }13 catch(ArithmeticException e1)14 {15 System.out.println("3/0: ");16 System.out.println("This is ArithmeticException");17 }18 19 try20 {21 System.out.println(a[2]);22 }23 catch(ArrayIndexOutOfBoundsException e2)24 {25 System.out.println("a[2] is out of Array: ");26 System.out.println("This is ArrayIndexOutOfBoundsException");27 }28 29 try30 {31 BufferedReader input = new BufferedReader(new FileReader(file));32 }33 catch (FileNotFoundException e3)34 {35 System.out.println("abc.txt is not found: ");36 System.out.println("This is FileNotFoundException");37 }38 catch(IOException e)39 {40 System.out.println("This is IOException");41 }42 43 }44 45 }
3/0:
This is ArithmeticException
a[2] is out of Array:
This is ArrayIndexOutOfBoundsException
abc.txt is not found:
This is FileNotFoundException
2. 如何抛出异常
编写代码过程中，如果不想在这段代码中捕捉和处理一个可能出现的异常，那么就需要将这个异常传递出去，传递给调用它的方法去处理该异常。这个时候就需要使用throw 和throws
- throws语句在方法声明中使用，抛出异常
- throw语句在方法体内部使用，抛出异常
注意： 方法体中若使用了throw语句抛出异常，则必须在该方法声明中，采用throws语句来声明该方法体中抛出的异常，同时，throws语句声明抛出的异常，必须是方法体中throw语句抛出的异常或该异常的父类。
1 import java.io.*; 2 3 public class ThrowTest { 4 5 public void throwTest1() throws ArithmeticException 6 { 7 System.out.println(3/0); 8 } 9 10 public void throwTest2() throws ArrayIndexOutOfBoundsException11 {12 int a[] ={1,2};13 System.out.println(a[2]);14 }15 16 public void throwTest3() throws FileNotFoundException17 {18 File file=new File("abc.txt");19 new BufferedReader(new FileReader(file));20 }21 22 public void throwTest4() throws FileNotFoundException23 {24 throw new FileNotFoundException("abc.txt");25 }26 27 public static void main(String[] args) {28 ThrowTest throwTest=new ThrowTest();29 30 try31 {32 throwTest.throwTest1();33 }34 catch (ArithmeticException e1)35 {36 System.out.println("3/0: ");37 System.out.println("This is ArithmeticException");38 }39 40 try 41 {42 throwTest.throwTest2();43 }44 catch(ArrayIndexOutOfBoundsException e2)45 {46 System.out.println("a[2] is out of Array: ");47 System.out.println("This is ArrayIndexOutOfBoundsException");48 }49 50 try51 {52 throwTest.throwTest3();53 }54 catch (FileNotFoundException e3)55 {56 System.out.println("abc.txt is not found: ");57 System.out.println("This is FileNotFoundException");58 }59 60 try61 {62 throwTest.throwTest4();63 }64 catch (FileNotFoundException e3)65 {66 System.out.println("abc.txt is not found: ");67 System.out.println("This is FileNotFoundException");68 }69 70 }71 72 }
3/0:
This is ArithmeticException
a[2] is out of Array:
This is ArrayIndexOutOfBoundsException
abc.txt is not found:
This is FileNotFoundException
abc.txt is not found:
This is FileNotFoundException
3. 自定义异常
建立自己的异常类，要做的只是根据需要，从Exception类或者从Exception类的子类中继承出需要的类。习惯上，会经常为每一个异常类，提供一个默认的和一个包含详细信息的构造器。需要注意的是，自定义异常类，必须由程序员使用throw语句抛出。
1 public class MyException { 2 3 public static void main(String[] args) { 4 String str="2abcde"; 5 6 try 7 { 8 char c=str.charAt(0); 9 if(c<'a'||c>'z'||c<'A'||c>'Z')10 throw new FirstLetterException();11 }12 catch (FirstLetterException e)13 {14 System.out.println("This is FirstLetterException");15 }16 17 }18 19 }20 21 class FirstLetterException extends Exception{22 public FirstLetterException()23 {24 super("The first char is not a letter");25 }26 27 public FirstLetterException(String str)28 {29 super(str);30 }31 }
This is FirstLetterException
1 public class MyException { 2 3 public static void main(String[] args) throws FirstLetterException{ 4 throw new FirstLetterException(); 5 } 6 } 7 8 class FirstLetterException extends Exception{ 9 public FirstLetterException()10 {11 super("The first char is not a letter");12 }13 14 public FirstLetterException(String str)15 {16 super(str);17 }18 }
Exception in thread "main" FirstLetterException: The first char is not a letter
at MyException.main(MyException.java:5)
4. 使用finally语句
在使用try...catch语句是，若try语句中的某一句出现异常情况，那么这部分try语句段中，从出现异常的语句开始，之后的所有语句都不会被执行，直到这部分try语句段结束。
但是在很多情况下，希望无论是否出现异常，某些语句都需要被执行。那么就可以把这部分代码放在finally语句段中，即使try或catch语句段中含有return语句，程序都会在异常抛出后先执行finally语句段，除非try或catch语句段中执行System.exit()方法，或者是出现Error错误，finally语句才不会被执行而退出程序。
1 import java.io.*; 2 3 public class FinallyTest { 4 5 public static void main(String[] args) { 6 File file=null; 7 BufferedReader input=null; 8 file=new File("abc.txt"); 9 10 try11 {12 input=new BufferedReader(new FileReader(file));13 }14 catch(FileNotFoundException e)15 {16 System.out.print("abc.txt is not found: ");17 System.out.println("This is FileNotFoundException");18 }19 finally20 {21 System.out.println("This is finally code part.");22 }23 24 }25 26 }
abc.txt is not found: This is FileNotFoundException
This is finally code part.