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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 try 10 { 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 try 20 { 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 try 30 { 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
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
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 ArrayIndexOutOfBoundsException 11 { 12 int a[] ={1,2}; 13 System.out.println(a[2]); 14 } 15 16 public void throwTest3() throws FileNotFoundException 17 { 18 File file=new File("abc.txt"); 19 new BufferedReader(new FileReader(file)); 20 } 21 22 public void throwTest4() throws FileNotFoundException 23 { 24 throw new FileNotFoundException("abc.txt"); 25 } 26 27 public static void main(String[] args) { 28 ThrowTest throwTest=new ThrowTest(); 29 30 try 31 { 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 try 51 { 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 try 61 { 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
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)
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 try 11 { 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 finally 20 { 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.
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原文地址：http://www.cnblogs.com/LilianChen/p/4639471.html
