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 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
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 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
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 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.