Java: 異常處理機制

時間 2019-11-09

標籤 java 異常處理機制欄目 Java 简体版

原文原文鏈接

1. 如何捕獲異常

try java

{程序員

可能會出現異常的代碼段；spa

}code

catch(異常類型名處理該異常對象)對象

{blog

異常處理代碼段；繼承

}input

 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 FileNotFoundExceptionit

2. 如何拋出異常

編寫代碼過程當中，若是不想在這段代碼中捕捉和處理一個可能出現的異常，那麼就須要將這個異常傳遞出去，傳遞給調用它的方法去處理該異常。這個時候就須要使用throw 和throwsio

throws語句在方法聲明中使用，拋出異常
throw語句在方法體內部使用，拋出異常

注意：方法體中若使用了throw語句拋出異常，則必須在該方法聲明中，採用throws語句來聲明該方法體中拋出的異常，同時，throws語句聲明拋出的異常，必須是方法體中throw語句拋出的異常或該異常的父類。

 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. 自定義異常

創建本身的異常類，要作的只是根據須要，從Exception類或者從Exception類的子類中繼承出須要的類。習慣上，會常常爲每個異常類，提供一個默認的和一個包含詳細信息的構造器。須要注意的是，自定義異常類，必須由程序員使用throw語句拋出。

 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語句

在使用try...catch語句是，若try語句中的某一句出現異常狀況，那麼這部分try語句段中，從出現異常的語句開始，以後的全部語句都不會被執行，直到這部分try語句段結束。

可是在不少狀況下，但願不管是否出現異常，某些語句都須要被執行。那麼就能夠把這部分代碼放在finally語句段中，即便try或catch語句段中含有return語句，程序都會在異常拋出後先執行finally語句段，除非try或catch語句段中執行System.exit()方法，或者是出現Error錯誤，finally語句纔不會被執行而退出程序。

 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 }