java开始到熟悉72-76

本次内容：异常机制

1、为什么需要异常

2、异常

3、error类

4、exception类

5、exception类中的unchecked exception

举例：

6、常用异常处理方法

a.try

注意：一个try语句块至少得带一个finally语句块或catch语句块

 1 package array;
 2 /**
 3  * exception
 4  * @author acer
 5  *
 6  */
 7 public class exception {
 8     public static void main(String[] args)
 9     {
10         try {
11             Thread.sleep(1000);
12         } catch (InterruptedException e) {
13             // TODO Auto-generated catch block
14             e.printStackTrace();
15         }
16     }
17 }

 1 package array;
 2 /**
 3  * try catch finally
 4  */
 5 import java.io.FileNotFoundException;
 6 import java.io.FileReader;
 7 import java.io.IOException;
 8 
 9 public class trycatch {
10     public static void main(String[] args)
11     {
12         FileReader f=null;
13         try {
14             f=new FileReader("d:/a.txt");
15             char c,d;
16             c = (char)f.read();
17             d = (char)f.read();
18             System.out.println(""+c+d);
19         } catch (FileNotFoundException e) {
20             e.printStackTrace();
21         } catch (IOException e) {
22             e.printStackTrace();
23         }finally{
24                 try {
25                     if(f!=null)
26                     f.close();
27                 } catch (IOException e) {
28                     e.printStackTrace();
29                 }
30             }
31     }
32 }

运行结果：
ab

（说明：a.txt文本中的内容：abcdefg）

不同内容：try，catch，finally，return执行顺序

代码1：

 1 package array;
 2 
 3 import java.io.FileInputStream;
 4 import java.io.FileNotFoundException;
 5 import java.io.IOException;
 6 
 7 /**
 8  * 测试try,catch,finally,return执行顺序
 9  * @author acer
10  *
11  */
12 public class exeshunxu {
13     public static void main(String[] args)
14     {
15         String str=new exeshunxu().openfile();
16         System.out.println(str);
17     }
18     String openfile()
19     {
20         try {
21             System.out.println("aaa");
22             FileInputStream fi=new FileInputStream("d:/a.txt");
23             int a=fi.read();
24             System.out.println("bbb");
25             return "try";
26             
27         } catch (FileNotFoundException e) {
28             System.out.println("catchingchild..................");
29             e.printStackTrace();
30             return "catch filenotfoundexception";
31         } catch (IOException e) {
32             System.out.println("catchingfather................");
33             e.printStackTrace();
34             return "catch ioexception";
35         }finally{
36             System.out.println("finally...............");
37             return "finally";
38         }
39     }
40 }
41

运行结果：
aaa
bbb
finally...............
finally

代码2：

 1 package array;
 2 
 3 import java.io.FileInputStream;
 4 import java.io.FileNotFoundException;
 5 import java.io.IOException;
 6 
 7 /**
 8  * 测试try,catch,finally,return执行顺序
 9  * @author acer
10  *
11  */
12 public class exeshunxu {
13     public static void main(String[] args)
14     {
15         String str=new exeshunxu().openfile();
16         System.out.println(str);
17     }
18     String openfile()
19     {
20         try {
21             System.out.println("aaa");
22             FileInputStream fi=new FileInputStream("d:/a.txt");
23             int a=fi.read();
24             System.out.println("bbb");
25             return "try";
26             
27         } catch (FileNotFoundException e) {
28             System.out.println("catchingchild..................");
29             e.printStackTrace();
30             return "catch filenotfoundexception";
31         } catch (IOException e) {
32             System.out.println("catchingfather................");
33             e.printStackTrace();
34             return "catch ioexception";
35         }finally{
36             System.out.println("finally...............");
37         }
38     }
39 }
40

运行结果：
aaa
bbb
finally...............
try

代码3：

 1 package array;
 2 
 3 import java.io.FileInputStream;
 4 import java.io.FileNotFoundException;
 5 import java.io.IOException;
 6 
 7 /**
 8  * 测试try,catch,finally,return执行顺序
 9  * @author acer
10  *
11  */
12 public class exeshunxu {
13     public static void main(String[] args)
14     {
15         String str=new exeshunxu().openfile();
16         System.out.println(str);
17     }
18     String openfile()
19     {
20         try {
21             System.out.println("aaa");
22             FileInputStream fi=new FileInputStream("d:/abcd.txt");
23             int a=fi.read();
24             System.out.println("bbb");
25             return "try";
26             
27         } catch (FileNotFoundException e) {
28             System.out.println("catchingchild..................");
29             e.printStackTrace();
30             return "catch filenotfoundexception";
31         } catch (IOException e) {
32             System.out.println("catchingfather................");
33             e.printStackTrace();
34             return "catch ioexception";
35         }finally{
36             System.out.println("finally...............");
37         }
38     }
39 }
40

运行结果：
aaa
catchingchild..................
java.io.FileNotFoundException: d:abcd.txt (系统找不到指定的文件。)
at java.io.FileInputStream.open(Native Method)
at java.io.FileInputStream.<init>(FileInputStream.java:138)
at java.io.FileInputStream.<init>(FileInputStream.java:97)
at array.exeshunxu.openfile(exeshunxu.java:22)
at array.exeshunxu.main(exeshunxu.java:15)
finally...............
catch filenotfoundexception

(注释：D盘中有a.txt文件，但没有abcd.txt文件)

由此可得到执行顺序为：

>1、执行try，catch，给返回值赋值；

>2、执行finally；

>3、return；

（一般不要在finally中使用return语句）

 1 package array;
 2 
 3 import java.io.FileNotFoundException;
 4 import java.io.FileReader;
 5 import java.io.IOException;
 6 /**
 7  * 抛出异常
 8  * @author acer
 9  *
10  */
11 public class throwexc {
12     public static void main(String[] args)
13     {    
14         try {
15             String str;
16             str = new throwexc().openfile();
17             System.out.println(str);
18         } catch (IOException e) {
19             e.printStackTrace();
20         }
21     }
22     String openfile() throws FileNotFoundException,IOException
23     {
24         FileReader fr=new FileReader("d:a.txt");
25         char c=(char)fr.read();
26         return ""+c;
27     }
28 }

 1 package array;
 2 
 3 import java.io.FileNotFoundException;
 4 import java.io.IOException;
 5 import java.text.ParseException;
 6 /**
 7  * 方法重写声明异常原则
 8  * @author acer
 9  *
10  */
11 class overexception{
12     public void method() throws IOException{}
13 }
14 class b extends overexception{
15     public void method() throws IOException{}
16 }
17 class c extends overexception{
18     public void method() throws FileNotFoundException{}
19 }
20 class d extends overexception{
21     public void method() throws IOException,ArithmeticException{}
22 }
23 class e extends overexception{
24     public void method() throws IOException,RuntimeException{}
25 }
26 class f extends overexception{
27     public void method() throws IOException,ParseException{}
28 }

 1 package array;
 2 
 3 import java.io.File;
 4 import java.io.FileNotFoundException;
 5 
 6 public class handthrows {
 7     public static void main(String[] args)
 8     {
 9         File fr=new File("d:/b.txt");
10         if(!fr.exists())
11         {
12             try{
13                 throw new FileNotFoundException("File is not exist!");
14             }catch(Exception e){
15                 e.printStackTrace();
16             }
17         }
18     }
19 }

运行结果：
java.io.FileNotFoundException: File is not exist!
at array.handthrows.main(handthrows.java:13)

（说明：手动抛出异常用的不多）

 1 package array;
 2 
 3 import java.io.IOException;
 4 /**
 5  * 自定义异常
 6  * @author acer
 7  *
 8  */
 9 class MyException extends IOException{
10     public MyException(){
11         
12     }
13     public MyException(String message){
14         super(message);
15     }
16 }
17 public class myexception {
18     public static void main(String[] args)
19     {
20         try{
21             new myexception().test();
22         }catch(MyException e){
23         }
24     }
25     public void test()throws MyException{}
26 }

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