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

 package array;
 /**
  * exception
  * @author acer
  *
  */
 public class exception {
     public static void main(String[] args)
     {
         try {
             Thread.sleep(1000);
         } catch (InterruptedException e) {
             // TODO Auto-generated catch block
             e.printStackTrace();
         }
     }
 }

 package array;
 /**
  * try catch finally
  */
 import java.io.FileNotFoundException;
 import java.io.FileReader;
 import java.io.IOException;

 public class trycatch {
     public static void main(String[] args)
     {
         FileReader f=null;
         try {
             f=new FileReader("d:/a.txt");
             char c,d;
             c = (char)f.read();
             d = (char)f.read();
             System.out.println(""+c+d);
         } catch (FileNotFoundException e) {
             e.printStackTrace();
         } catch (IOException e) {
             e.printStackTrace();
         }finally{
                 try {
                     if(f!=null)
                     f.close();
                 } catch (IOException e) {
                     e.printStackTrace();
                 }
             }
     }
 }

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：

 package array;

 import java.io.FileInputStream;
 import java.io.FileNotFoundException;
 import java.io.IOException;

 /**
  * 测试try,catch,finally,return执行顺序
  * @author acer
  *
  */
 public class exeshunxu {
     public static void main(String[] args)
     {
         String str=new exeshunxu().openfile();
         System.out.println(str);
     }
     String openfile()
     {
         try {
             System.out.println("aaa");
             FileInputStream fi=new FileInputStream("d:/a.txt");
             int a=fi.read();
             System.out.println("bbb");
             return "try";

         } catch (FileNotFoundException e) {
             System.out.println("catchingchild..................");
             e.printStackTrace();
             return "catch filenotfoundexception";
         } catch (IOException e) {
             System.out.println("catchingfather................");
             e.printStackTrace();
             return "catch ioexception";
         }finally{
             System.out.println("finally...............");
             return "finally";
         }
     }
 }
     

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

代码2：

 package array;

 import java.io.FileInputStream;
 import java.io.FileNotFoundException;
 import java.io.IOException;

 /**
  * 测试try,catch,finally,return执行顺序
  * @author acer
  *
  */
 public class exeshunxu {
     public static void main(String[] args)
     {
         String str=new exeshunxu().openfile();
         System.out.println(str);
     }
     String openfile()
     {
         try {
             System.out.println("aaa");
             FileInputStream fi=new FileInputStream("d:/a.txt");
             int a=fi.read();
             System.out.println("bbb");
             return "try";

         } catch (FileNotFoundException e) {
             System.out.println("catchingchild..................");
             e.printStackTrace();
             return "catch filenotfoundexception";
         } catch (IOException e) {
             System.out.println("catchingfather................");
             e.printStackTrace();
             return "catch ioexception";
         }finally{
             System.out.println("finally...............");
         }
     }
 }
     

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

代码3：

 package array;

 import java.io.FileInputStream;
 import java.io.FileNotFoundException;
 import java.io.IOException;

 /**
  * 测试try,catch,finally,return执行顺序
  * @author acer
  *
  */
 public class exeshunxu {
     public static void main(String[] args)
     {
         String str=new exeshunxu().openfile();
         System.out.println(str);
     }
     String openfile()
     {
         try {
             System.out.println("aaa");
             FileInputStream fi=new FileInputStream("d:/abcd.txt");
             int a=fi.read();
             System.out.println("bbb");
             return "try";

         } catch (FileNotFoundException e) {
             System.out.println("catchingchild..................");
             e.printStackTrace();
             return "catch filenotfoundexception";
         } catch (IOException e) {
             System.out.println("catchingfather................");
             e.printStackTrace();
             return "catch ioexception";
         }finally{
             System.out.println("finally...............");
         }
     }
 }
     

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）

7.

 package array;

 import java.io.FileNotFoundException;
 import java.io.FileReader;
 import java.io.IOException;
 /**
  * 抛出异常
  * @author acer
  *
  */
 public class throwexc {
     public static void main(String[] args)
     {
         try {
             String str;
             str = new throwexc().openfile();
             System.out.println(str);
         } catch (IOException e) {
             e.printStackTrace();
         }
     }
     String openfile() throws FileNotFoundException,IOException
     {
         FileReader fr=new FileReader("d:a.txt");
         char c=(char)fr.read();
         return ""+c;
     }
 }

 package array;

 import java.io.FileNotFoundException;
 import java.io.IOException;
 import java.text.ParseException;
 /**
  * 方法重写声明异常原则
  * @author acer
  *
  */
 class overexception{
     public void method() throws IOException{}
 }
 class b extends overexception{
     public void method() throws IOException{}
 }
 class c extends overexception{
     public void method() throws FileNotFoundException{}
 }
 class d extends overexception{
     public void method() throws IOException,ArithmeticException{}
 }
 class e extends overexception{
     public void method() throws IOException,RuntimeException{}
 }
 class f extends overexception{
     public void method() throws IOException,ParseException{}
 }

8.

 package array;

 import java.io.File;
 import java.io.FileNotFoundException;

 public class handthrows {
     public static void main(String[] args)
     {
         File fr=new File("d:/b.txt");
         if(!fr.exists())
         {
             try{
                 throw new FileNotFoundException("File is not exist!");
             }catch(Exception e){
                 e.printStackTrace();
             }
         }
     }
 }

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）

9.

 package array;

 import java.io.IOException;
 /**
  * 自定义异常
  * @author acer
  *
  */
 class MyException extends IOException{
     public MyException(){

     }
     public MyException(String message){
         super(message);
     }
 }
 public class myexception {
     public static void main(String[] args)
     {
         try{
             new myexception().test();
         }catch(MyException e){
         }
     }
     public void test()throws MyException{}
 }

10.

11.