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.