JAVA基础--异常
异常的分类:
1. Throwable: 根类
1) Error:系统错误, 由java虚拟机生成并抛出, 无法处理
2) Exception: 所有异常类的父类, 可以处理的错误, 可以catch到
1) RuntimeException:经常出现的错误, 特殊的异常, 比如被0除, 数组下标超范围等, 产生频繁, 处理麻烦, , 可以catch, 也可以不catch, 比如ArithmeticException,BufferOverflowExcetpion, IndexOutOfBoundsExcetpion
2) 其他Exception: 必须要catch的错误, 如IOException
所以非RuntimeExcetpion必须要catch
异常的5个关键字: try, catch, finally, throws, throw
如何抛:
1. throws:已知错误类型
2. throw: 手动抛, 后面加异常对象
import java.io.*; public class TestEx { public static void main(String[] args) { try { new TestEx().f2(); } catch (IOException e) { e.printStackTrace(); } /* int[] arr = {1, 2, 3}; System.out.println(arr[2]); try { System.out.println(2/0); } catch (ArithmeticException e) { System.out.println("系统正在维护,请与管理员联系"); e.printStackTrace(); } */ //TestEx te = new TestEx(); //te.m(0); /* try { new TestEx().m(0); } catch (ArithmeticException ae) { ae.printStackTrace(); System.out.println("出错了"); } */ FileInputStream in = null; try { in = new FileInputStream("myfile.txt"); int b; b = in.read(); while (b != -1) { System.out.print((char) b); b = in.read(); } } catch (IOException e) { System.out.println(e.getMessage()); } catch (FileNotFoundException e) { e.printStackTrace(); } finally { try { in.close(); } catch (IOException e) { e.printStackTrace(); } } } void m(int i) throws ArithmeticException { if(i==0) throw new ArithmeticException("被除数为0"); } void f() throws FileNotFoundException , IOException { FileInputStream in = new FileInputStream("myfile.txt"); int b; b = in.read(); while (b != -1) { System.out.print((char) b); b = in.read(); } } void f2() throws IOException { /* try { f(); } catch (FileNotFoundException e) { System.out.println(e.getMessage()); } catch (IOException e) { e.printStackTrace(); } */ f(); } }
try,catch,finally..
通常在finally语句中进行资源的清除工作, 比如关闭打开的文件, 删除临时文件.
FileInputStream in = null; try { in = new FileInputStream("myfile.txt"); int b; b = in.read(); while (b != -1) { System.out.print((char) b); b = in.read(); } } catch (IOException e) { System.out.println(e.getMessage()); } catch (FileNotFoundException e) { e.printStackTrace(); } finally { try { in.close(); } catch (IOException e) { e.printStackTrace(); } }
异常的捕获和处理:
方法持续往上抛异常, 用throws关键字
接收了以后, 在catch里必须处理.
void f() throws FileNotFoundException , IOException { FileInputStream in = new FileInputStream("myfile.txt"); int b; b = in.read(); while (b != -1) { System.out.print((char) b); b = in.read(); } } void f2(){ try { f(); } catch (FileNotFoundException e) { System.out.println(e.getMessage()); } catch (IOException e) { e.printStackTrace(); } f(); }
也可以继续往上抛:
IOException包含了FileNotFoundException, 所以直接写IOException就可以了.
void f() throws FileNotFoundException , IOException { FileInputStream in = new FileInputStream("myfile.txt"); int b; b = in.read(); while (b != -1) { System.out.print((char) b); b = in.read(); } } void f2() throws IOException { f(); }
再继续往上到main的时候要进行try catch处理:
public class TestEx { public static void main(String[] args) { try { new TestEx().f2(); } catch (IOException e) { e.printStackTrace(); }
}
}
还可以不进行处理直接交给main, 但是不建议这样:
public class TestEx { public static void main(String[] args) throws Exception{ new TestEx().f2(); } }
throw: 手动抛, 后面加异常对象:
void m(int i) throws ArithmeticException { if(i==0) throw new ArithmeticException("被除数为0"); }
异常捕获时, 先捕获小的, 再捕获大的
重写方法需要抛出与原方法所抛出异常类型一致的异常, 或者不抛出异常.
自定义异常:
1. 通过继承Exception类声明自己的异常类
2. 在方法适当的位置生成自定义异常的实例, 并用throw抛出
3. 在方法的声明部分用throws语句声明该方法可能抛出的异常
class MyException extends Exception { private int id; public MyException(String message, int id){ super(message); this.id = id; } public int getId(){ return id; } } public class Test{ public void regist(int num) throws MyException{ if(num<0){ throw new MyException("人数为负值,不合理",3); } System.out.println("登记人数 "+num); } public void manager(){ try{ regist(100); } catch (MyException e){ System.out.println("登记失败, 出错类型码= "+e.getId()); e.printStackTrace(); } System.out.println("操作结束 "); } public static void main(String[] args) { Test t = new Test(); t.manager(); } }