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();
}
}