章节笔记——异常处理

异常出现，程序变得强大，异常是处理错误的机制

1. 用try...catch处理异常

处理程序员控制和用户输入有误的情况
举例：

import java.util.Scanner;

public class Main {

    public static void main(String[] args) {

        int a,b,c;
        Scanner sc = new Scanner (System.in );
        a=sc.nextInt();
        b=sc.nextInt();
        c=a%b;
        System.out.printf("余数是"+c);
    
    }
}

如果输入aa会出现报错

aa
Exception in thread "main" java.util.InputMismatchException
    at java.util.Scanner.throwFor(Unknown Source)
    at java.util.Scanner.next(Unknown Source)
    at java.util.Scanner.nextInt(Unknown Source)
    at java.util.Scanner.nextInt(Unknown Source)
    at Main.main(Main.java:9)

或者被除数为0时会出现

10
0
Exception in thread "main" java.lang.ArithmeticException: / by zero
    at Main.main(Main.java:11)

此时可以用到try...catch
try{
有可能出现问题的语句
}
catch(Exception e){
出现问题的处理语句
}

增加语句

import java.util.Scanner;

public class Main {

    public static void main(String[] args) {

        try {
        int a,b,c;
        Scanner sc = new Scanner (System.in );
        a=sc.nextInt();
        b=sc.nextInt();
        c=a%b;
        System.out.printf("余数是"+c);
        }
        catch(Exception e) {
            System.out.printf("出现异常");
        }
    }
}

输出结果

100
0
出现异常

werwf
出现异常

拓展：Exception e 异常的分类

2. 用throws处理异常

抛给上一级处理，如果不行再向上一级，最后交个虚拟机处理，虚拟机终止
一般处理无法避免和处理的异常
举例：
Main类

import java.util.Scanner;

public class Main {

    public static void main(String[] args) {

        LeiA aa= new LeiA();
        try{
            aa.a();
            }
        catch(Exception e){
            System.out.printf("出现异常");
        }
        
    }
}

LeiA类

import java.util.Scanner;

public class LeiA {
    void a (){
        int a,b,c;
        Scanner sc = new Scanner (System.in);
        a=sc.nextInt();
        b=sc.nextInt();
        c=a%b;
        System.out.printf("余数是"+c);
    }
}

增加throws
Main类

import java.util.Scanner;

public class Main {

    public static void main(String[] args) throws Exception {

        LeiA aa= new LeiA();
//      try{
            aa.a();
//          }
//      catch(Exception e){
            System.out.printf("出现异常");
//      }
        
    }
}

LeiA类

import java.util.Scanner;

public class LeiA {
    void a ()throws Exception 
    {
        int a,b,c;
        Scanner sc = new Scanner (System.in);
        a=sc.nextInt();
        b=sc.nextInt();
        c=a%b;
        System.out.printf("余数是"+c);
    }
}

结果

aa
Exception in thread "main" java.util.InputMismatchException
    at java.util.Scanner.throwFor(Unknown Source)
    at java.util.Scanner.next(Unknown Source)
    at java.util.Scanner.nextInt(Unknown Source)
    at java.util.Scanner.nextInt(Unknown Source)
    at LeiA.a(LeiA.java:8)
    at Main.main(Main.java:9)

3. Finally

Finally起到异常处理出口的作用，用在try...catch语句最后，无论catch是否执行，Finally都被执行

import java.util.Scanner;

public class Main {

    public static void main(String[] args) throws Exception {
        int a,b,c;
        Scanner sc = new Scanner (System.in);
        try{
        a=sc.nextInt();
        b=sc.nextInt();
        c=a%b;
        System.out.printf("余数是"+c);
        }
        catch(Exception e){
            System.out.printf("出现异常");
        }
        finally {
            System.out.printf("无论怎样都出现");
        }   
    }
}

运行结果

aa
出现异常无论怎样都出现