四、异常

1、为什么要有异常?

     System.out.println(6 / 0);
     //问题：程序出错后则不再往下运行
     System.out.println("---------------");

2、异常体系结构

   Throwable
      |
      |--编译时异常(受检异常checkedException)：
          Exception：
               |--FileNotFoundException：文件找不到
               |--IOException：io异常
               |--ClassNotFoundException：找不到类
      |--运行时异常(非受检异常UncheckedException)：
          RuntimeException:
              |--ArrayIndexOutOfBounds：下标越界异常
              |--NullPointerException：空指针异常
              |--ArithmeticException：算数运算异常
              |--ClassCastException：类型转型异常
              |--NumberFormatException：数字格式化异常

3、异常的处理方式

    1、try-catch方式

        try{
            //可能发生异常的语句
        }catch(异常类型1 变量名1){
            //处理异常1
        }catch(异常类型2 变量名2){
            //处理异常2
        }finally{
            //无条件执行的语句
        }
        注意：finally在return之后返回调用处之前

package com.hg.tryCatch;

public class TryCatchFinallyTest2 {
    public static void main(String[] args) {
        //test2();
        System.out.println("test1()" + test1());
    }

    private static void test2() {
        try {
            int a =10;
            int b =0;
            System.out.println(a/b);
            System.out.println("hello------------");
        }catch (ArithmeticException e){
            e.printStackTrace();
            System.out.println("这是catch");
            int[] arr = new int[10];
            System.out.println(arr[10]);
        }finally {
            System.out.println("这是finally");
        }
    }

    private static int test1() {
        int a = 1;
        try {
            System.out.println("try中的代码块");
            //返回到方法调用处
            return a += 1;//a=2
        }catch (Exception e){
            System.out.println("catch中的代码块");
        }finally {
            System.out.println("finally中的代码块");
            if (a > 1){
                System.out.println("a > 1,a=" + a);
            }
        }
        return a += 5;
    }
}

    2、throws方式

        public void method() throws 异常类型1,异常类型2{
        }

package com.hg.Throws;

import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.util.Scanner;

public class ThrowsTest {
    public static void main(String[] args) {
        //inputAge();
        try {
            readTxt("D:/a.txt");
        } catch (FileNotFoundException e) {
            System.out.println("读取文件出错了：" + e.getMessage());
        }
    }

    private static void readTxt(String txt) throws FileNotFoundException {
        method(txt);
    }

    private static void method(String txt) throws FileNotFoundException {
        new FileInputStream(txt);
    }

    private static void inputAge() {
        Scanner scanner = new Scanner(System.in);
        int age;
        while (true){
            try {
                System.out.println("请输入您的年龄：");
                String input = scanner.next();
                age = Integer.valueOf(input);
                break;
            }catch (Exception e){
                System.out.println("输入错误，请重新输入：");
            }
        }
        System.out.println("您的年龄是：" + age);
        scanner.close();
    }
}

4、手动创建异常

    IOException e = new IOException();
    throw e;//手动创建异常，等价于6/0
    或
    throw new IOException();

package com.hg.Throw;

public class ThrowTest {
    public static void main(String[] args) {
        try {
            register("admin","1111");
        } catch (Exception e) {
            e.printStackTrace();
        }

    }


    private static void register(String username, String password) {
        if (username == null){
            //此时，不希望后续程序再执行，要throw异常
            throw new IllegalArgumentException("账户不能为空");
        }
        if (password == null){
            throw new IllegalArgumentException("密码不能为空");
        }
        if (password.length() < 4){
            throw new RuntimeException("密码要求最少4位");
        }
        System.out.println("INSERT INTO t_user(username,password) VALUES("+username+ ","+password+")");
    }
}

5、使用场景

    try-catch       后续还有程序要执行
    throws          把异常交给调用者处理
    throw           不希望后续程序继续执行

6、自定义异常

    class MyException extends RuntimeException{
        public MyException(String s) {
                super(s);
        }
    }

package com.hg.CustomizeException;

public class CustomizeExceptionTest {
    public static void main(String[] args) {
        try {
            login("admin","111");
        } catch (Exception e) {
            e.printStackTrace();
            System.out.println("登录错误：" + e.getMessage());
        }
    }

    private static void login(String inputUsername, String inputPassword) {
        String username = "admin";
        String password = "1111";
        //账户不能为空
        if (inputUsername == null){
            throw new IllegalArgumentException("账户不能为空");
        }
        //密码不能为空
        if (inputPassword == null){
            throw new IllegalArgumentException("密码不能为空");
        }
        //账号正确
        if (inputUsername != username){
            throw new UsernameErrorException("账号错误");
        }
        //密码正确
        if (inputPassword != password){
            throw new PasswordErrorException("密码错误");
        }
        System.out.println("登录成功");
    }
}

class UsernameErrorException extends RuntimeException{
    public UsernameErrorException() {
        super();
    }

    public UsernameErrorException(String s) {
        super(s);
    }
}

class PasswordErrorException extends RuntimeException{
    public PasswordErrorException() {
        super();
    }

    public PasswordErrorException(String s) {
        super(s);
    }
}