Java---异常及处理

一.异常

1.概念

程序的非正常执行。高级语言都有异常处理机制（C++，Java）

2.一般处理异常的方法

java">Scanner sc = new Scanner(System.in);

System.out.println("请输入一个数字:");

String s = sc.nextLine();

if (s.matches("[0-9]+")) {
    int x = Integer.parseInt(s);
    System.out.println("输入的数字是:" + x);
} else {
    System.out.println("程序异常");
}

3.异常方式

java">Scanner sc = new Scanner(System.in);

System.out.println("请输入一个数字:");

String s = sc.nextLine();

if (s.matches("[0-9]+")) {
    int x = Integer.parseInt(s);
    System.out.println("输入的数字是:" + x);
} else {
    System.out.println("程序异常");
}

4.异常机制

4.1 try

探测代码块是否有异常

java">try{
    语句块...
}

4.2 catch

异常捕获

java">cacth(异常对象 e){

}

4.3 finally

有无异常一定执行。异常机制中

在‌Java中，finally块通常用于释放资源、执行清理操作或记录错误。finally块总是会在try或‌catch块之后执行，无论是否出现异常。如果try块中没有出现异常，finally块将在try块之后立即执行；如果try块中出现异常，finally块将紧随catch块执行。

4.4 throws

方法上，声明方法内有异常，异常类型

4.5 throw

抛出异常对象

throw new 异常()；

java">public static void f() throws Exception{
   int a = 1;
   int b = 0;
   
   if(b==0) throw new Exception("除0异常");
}

5.异常体系

6.自定义异常

java">package com.ffyc.exception;

public class MyException01 extends Exception{
    public MyException01(String msg){
        super(msg);
    }
}

class MyException02 extends RuntimeException{
    public MyException02(String msg){
        super(msg);
    }
}

class testMyException01{
    public static void test01() throws MyException01{
        throw new MyException01("自定义异常继承Exception");
    }
    public static void test02(){
        throw new MyException02("自定义异常继承RuntimeException");
    }

    public static void main(String[] args) {
        test02();
        try {
            test01();
        } catch (MyException01 e) {
            throw new RuntimeException(e);
        }
    }
}

7.图书系统

7.1借书

业务流程，电脑查编号，bookshell找书

java">package com.ffyc.service;

import com.ffyc.entity.Book;

//接口
public interface IBookService {
    Book borrowBook(String title);
}

java">package com.ffyc.exception;

//实现
public class BookException extends RuntimeException{
    public BookException(String msg){
        super(msg);
    }
}

java">package com.ffyc.entity;

import java.util.Date;

public class Book {
    private String title;
    private String author;
    private String content;
    private String status;
    private Date borrowDate;
    private double price;

    public Book(){

    }
    public Book(String title, String author, String content, String status, Date borrowDate, double price) {
        this.title = title;
        this.author = author;
        this.content = content;
        this.status = status;
        this.borrowDate = borrowDate;
        this.price = price;
    }
    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = title;
    }

    public String getAuthor() {
        return author;
    }

    public void setAuthor(String author) {
        this.author = author;
    }

    public String getContent() {
        return content;
    }

    public void setContent(String content) {
        this.content = content;
    }

    public String getStatus() {
        return status;
    }

    public void setStatus(String status) {
        this.status = status;
    }

    public Date getBorrowDate() {
        return borrowDate;
    }

    public void setBorrowDate(Date borrowDate) {
        this.borrowDate = borrowDate;
    }

    public double getPrice() {
        return price;
    }

    public void setPrice(double price) {
        this.price = price;
    }

    @Override
    public String toString() {
        return "Book{" +
                "title='" + title + '\'' +
                ", author='" + author + '\'' +
                ", content='" + content + '\'' +
                ", status='" + status + '\'' +
                ", borrowDate=" + borrowDate +
                ", price=" + price +
                '}';
    }
}

java">package com.ffyc.service.impl;

import com.ffyc.entity.Book;
import com.ffyc.exception.BookException;
import com.ffyc.service.IBookService;

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

public class BookServiceImpl implements IBookService {
    private List<Book> books = new ArrayList<Book>();

    {
        books.add(new Book("西游记","武建军","我的大学生活","在库",null,33.44));
    }

    public Book borrowBook(String title)  {
        Iterator<Book> it = books.iterator();
        while (it.hasNext()){
            Book book = it.next();
            if (book.getTitle().equals(title) && book.getStatus().equals("在库")){
                return book;
            }
        }
        throw new BookException("图书已借出或丢失");
    }

    public static void main(String[] args) {
        IBookService ibs = new BookServiceImpl();

        Book book = ibs.borrowBook("西游记");
        System.out.println(book);
    }
}