图书类

public class Book {
     public String bookname;
     public int booknum;
     Book(String bookname,int booknum){
         this.bookname=bookname;
         this.booknum=booknum;
     }
}

自定义异常类

public class NoexistException extends Exception {
    public NoexistException () {};
    public NoexistException (String message) {
        super(message);
    };
}

主程序

import java.util.Scanner;

public class borrowbook {

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        Book book[] = { new Book("高数", 1), new Book("数据结构", 2) };       
        while(true){
            System.out.println("请输入数字命令：1-按照书名查找图书；2-按照序号查找图书");
        Scanner sn = new Scanner(System.in);
        try {
            int input_in = sn.nextInt();
            if (input_in == 2) {
                System.out.println("请输入图书序号：");
                int input_in_num = sn.nextInt();
                boolean num = false;
                for (int i = 0; i < book.length; i++) {
                    if (input_in_num == book[i].booknum) {
                        System.out.println("book:" + book[i].booknum);
                        num = true;
                    }
                }
                if (!num) {
                    warn();
                }           
            } else if(input_in == 1) {
                System.out.println("请输入图书名称：");
                String input_in_name = sn.next();
                boolean name = false;
                for (int i = 0; i < book.length; i++) {
                    if (input_in_name.equals(book[i].bookname)) {
                        System.out.println("book:" + book[i].bookname);
                        name = true;
                    }
                }
                if (!name) {
                    warn();
                }           
            }
            else {
                System.out.println("命令输入错误！请输入数字命令！");              
            }

        } catch (NoexistException e) {
            // TODO: handle exception
            System.out.println("图书不存在！");       
        } catch (Exception e) {
            // TODO: handle exception
            System.out.println("命令输入错误！请输入数字命令！");                  
        }
    }

    }
    public static void warn() throws NoexistException {
        throw new NoexistException("图书不存在！");
    }

}

运行结果

请输入数字命令：1-按照书名查找图书；2-按照序号查找图书
1
请输入图书名称：
4
图书不存在！
请输入数字命令：1-按照书名查找图书；2-按照序号查找图书
1
请输入图书名称：
高数
book:高数
请输入数字命令：1-按照书名查找图书；2-按照序号查找图书