写的有点麻烦，以后会简化！

Book.java文件

public class Book {
    private String[] books = {
        "C语言",  "C++语言","Java语言","数据结构","线性代数",
        "离散数学","微积分",   "大学物理","大学英语","操作系统",   
        "嵌入式系统","马克思主义","毛泽东思想","邓小平理论"
    };

    public String[] getBooks() {
        return books;
    }

    public void setBooks(String[] books) {
        this.books = books;
    }

}

BookNotExistException.java文件

public class BookNotExistException extends Exception {
    public BookNotExistException(){
        System.out.println("你所查找的图书不存在！");
    }

    public BookNotExistException(String message){
        super(message);
    }
}

DemandNotExistException.java文件

public class DemandNotExistException extends Exception{

    DemandNotExistException(String message){
        super(message);
    }

    public DemandNotExistException() {
        System.out.println("命令输入错误！");
    }
}

MainSystem.java文件

import java.util.Scanner;

public class MainSystem {

    public static void main(String[] args) {
        System.out.println("欢迎来到借书系统！");
        while (true) {
            Step1 step1 = new Step1();
            try {
                step1.entrance();
            } catch (DemandNotExistException e) {
                // TODO Auto-generated catch block
                continue;
            }
        }
    }

}

Step1.java文件

import java.util.Scanner;

public class Step1 {
    public void entrance() throws DemandNotExistException{
        System.out.println("输入命令：1：按名称查找图书  2.按序号查找图书");
        Scanner scanner = new Scanner(System.in);
        int demand = scanner.nextInt();
        if (demand == 1) {
            System.out.println("输入图书名称：");
            Step3 step3 = new Step3();
            try {
                step3.findBookByName();
            } catch (BookNotExistException e) {
                // TODO Auto-generated catch block
            }
        }else if (demand == 2) {
            System.out.println("输入图书序号：");
            Step2 step2 = new Step2();
            try {
                step2.findBookByNum();
            } catch (BookNotExistException e) {
                // TODO Auto-generated catch block
            }
        }else {
            throw new DemandNotExistException();
        }
    }
}

Step2.java文件

import java.util.Scanner;

public class Step2 {
    public void findBookByNum() throws BookNotExistException {
        // TODO Auto-generated method stub
        System.out.println("请输入图书的序号(从1开始)：");
        Scanner scanner = new Scanner(System.in);
        int num = scanner.nextInt();
        if (num>=1&&num<=(new Book().getBooks().length + 1)) {
            System.out.println("book："+new Book().getBooks()[num-1]);
        }else {
            throw new BookNotExistException();
        }

    }
}

Step3.java文件

import java.util.Scanner;

public class Step3 {
    public void findBookByName() throws BookNotExistException {
        // TODO Auto-generated method stub
        System.out.println("请输入图书的书名：");
        Scanner scanner = new Scanner(System.in);
        String name = scanner.next();
        int i = 0;
        for (String bookName : new Book().getBooks()) {
            if (name.equals(bookName)) {
                System.out.println("book："+bookName);
                break;
            }else {
                i++;
            }
        }
        if (i == new Book().getBooks().length) {
            throw new BookNotExistException();
        }
    }
}