package?test1;

import?java.util.Scanner;

public?class?test?{
	Scanner?cmd?=?new?Scanner(System.in);
	String?[]?bookName?=?{"高數","數據結構"};
	
	public??int?inputCommand(){
		int?command;
		try?{
			command?=?cmd.nextInt();
			return?command;
		}?catch?(Exception?e)?{
			cmd?=?new?Scanner(System.in);
			return?-1;
		}
	}
	
	public?String?searchSerial()?throws?Exception{
		System.out.println("輸入序號:");
		while(true)
		{
		try?{
		int?serial?=?cmd.nextInt();
			return?bookName[serial];
		}catch(Exception?e)?{
//			cmd?=?new?Scanner(System.in);??//如果注釋掉這一行就會發生圖中的錯誤
			throw?new?Exception("圖書不存在?s");
		}
		}
	}
	
	public?static?void?main(String[]?args)?{
		test?t?=new?test();
		while(true)
		{
			try?{
				System.out.println("輸入命令，1-按名稱查找，2-按序號查找");
				int?i=t.inputCommand();
				switch?(i)
				{
				case?2:
				{
					System.out.println("book:"+t.searchSerial());
					break;
				}
				case?-1:
				{
					System.out.println("命令輸入錯誤！請根據提示輸入數字命令！");
					continue;
				}
				default:
				{
					System.out.println("命令輸入錯誤！");
					continue;
				}
				}
				break;
			}catch(Exception?e)
			{
				System.out.println(e.getMessage());
				continue;
			}
		}
	}
}

注釋的那一行cmd?=?new?Scanner(System.in);

取消注釋后結果

請大神給我講講內部的原理，個人理解是輸入的值還被保存著但是不知道為啥在執行一次就沒了。。