Java - 用于菜单选择的 while 循环(基于控制台的程序)

Question

更新用户第一次做出诸如"1"之类的选择时，将再次显示菜单。下次进行选择时，付款信息开始循环。循环完成后，菜单再次显示，它将正常工作。此外，当选择开始循环时，前两年是输出，而不仅仅是第一年，但随后按预期一次输出一年。

//create scanner object for choosing a loan, then prompt for and accept input
    Scanner choose = new Scanner(System.in);
    String choice;
    System.out.println("nType 1, 2, or 3 and press enter to see the monthly payment information for the respective loan. To end the program type "end".");
    choice = choose.next();
    //cycle loan 1 payment information
    //create scanner object to advance to the next year's payments
    //loop for cycling payment information
    //initialize loan principal to variable
    while (!"end".equals(choice)) {
        System.out.println("nType 1, 2, or 3 and press enter to see the monthly payment information for the respective loan. To end the program type "end".");
        choice = null;
        choice = choose.next();
        if ("1".equals(choice)) {
           //calculation code
                    }
                if (j < 6) {
                    System.out.println("Press enter to get the mortgage information for year " + (j + 2));
                    choice = choose.nextLine();
                } else {
                    System.out.println("Congratulations, your mortgage has been paid off.");
                }
            }
            choice = null;
        }
        if ("2".equals(choice)) {
            //calculation code
                }
                if (j < 14) {
                    System.out.println("Press enter to get the mortgage information for year " + (j + 2));
                    choice = choose.nextLine();
                } else {
                    System.out.println("Congratulations, your mortgage has been paid off.");
                }
            }
            choice = null;
        }
        if ("3".equals(choice)) {
            //calculation code
                }
                if (j < 29) {
                    System.out.println("Press enter to get the mortgage information for year " + (j + 2));
                    choice = next.nextLine();
                } else {
                    System.out.println("Congratulations, your mortgage has been paid off.");
                }
            }
            choice = null;
        }
    }
    choose.close();
}

}

Answer 1

我可以预先看到三个问题：

1. 您不需要两个扫描仪即可System.in流。Scanner next = new Scanner(System.in);删除此语句并使用choose实例。

2. 如果按next.close();关闭输入扫描程序，它也会System.in关闭输入流，并且您可能无法再次读取流。确保仅在完全完成程序后关闭流。

3. 使用equals方法将while中的条件比较为while(!"end".equals(choice))。将文字"end"作为第一个参数将处理 choice 的空值。

编辑：

您修改的代码在高级别：

    Scanner choose = new Scanner(System.in);
    String choice= null;
    int j = 0;
    while (!"end".equals(choice)) {
        System.out.println("nType 1, 2, or 3 and press enter to see the monthly payment information for the respective loan. To end the program type "end".");
        choice = choose.nextLine();
        if ("1".equals(choice)) {
           //calculation code
               if (j < 6) {
                    System.out.println("Press enter to get the mortgage information for year " + (j + 2));
                    choice = choose.nextLine();
                } else {
                    System.out.println("Congratulations, your mortgage has been paid off.");
                }
            choice = null;
        }
        if ("2".equals(choice)) {
                if (j < 14) {
                    System.out.println("Press enter to get the mortgage information for year " + (j + 2));
                    choice = choose.nextLine();
                } else {
                    System.out.println("Congratulations, your mortgage has been paid off.");
                }
            choice = null;
        }
        if ("3".equals(choice)) {
                if (j < 29) {
                    System.out.println("Press enter to get the mortgage information for year " + (j + 2));
                    choice = choose.nextLine();
                } else {
                    System.out.println("Congratulations, your mortgage has been paid off.");
                }
            choice = null;
        }
    }
    choose.close();

Answer 2

一个错误是字符串相等性与 == 不同。你应该使用 .equals（）：

while (!choice.equals("end")) {

希望对您有所帮助！

Answer 3

您将choice设置为null，因此choice != "end"始终为真。将next.close(); choice = null移到 while 循环的外部。另外，weolfe91说的。

Answer 4

import java.util.Scanner;

类 主要

{

public static void main(String [] args)
  {
    Scanner sc=new Scanner(System.in);
    System.out.println("Enter no.1:");
    int a=sc.nextInt();
    System.out.println("Enter no.2:");
    int b=sc.nextInt();
    boolean exit=false;
    do
    {
      System.out.println("1.addition");
      System.out.println("2.subtraction");
      System.out.println("3.multiplication");
      System.out.println("4.division");
      System.out.println("5.exit");
      System.out.println("choose one!");
      Scanner sd=new Scanner(System.in);
      System.out.println("enter your choice");
      int num=sd.nextInt();
      switch(num)
     {
       case 1:
       int add=a+b;
       System.out.println("addition="+add);
       System.out.println("n");
       break;
       case 2:
       int sub=a-b;
       System.out.println("subtraction="+sub);
       System.out.println("n");
       break;
       case 3:
       int mul=a*b;
       System.out.println("multilpication="+mul);
       System.out.println("n");
       break;
       case 4:
       int div=a/b;
       System.out.println("division="+div);
       System.out.println("n");
       break;
       case 5:
       exit=true;
       break;
      }
    }while(!exit);
}

}