如何避免在while循环中使用if语句重复打印语句

Question

我是 * 的新手，想知道为什么当我在我的 while 循环中引入 if 语句@“如果完成，请键入“返回””时，我的语句会不断重复两次。其次，有人能告诉我为什么当我只向 ArrayList 添加一项时，ArrayList 在索引 0 处保留一个空字符串吗？谢谢！

代码如下：

包 com.codewithrichard;

导入 java.util.ArrayList; 导入 java.util.Scanner;

公共类主{

public static void main(String[] args) {
 Scanner input = new Scanner(System.in);

 //global variables
  boolean appIsStillOn = true;


 ArrayList <String> shoppingList = new ArrayList<>();

 System.out.println("Welcome to your mobile shopping list" + "\n" + "Your options are");
 System.out.println("1) add item to list");
 System.out.println("2) display list and amount of items in it");
 System.out.println("3) quit!");

 while (appIsStillOn) {
     System.out.println("Option (1-4): ");
     int option1 = input.nextInt();
     if (option1 == 1) {
         while (true) {
             System.out.println("item (if done, type \"back\"): ");
             String itemAdded = input.nextLine().toLowerCase();
             if (!itemAdded.equals("back")) {
                 shoppingList.add(itemAdded);
             } else {
                 break;
             }
         }
     }
     else if (option1 ==2){
         System.out.println(shoppingList);
         System.out.println("size of shopping list: "  + shoppingList.size());
     }
     else {
         System.out.println("Can't wait for you to come back!");
         appIsStillOn = false;
     }









 }










}

}

Answer 1

在输入input.nextInt() 后，当您按下输入时，input.nextLine().toLowerCase() 会获取该行的数据，因为input.nextInt() 不会占用\n（换行符）。

在input.nextInt()之后阅读换行符以跳过它

int option1 = input.nextInt();
input.nextLine();

Answer 2

Scanner#nextInt() 方法（以及许多其他 next...() 方法）不会消耗扫描仪缓冲区中的换行符ENTER 键被按下。 Scanner#nextLine() 方法会在 Scanner#nextInt() 方法之后使用它，因此给人的印象是提示输入被跳过了。

还要考虑这个...

如果用户不小心输入了字母字符而不是菜单选择数字，会发生什么？没错，您的应用程序会因为 InputMismatchException 而崩溃。

您应该始终对用户输入进行某种形式的验证，如果验证失败，请允许用户进行正确的输入。在使用您的应用程序时，这显然会促进一个更无故障的环境。使用您当前的模型，这是一个示例：

java.util.Scanner input = new java.util.Scanner(System.in);

//global variables
boolean appIsStillOn = true;
ArrayList<String> shoppingList = new ArrayList<>();

System.out.println("Welcome to your mobile shopping list.");

while (appIsStillOn) {
    int option1 = 0;
    while (option1 < 1 || option1 > 3) {
        System.out.println();
        System.out.println("Your Shopping List options are:");
        System.out.println("  1) Add item to list.");
        System.out.println("  2) Display list and amount of items in it.");
        System.out.println("  3) Quit!");
        System.out.print("Choice (1-3): --> ");
        try{
            option1 = input.nextInt();
            if (option1 < 1 || option1 > 3) {
                throw new java.util.InputMismatchException();
            }
            /* Consume the enter key hit (newline char) in case
               a Scanner#nextLine() prompt is next so that it 
               doesn't get consumed by that method.   */
            input.nextLine(); 
        }
        catch (java.util.InputMismatchException ex) {
            System.err.println("Invalid menu choice supplied! Try again...");
            /* Consume the enter key hit (newline char) in case
               a Scanner#nextLine() prompt is next so that it 
               doesn't get consumed by that method. It is also
               required here in case an exception has bypassed 
               the above 'input.nextLine()' call.*/
            input.nextLine(); // Consume the enter key hit (newline char)
        }
    }

    if (option1 == 1) {
        while (true) {
            System.out.println();
            System.out.println("Enter the item to add (when done, enter \"back\"): ");
            System.out.print("Item: --> ");
            String itemToAdd = input.nextLine();
            if (itemToAdd.trim().equals("")) {
                System.err.println("Invalid Item String! You must supply something!");
                continue;
            }
            else if (itemToAdd.equalsIgnoreCase("back")) {
                break;
            }
            shoppingList.add(itemToAdd);
        }
    }
    else if (option1 == 2) {
        System.out.println();
        System.out.println(shoppingList);
        System.out.println("Number of Items in shopping list: " + shoppingList.size());
    }
    else {
        System.out.println();
        System.out.println("Bye-Bye - Can't wait for you to come back!");
        appIsStillOn = false;
    }
}