scanner.nextline()在do中，循环.退一步

Question

我有2个问题。 1.如果用户不输入正确的命令，我有问题要找到一种方法"做，循环"。 2.这样做之后，我想在开关循环中使用用户输入，因此可以处理所选的命令，但是我不确定该将什么放入括号中作为测试变量，因为它无法识别用户输入。p>这是我到目前为止所做的。我已经尝试了很多事情，但是它们没有解决。

  `package assignment6;
   public class Enumeracija {
   public enum Commands {
   PROPERTIES, NEW_FOLDER, RENAME, COPY, CUT, DELETE, UNSPECIFIED 
   }}  

   package assignment6;
   import assignment6.Enumeracija.Commands;
   import java.io.*;
   import java.util.Scanner;
   public class Assignment6 {
   public static boolean main(String[] args) throws FileNotFoundException, 
   IOException {
    //Showing commands to user
    for (int i = 0; i<Commands.values().length-1; i++){
        System.out.println(Commands.values()[i]);
    }
    //Reading users input
        Scanner scan = new Scanner(System.in); 
        System.out.println("Enter one of the given commands: ");
        String userInput;
    /* WHILE DO LOOP so user can make a mistake and program will not stop 
    running, until he writes it correctly. But Im getting main class not 
    found problem, when I want to run it.*/ 
    do {
        userInput = scan.nextLine().trim().toUpperCase();
        if (userInput.equals(Commands.values())) {
        return true;} 
        else {
        System.out.println("Input error. Keep trying and make sure you write command correctly.");    
        return false;}
    } while (false);
    switch(userInput) {
    /*If I use userInput, it does not recognise commands 
    that user enters. Every case is being unrecognized.*/ 
        case PROPERTIES:
        //;
        break;
        case NEW_FOLDER:
        //;
        break;
        case RENAME:
        //;
        break;
        case COPY:
        //;
        break;
        case CUT:
        //;
        break;
        case DELETE:
        //;
        break;
        case UNSPECIFIED: 
        //;
        break;*/

Answer 1

首先，java中主方法的返回类型应始终无效。

public static void main(String[] args) throws FileNotFoundException, IOException

第二，如果用户输入正确的命令，则程序将在您使用返回语句时结束。如果您将布尔变量用作标志，最好将其值设置为true，然后突破循环。

boolean flag = false;
do {
    userInput = scan.nextLine().trim().toUpperCase();
    if (userInput.equals(Commands.values())) {
    flag = true;
    break;} 
    else {
    System.out.println("Input error. Keep trying and make sure you write command correctly.");    
    }
} while (flag);

这将使用户保持在循环中，直到输入正确的命令为止。

现在，来到Switch语句，由于命令是枚举的形式，您需要正确参考它们。

switch(userInput) {
    case Commands.PROPERTIES:
    //;
    break;
    case Commands.NEW_FOLDER:
    //;
    break;
    case Commands.RENAME:
    //;
    break;
    case Commands.COPY:
    //;
    break;
    case Commands.CUT:
    //;
    break;
    case Commands.DELETE:
    //;
    break;
    case Commands.UNSPECIFIED: 
    //;
    break;

让我知道这是否有效。

Answer 2

do {
    userInput = scan.nextLine().trim().toUpperCase();
    // telling you what to do here will take the entire fun out of the problem
    // HINT: check what Command.values() actually does and also what userInput.equals() actually does.
    // HINT: once you have figured out how to interpret the user input, you need to store it in a variable to be used in the switch statement
    // HINT: the variable that you store it in should be of the Command type (this should be more than sufficient a hint)
    if (userInput.equals(Commands.values())) {
        // do not return here. Return will cause the execution of the main method to stop
        // what you actually want over here is break;
        // this will allow you to break out of the infinite loop once your conditions are met.
    } 
    else {
        System.out.println("Input error. Keep trying and make sure you write command correctly.");   
        // remove the return condition here. This will stop execution of the main method
    }
} while (true); // change condition to true so that loop is infinite

Answer 3

您正在通过返回true/false停止程序流。而是保持变量以保持跟踪

 do {
        boolean validInput = false; // this will keep track
        userInput = scan.nextLine().trim().toUpperCase();
        if (userInput.equals(Commands.values())) {
           validInput = true; // make flag as true on valid input
        } 
        else {
           System.out.println("Input error. Keep trying and make sure you write command correctly.");    
           validInput = false; // make flag as false on invalid input
       }
    } while (!validInput); //repeat the loop if invalid input

开关语句的问题是，您将字符串传递给切换，而案例是枚举。将字符串转换为枚举对象

Commands value = Commands.valueOf(userInput);
switch(value){
   //processing
}

Answer 4

1. 您可以使用while循环，而不是do while循环。这使代码更可读，因此更易于调试。

   boolean flag = true;
   while(flag){
    userInput = scan.nextLine().trim().toUpperCase();
    if (userInput.equals(Commands.values())) 
        flag = false;
    else 
        System.out.println("Input error..."); 
   }  
   

2. 您使用的是Switch语句中的String对象的UserInput。单击此处以对此进行说明。

主类的签名也是：

public static void (String[] whateverHere)