Scanner.nextLine() in do, while loop. One step back

Question

I have 2 questions. 1. I have problem to find a way going one step back in 'do, while loop' if user do not enter correct command into console. 2. After doing that, I want to use Users input in switch loop, so the chosen command can be processed, but I am not sure what to put into brackets as a variable to test, because it does not recognise Users input.

Here is what I have done so far. I've tried a lot of things, but non of them worked out.

  `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

First of all, the return type of main method in Java should always be void.

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

Second, if the user enters the correct command, the program will end as you have used the return statement. It's better if you use a boolean variable as a flag, set its value to true and just break out of the loop.

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);

This will keep the user in the loop till the right command is entered.

Now, coming to the switch statement, since the commands are in the form of enums, you need to reference them properly.

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;

Let me know if this works.

Answer 2

You are stopping the flow of program by returning true/false. Instead maintain a variable to keep track

 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

The problem with your switch statement is that you are passing String to switch whereas the cases are Enums. Convert your String to an Enum object

Commands value = Commands.valueOf(userInput);

switch(value){
   //processing
}

Answer 3

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 4

1. You can use a while loop as opposed to a do while loop. This makes the code more readable and hence easier to debug.

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

2. You are using userInput which is a String object in the switch statement. Click here for an explanation on that.

Also the signature of the main class is:

public static void (String[] whateverHere)