Java：使用扫描仪退出 try-catch 块时，Int 变量始终恢复为旧值

Question

I'm absolutely stumped here and after many hours of fiddling I just can't seem to get it to work:

The function below is basically a menu. It will ask you to input either 1, 2 or 3 and return that int.

If you input anything other than an int, it'll catch it and launch the menu again. If you input an int other than 1-2-3 it will tell you, and launch the menu again.

The problem is that if you make an invalid input, it tells you the right thing, relaunches the menu, but then if you make a CORRECT input, it will return the invalid value you tried before instead of the new, good one.

I followed it line-by-line and it will hit the return from my if, and then hit the return at the end of the try-catch and "choixDeMenu" reverts to the old invalid value and it returns that. If you were notified that it wasn't an int, it returns 0, but if it was an invalid int, it will return that invalid int.

If you make a VALID input right away, the function works and returns your valid input. It's only when you try invalid things first that I get problems.

public static int menu()
{
  int choixDeMenu = 0;
  System.out.println ("Sélectionner le fichier à importer :");
  System.out.println ("1. Partie 1.");
  System.out.println ("2. Partie 2.");
  System.out.println ("3. Partie 3.");
  
  Scanner waitingForChoice = new Scanner(System.in);

  try
    {
      choixDeMenu = waitingForChoice.nextInt();
      if (choixDeMenu < 4 && choixDeMenu > 0)
      {
        waitingForChoice.close();
        return choixDeMenu;
      }
        else
        {
          System.out.println("Entrée invalide. Entrer 1, 2, ou 3.");
          menu();
        }  
    } 
      catch (InputMismatchException e)
      {
        System.out.println("Doit être un chiffre. Entrer 1, 2, ou 3.");
        menu();
      }
      catch (Exception e)
      {
        e.printStackTrace();
      }
      
  return choixDeMenu;
}

Answer 1

Any time you create a menu like this, you should contain it within a while loop. So, rather than calling a new method ( menu() ) each time the user is prompted to make a choice, you are simply circling back to the beginning of the block and repeating the same actions ( System.out.println("1..) ).

If you're required to do this using only try/catch, this can still take place within a while loop.

From there you can use either a switch statement or a group if-else to validate each possible choice and return the value. If you're required to use a try/catch, you can contain the switch or if-else within that.

Answer 2

With the help of the commenters, here is how I modified it so it works:

Dumped everything in a while loop, and since the validity of the choice depends on either the right type of input OR an input of the right type in the proper range, I added a boolean to take note of that and operate the while loop using it.

Thanks for everything.

public static int menu()
{
  int choixDeMenu = 0;
  boolean validChoice = false;
  while (validChoice == false)
  {
    System.out.println ("Sélectionner le fichier à importer :");
    System.out.println ("1. Partie 1.");
    System.out.println ("2. Partie 2.");
    System.out.println ("3. Partie 3.");
    Scanner waitingForChoice = new Scanner(System.in);
    
    try
    {      
      choixDeMenu = waitingForChoice.nextInt();
      if (choixDeMenu < 4 && choixDeMenu > 0)
      {
        waitingForChoice.close();
        validChoice = true;
        return choixDeMenu;
      }
        else
        {
          validChoice = false;
          System.out.println("Entrée invalide. Entrer 1, 2, ou 3.");
        }  
    } 
      catch (InputMismatchException e)
      {
        validChoice = false;
        System.out.println("Doit être un chiffre. Entrer 1, 2, ou 3.");
      }
      catch (Exception e)
      {
        e.printStackTrace();
      }
  } 
  return choixDeMenu;
}

Answer 3

Try this code hope this help

    public static int menu()
{
  int choixDeMenu = 0;
  System.out.println ("Sélectionner le fichier à importer :");
  System.out.println ("1. Partie 1.");
  System.out.println ("2. Partie 2.");
  System.out.println ("3. Partie 3.");

  Scanner sc = new Scanner(System.in);

  int prev=0;
  int flg=0;

  while(true){
    try{
    choixDeMenu = sc.nextInt();
    if(choixDeMenu>0 && choixDeMenu<4){
      if(flg==0){
        return choixDeMenu;
      }
      else{
        return prev;
      }
    }
    else{
      System.out.println("Entrée invalide. Entrer 1, 2, ou 3.");
      prev = choixDeMenu;
      flg=1;
      continue;
    }
  }
  catch(InputMismatchException e)
      {
        System.out.println("Doit être un chiffre. Entrer 1, 2, ou 3.");
        sc.next();
        continue;
      }
  }

}