Java: show different results depending on User Console Input without try...catch statement-CodePudding

I try to write a programm in Java that gets user input using Scanner Class. The user has to enter any positive integer number. Depending on user actions, the results should be as follows:

The user has entered not an integer number -> the programm prints the message

Oops! You entered something different, but not an integer number, try again

The user has entered not a positive integer number -> the programm prints the message
```
You entered not a positive integer number, try again
```
The user has entered a positive integer number -> the programm prints the number
```
User's positive integer number - ...
```

I have written some code using loop and Scanner class

public static void main(String[] args) {
        
        int userIntNum;
        boolean isUserInputCorrect;
        
        @SuppressWarnings("resource")       
        Scanner sc = new Scanner(System.in);
        userIntNum = 0;
        
        System.out.println("Please, enter a positive integer number");
        
        isUserInputCorrect = (sc.hasNextInt() && (userIntNum > 0)); 
                
        // double console input for correct integer number
        while (isUserInputCorrect == false) {
            if (!sc.hasNextInt()) {
                System.out.println("Oops! You entered something different, but not an integer number, try again");
                sc.nextLine();
            } else if (sc.nextInt() <= 0) {
                System.out.println("You entered not a positive integer number, try again");
                sc.nextLine();
            } else {
                break;
            }
        }
        userIntNum = sc.nextInt();
                
        System.out.println("User's positive integer number - "   userIntNum);

When I put in the console a positive integer number (the correct input), the programm, for some reason, asks me to enter this number twice. Moreover, if I put first an integer number and then any non-positive number separated by space, it will print this incorrect number. And if I put first an integer number and then not an integer number separated by space, it will throw an exception.

Why does it happen and how can I fix these errors?

CodePudding user response：

First, I would eliminate isUserInputCorrect. You are trying to do too much with it, instead I would loop while userIntNum is less than or equal to zero. Also, try and limit variable scope. Something like,

Scanner sc = new Scanner(System.in);
int userIntNum = -1;

System.out.println("Please, enter a positive integer number");
// double console input for correct integer number
while (userIntNum <= 0) {
    if (sc.hasNextInt()) {
        userIntNum = sc.nextInt();
    } else {
        System.out.println("Oops! You entered something different, "
                  "but not an integer number, try again");
        sc.nextLine();
    }
    if (userIntNum <= 0) {
        System.out.println("You entered not a positive integer number, try again");
    }
}
System.out.println("User's positive integer number - "   userIntNum);

CodePudding user response：

Expanding on Elliott's answer above, I wanted to provide an alternative solution that addresses your following point:

Moreover, if I put first an integer number and then any non-positive number separated by space, it will print this incorrect number. And if I put first an integer number and then not an integer number separated by space, it will throw an exception.

The Scanner class will read tokens individually in a temporal fashion. If you look at the nextInt() function you will see it that throws InputMismatchException which you can explicitly catch.

Additionally, take a look at the Java modulus operator. Since you are explicitly looking for even values, the modulus operator is extremely valuable here.

public static void main(String[] args) {

    int userIntNum = 0;
    boolean isUserInputCorrect = false;

    @SuppressWarnings("resource")
    Scanner sc = new Scanner(System.in);

    do {
        System.out.println("Please, enter a positive integer number");
        try {
            userIntNum = sc.nextInt();
            isUserInputCorrect = (userIntNum > 0 && userIntNum % 2 == 0);
        } catch (InputMismatchException ex) {
            System.out.println("Invalid input: please enter an integer value");
            sc.next();
        }
    } while(!isUserInputCorrect);
        System.out.println("User's positive integer number - "   userIntNum);
}