Proper way to use Scanner with multiple functions?

Question

I have a console application with a main method and some methods that I call from main. There, I want to ask the user for some input, for which I use the Scanner class.

Here's my problem:

I find there is no way to use Scanner when reading inputs from outside main without random exceptions or unexpected behaviour. I have tried two approaches:

1. Having a Scanner global variable in the class containing main. Then I use this same Scanner in all functions in that same class.
2. In every function I need to ask for input, I declare a new Scanner variable, use it, and close it before exiting the function.

1. makes Scanner try to read twice. I mean, I have a sc.readLine in a function and, when I exit that function, I have another sc.readLine in main. I input once and the two readLine lines get executed, the second one reading an empty String.

2. throws Exception (base class Exception) when I call any sc.readLine for a second time during the execution of the program.

I have also noticed that any other method other than readLine is going to read various items on the same line. For example, line "10 20 30 40" would execute 4 sc.nextInt calls.

TL;DR: how do you use Scanner in a console application?

Answer 1

One way:

import java.util.Scanner;

public class Main {
    private Scanner scanner = new Scanner(System.in);

    public Scanner getScanner() {
        return scanner;
    }

    void fun1() {
        Scanner in = getScanner();
        System.out.print("Enter a string: ");
        System.out.println("You entered: " + in.nextLine());
    }

    void fun2() {
        Scanner in = getScanner();
        System.out.print("Enter an integer: ");
        int n = 0;
        try {
            n = Integer.parseInt(in.nextLine());
        } catch (NumberFormatException e) {
            e.printStackTrace();
        }
        System.out.println(n + " + 10 = " + (n + 10));
    }

    public static void main(String[] args) {
        Main m = new Main();
        m.fun1();
        m.fun2();
    }
}

A sample run:

Enter a string: Hello world!
You entered: Hello world!
Enter an integer: 25
25 + 10 = 35

Another way:

import java.util.Scanner;

public class Main {
    static void fun1(Scanner in) {
        System.out.print("Enter a string: ");
        System.out.println("You entered: " + in.nextLine());
    }

    static void fun2(Scanner in) {
        System.out.print("Enter an integer: ");
        int n = 0;
        try {
            n = Integer.parseInt(in.nextLine());
        } catch (NumberFormatException e) {
            e.printStackTrace();
        }
        System.out.println(n + " + 10 = " + (n + 10));
    }

    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        fun1(in);
        fun2(in);
    }
}

A sample run:

Enter a string: Hello world!
You entered: Hello world!
Enter an integer: 25
25 + 10 = 35

Regarding your problem with next() or nextInt(): Given below is the recommended way for multiple inputs in one go.

import java.util.Arrays;
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        boolean valid = true;
        System.out.print("Enter some intgers: ");
        String strNum = in.nextLine();
        String[] strNumArr = strNum.split("\\s+");
        int[] numArr = new int[strNumArr.length];
        for (int i = 0; i < strNumArr.length; i++) {
            try {
                numArr[i] = Integer.parseInt(strNumArr[i]);
            } catch (NumberFormatException e) {
                numArr[i] = Integer.MIN_VALUE;
                valid = false;
            }
        }
        System.out.println(Arrays.toString(numArr));
        if (!valid) {
            System.out.println("Note: invalid inputs have been reset to " + Integer.MIN_VALUE);
        }
    }
}

A sample run:

Enter some intgers: 10 5 20 15
[10, 5, 20, 15]

Another sample run:

Enter some intgers: 4     40 a    20   b   15
[4, 40, -2147483648, 20, -2147483648, 15]
Note: invalid inputs have been reset to -2147483648

Check Scanner is skipping nextLine() after using next() or nextFoo()? for more information about console input using Scanner.

Answer 2

to use a single instance of Scanner (or any other datatype or class), you can use the design pattern "Singleton" which consist to instantiate a single instance of your Object for the whole project.

The Singleton definition (a new class) :

import java.util.Scanner;

public class ScannerSingleton {
    private static Scanner sc = null;

    private ScannerSingleton() {
        sc = new Scanner(System.in);

    }

    public static Scanner getInstance() {
        if (sc == null) {
            synchronized(ScannerSingleton.class) {
                if (sc == null)
                    sc = new ScannerSingleton();
            }
        }

    }
}

and each time you want to use your scanner anywhere you only have to call ScannerSingleton.getInstance() which will return your single instance of scanner

example of use:

String test = ScannerSingleton.getInstance().nextLine();

Answer 3

I suggest to pass your Scanner object as another parameter for your functions.

public static void main(String[] args)
{
  Scanner scanner=new Scanner(System.in);
  String answer = ask(scanner, "what is your name?");
  System.out.println("Oh! Hello dear " + answer + "!");
  scanner.close();
}

private static String ask(Scanner scanner, String question)
{
  System.out.println(question);
  return scanner.nextLine();
}