JUnit testing with simulated user input

Question

I am trying to create some JUnit tests for a method that requires user input. The method under test looks somewhat like the following method:

public static int testUserInput() {
    Scanner keyboard = new Scanner(System.in);
    System.out.println("Give a number between 1 and 10");
    int input = keyboard.nextInt();

    while (input < 1 || input > 10) {
        System.out.println("Wrong number, try again.");
        input = keyboard.nextInt();
    }

    return input;
}

Is there a possible way to automatically pass the program an int instead of me or someone else doing this manually in the JUnit test method? Like simulating the user input?

Answer 1

You can replace System.in with you own stream by calling System.setIn(InputStream in). InputStream can be a byte array:

InputStream sysInBackup = System.in; // backup System.in to restore it later
ByteArrayInputStream in = new ByteArrayInputStream("My string".getBytes());
System.setIn(in);

// do your thing

// optionally, reset System.in to its original
System.setIn(sysInBackup);

Different approach can be make this method more testable by passing IN and OUT as parameters:

public static int testUserInput(InputStream in,PrintStream out) {
    Scanner keyboard = new Scanner(in);
    out.println("Give a number between 1 and 10");
    int input = keyboard.nextInt();

    while (input < 1 || input > 10) {
        out.println("Wrong number, try again.");
        input = keyboard.nextInt();
    }

    return input;
}