Junit test of method with Scanner and System.in (Java)

Question

I'm new to programmming and I have this simple method:

public double input() {
        double result = 0;
        Scanner scanner = new Scanner(System.in);
        if (scanner.hasNextDouble()) {
            result = scanner.nextDouble();
        } else {
            System.out.print("Please, type numbers!\n");
        }
        return result;
    }

The question is how to simulate (emulate) user's input from the keyboard in junit test.

Answer 1

Pass a Scanner as input parameter to the method you want to test. In your test code, you can create a Scanner instance from a string:

Scanner scanner = new Scanner("the sample user input");

And then in the production code, you can pass new Scanner(System.in) to the method.

Answer 2

You should read more about dependency injection .

Your Class shouldn't be tightly coupled with other classes. Dependencies can be provided to objects at multiple levels according to the need.

1. Using constructor/setter if it is a field.
2. Using method parameters if the scope is just in a method.

In your case as soon as you say:-

Scanner scanner = new Scanner(System.in);

Now your code is fully coupled with System.in stream. Instead you should inject this as a parameter to your method in below format.

public double input(InputStream inputStream) {
    double result = 0;
    Scanner scanner = new Scanner(inputStream);
    if (scanner.hasNextDouble()) {
        result = scanner.nextDouble();
    } else {
        System.out.print("Please, type numbers!\n");
    }
    return result;
}

Now from your main code you can call it with System.in . From your test class you call it with any InputStream. Mostly we use a mock/stub for it.

Note:- Above it just an example, and can be change according to need.