i'm a beginner, Need help, Please!!!

I want to read optional number "a" from console and then store it in variable to use as passing to a different class (different .java file). and pint the sum separetely by optional inputting.

How do i code the 2 classes? thanks

/*
* DemoApp.java
*/

public class DemoApp {

    public static void main(String[] args) {

        Scanner input = new Scanner(System.in);
        int a;
        System.out.println("Input one of the following 3 numbers: 100, 200, 300");
        System.out.print("Enter: ");
        a = input.nextInt();
        TestApplication testapp = new TestApplication();
        testapp.test(a);

    }
}


/*
 * TestApplication.java
 *
 */

public class TestApplication {

    private int a; 

    public void test(int a) {
        this.a = a; // TODO: where to get the "a"? (entered by users from console)

        System.out.println("The number_a was passed in: "+a);
    }

    protected void printNum() throws Exception {

        int num;


        switch (a) {
        case 100:
            num = num + 10;
            break;

        case 200:
            num = num + 20;
            break;

        case 300:
            num = num + 30;
            break;

        default:
            // TODO: unexpected number input. throw();
            break;

        }

                System.out.println("I got a sum number"+num);

    }

}