Simple Java addition calculator
You have written some good code, but what if the user does not enter a number, but makes a mistakes? You can, and should, catch that error. What about this:

import java.util.InputMismatchException;
import java.util.Scanner;

public class Calculator
{
    static Scanner scanner;

    public static void main(String args[])
    {
       try
       {
           scanner = new Scanner(System.in);
           double firstNum, secondNum, answer;

           System.out.println("Enter your first number: ");
           firstNum = scanner.nextDouble();

           System.out.println("Enter your second number: ");
           secondNum = scanner.nextDouble();

           answer = firstNum + secondNum;
           System.out.println(answer);
       }
       catch (InputMismatchException e)
       {
           System.out.println("Invalid input");
       }
       finally
       {
           scanner.close();
       }

}


}

I think a try - catch - finally is a great way of getting user input. This way your problem of the Scanner not being closed, is solved. If this is a little over you head right now, don't worry. It does what it says: we "try" the code in the try block, but if something goes wrong, in this case a "Inputmismatchexception", which means if the user inputs something wrong, we catch it and we display a message saying something went wrong.

Note that I have also changed the variable names. By reading the name, you should immediately know what it is and so names like firstNum and answer are a little better than inInt1 and stuff.

Hope you're still with me. If something is not clear, please feel free to ask some more questions. I myself am a beginner to Java and programming in general and this is the first question I answered.