File Input

Question

Maybe someone can clarify,
I am looking for examples where a program allows me to input data from a file,
http://chortle.ccsu.edu/CS151/Notes/chap22/ch22_8.html
on the above webpage there is a good explanation, but it's still not clear when I am writing a program how to fit this together. Should I write a program to create the file first? And what if I want to send the output to another file?
import java.io.*;
class AddTwo
  public static void main ( String[] args ) throws IOException
    int numberA, numberB;
    String line;
    BufferedReader stdin = new BufferedReader(
        new InputStreamReader( System.in ) );
    System.out.println("Enter first number:");
    line      = stdin.readLine();
    numberA   = Integer.parseInt( line );
    System.out.println("Enter second number:");
    line      = stdin.readLine();
    numberB   = Integer.parseInt( line );
    System.out.println( "Sum: " + (numberA + numberB) );
}

Answer 1

My question is how to fit these together into a program that does what I want it to,
it's not clear to me how to produce a program that inputs the data from the file.
On the next page the text gives the following, but it does not explain how to:
create a program with the input.txt file or another type of file
how to read Line 1 A * B, Line 2 A + B
how to output the data
with a GUI or without
how to output the data to another file?
I want to enter A * B, C *D, and then calculate totals.
with a file, rather than entering all the data by hand.
The next page shows the following:
Here is an input file for use with this program. The file is named input.txt.
12
7
Here is a sample run of the program with this data file:
C:\users\default\JavaLessons>java AddTwo < input.txt
Enter first number:
Enter second number:
Sum: 19
C:\users\default\JavaLessons>