Add.java

import java.io.*;
import java.util.*;

public class Add {
    public static void main(String[] args) throws IOException {
        BufferedReader reader1 = new BufferedReader(new FileReader("1.txt"));
        BufferedReader reader2 = new BufferedReader(new FileReader("2.txt"));
        //BufferedWriter writer = new BufferedWriter(new FileOutputStream("3.txt"));
        while(true) {
            int sum = 0;
            String line1 = reader1.readLine();
            String line2 = reader2.readLine();
            if (line1 == null && line2 == null) {
                break;
            }
            if (line1 != null) {
                sum += Integer.valueOf(line1);
            }
            if (line2 != null) {
                sum += Integer.valueOf(line2);
            }
            System.out.println(sum);

        }
        reader1.close();
        reader2.close();
    }
}

import java.io.*;
import java.util.*;

public class Add {
    public static void main(String[] args) throws IOException {
        Scanner in1 = new Scanner(new FileReader("1.txt"));
        Scanner in2 = new Scanner(new FileReader("2.txt"));
        //BufferedWriter writer = new BufferedWriter(new FileOutputStream("3.txt"));
        while(in1.hasNext() || in2.hasNext()) {
            int sum = 0;
            if (in1.hasNext()) {
                sum += Integer.valueOf(in1.nextLine());
            }
            if (in2.hasNext()) {
                sum += Integer.valueOf(in2.nextLine());
            }
            System.out.println(sum);

        }
        in1.close();
        in2.close();
    }
}