Java program to perform binary addition

import java.util.*;
public class BinaryAddition {
    private Scanner scan;
    private String num1, num2, binary;
    public BinaryAddition() {
        scan = new Scanner(System.in);
        num1 = num2 = binary = "";
    }
    public void input() {
        try {
            System.out.printf("Enter First Number: ");
            num1 = scan.next();
            if (!isBinary(num1)) {
                System.err.printf("Binary Number Expected.");
                System.exit(0);
            }
            System.out.printf("Enter Second Number: ");
            num2 = scan.next();
            if (!isBinary(num2)) {
                System.err.printf("Binary Number Expected.");
                System.exit(0);
            }
        } catch (InputMismatchException | NumberFormatException e) {
            System.err.println("Error Occurred!\n" + e.getMessage());
        }
    }
    private boolean isBinary(String num) {
        try {
            char ch;
            for (int i = 0; i < num.length(); i++) {
                ch = num.charAt(i);
                if (Integer.parseInt(Character.toString(ch)) != 0 && Integer.parseInt(Character.toString(ch)) != 1) {
                    return false;
                }
            }
        } catch (IndexOutOfBoundsException | NullPointerException e) {
            System.err.println("Error Occurred!\n" + e.getMessage());
            System.exit(0);
        }
        return true;
    }
    public void compute() {
        try {
            long len1, len2;
            len1 = num1.length();
            len2 = num2.length();
            if (len1 != len2) {
                if (len1 > len2) {
                    int dif = (int) ((int) len1 - len2);
                    String d = "";
                    for (int i = 0; i < dif; i++) {
                        d += "0";
                    }
                    num2 = d + num2;
                } else {
                    int dif = (int) ((int) len2 - len1);
                    String d = "";
                    for (int i = 0; i < dif; i++) {
                        d += "0";
                    }
                    num1 = d + num1;
                }
            }
            System.out.println("Binary Addition Rules:-\n1. 0 + 1 = 1\n2. 1 + 0 = 1\n3. 1 + 1 = 10\n4. 1 + 1 + 1 = 11");
            addition(num1, num2);
        } catch (NullPointerException | IndexOutOfBoundsException | NumberFormatException e) {
            System.err.println("Error Occurred!\n" + e.getMessage());
            System.exit(0);
        }
    }
    public void addition(String num1, String num2) {
        String binary = "", carry = "0";
        num1 = "0" + num1;
        num2 = "0" + num2;
        for (int i = num1.length() - 1; i >= 0; i--) {
            int sum;
            sum = Integer.parseInt(carry) + Integer.parseInt(Character.toString(num1.charAt(i))) + Integer.parseInt(Character.toString(num2.charAt(i)));
            if (sum == 2) {
                binary += "0";
                carry = "1";
            } else if (sum == 3) {
                binary += "1";
                carry = "1";
            } else {
                binary += Integer.toString(sum);
                carry = "0";
            }
        }
        this.binary = new String(new StringBuffer(binary).reverse());
        while (this.binary.startsWith("0")) {
            this.binary = this.binary.substring(1);
        }
        System.out.println("Result: " + this.binary);
    }
    public static void main(String[] args) {
        BinaryAddition ba = new BinaryAddition();
        ba.input();
        ba.compute();
    }
}

Output:-
Enter First Number: 1111
Enter Second Number: 1111
Binary Addition Rules:-
1. 0 + 1 = 1
2. 1 + 0 = 1
3. 1 + 1 = 10
4. 1 + 1 + 1 = 11
Result: 11110