Understanding binary addition program

Question

This program is about binary addition in Java:

public static String binaryAddition(String s1, String s2) {
    if (s1 == null || s2 == null) return "";
    int first = s1.length() - 1;
    int second = s2.length() - 1;
    StringBuilder sb = new StringBuilder();
    int carry = 0;
    while (first >= 0 || second >= 0) {
        int sum = carry;
        if (first >= 0) {
            sum += s1.charAt(first) - '0';
            first--;
        }
        if (second >= 0) {
            sum += s2.charAt(second) - '0';
            second--;
        }
        carry = sum >> 1;
        sum = sum & 1;
        sb.append(sum == 0 ? '0' : '1');
    }
    if (carry > 0)
        sb.append('1');

    sb.reverse();
    return String.valueOf(sb);
}

I am having difficulty in understanding this program.

Why we have to substract with 0 here?

sum += s1.charAt(first) - '0';

What is the use of >> operator and & operator here:

        carry = sum >> 1;
        sum = sum & 1;
        sb.append(sum == 0 ? '0' : '1');

Can you please help me in understanding this program.

Community · Accepted Answer · 2020-06-20 09:12:55Z

0

Why we have to subtract with 0 here?
 sum += s1.charAt(first) - '0';

That is converting the characters '0' and '1' to the numbers 0 and 1.

What is the use of >> operator and & operator here:

The >> is shifting one bit to the right, and the & is masking out all bits apart from the rightmost one.

Can I recommend that you find and read a tutorial on how bitwise operations work in Java. This is very basic stuff. Once you understand the basics, then you should be able to read and understand code like the code you have shown us.

edited Jun 20, 2020 at 9:12

CommunityBot

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answered Apr 2, 2017 at 6:42

Stephen C

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2 Comments

Learner Over a year ago

Thank you, can you please explain how the >> and & in solving binary addition here. I understand how the operator works but not clear how this is solving this addition program.

Stephen C Over a year ago

Clearly, those two statements are figuring out what the carry bit is. Have you read anything about how binary addition works? en.wikipedia.org/wiki/Binary_number#Addition

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