Revisiting Two's complement: A demonstration using Java

public class Main {
    public static void main(String[] args) {
        int number = 1;
        int invertedNumber = ~number; // Invert bits using the ~ operator
        int invertedNumberPlusOne = invertedNumber + 1;
        System.out.println("Number = " + number + " Binary value: " + Integer.toBinaryString(number));
        System.out.println("Inverted number = " + invertedNumber + " Binary value: " + Integer.toBinaryString(invertedNumber));
        System.out.println("Negative number = " + invertedNumberPlusOne + " Binary value: " + Integer.toBinaryString(invertedNumberPlusOne));
    }
}

Output:

Number = 1 Binary value: 1
Inverted number = -2 Binary value: 11111111111111111111111111111110
Negative number = -1 Binary value: 11111111111111111111111111111111

At first glance, this code might seem nonsensical, but it demonstrates how computers represent negative numbers in binary using two's complement. In this example, the number 1 is converted to -1 by inverting its bits and adding 1.

How Does Two's Complement Work?

Two's complement is a mathematical trick for efficiently representing negative numbers in binary. The process involves:

1. Inverting the bits of the binary representation of the number.
2. Adding 1 to the inverted result.

But why does this work? It’s tied to how binary subtraction operates. When transforming a number into its negative counterpart, you’re essentially subtracting the number from 0. For example:

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Similar to decimal subtraction, binary subtraction follows the borrowing rule (reference here). Notice how subtracting a number from 0 involves flipping its bits and then adding 1.

In most systems, binary numbers are stored in fixed-width variables (e.g., 8 bits). Any extra bits from the computation are simply discarded, which means subtracting 01001011 from 100000000 (1 followed by 8 zeros) is functionally identical to subtracting it from 0.

Here’s what happens when you carry out this subtraction:

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By splitting the computation into two steps:

1. Flip the bits: 11111111.
2. Add 1: 1 + 11111111 = 100000000.

The result is the same as performing a direct subtraction.

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In essence, subtracting a binary number ( A ) from an all-ones number is equivalent to inverting the bits of ( A ). The Java program leverages this trick to efficiently compute the negative of a number using two's complement.

Key Takeaway:

The Java program illustrates a clever use of two's complement for converting a binary number into its negative counterpart.

Source: Cornell University: Two's Complement Notes