Java: Count the number of set bits in a 32-bit integer

Last update on May 13 2025 12:59:53 (UTC/GMT +8 hours)

Hamming Weight of Integer (Count Set Bits)

From Wikipedia,
The Hamming weight of a string is the number of symbols that are different from the zero-symbol of the alphabet used. It is thus equivalent to the Hamming distance from the all-zero string of the same length. For the most typical case, a string of bits, this is the number of 1's in the string, or the digit sum of the binary representation of a given number and the ℓ_₁ norm of a bit vector. In this binary case, it is also called the population count, popcount, sideways sum, or bit summation.

Example:

Sample Solution:

Java Code:

// Importing the Scanner class from the java.util package to read user input
import java.util.Scanner;

// Defining a class named "solution"
public class solution {

    // Method to count the number of set bits (1s) in the binary representation of a number
    static int count_Set_Bits(int num) {
        int ctr = 0;

        // Looping until the number becomes zero
        while (num != 0) {
            // Using bitwise AND operation to clear the rightmost set bit and incrementing the counter
            num = num & (num - 1);
            ctr++;
        }

        // Returning the count of set bits
        return ctr;
    }

    // Main method, the entry point of the program
    public static void main(String args[]) {
        // Creating a Scanner object for user input
        Scanner sc = new Scanner(System.in);

        // Prompting the user to input a number
        System.out.print("Input a number: ");

        // Reading the input number from the user
        int num = sc.nextInt();

        // Calling the count_Set_Bits method and printing the result
        System.out.println(count_Set_Bits(num));

        // Closing the Scanner to avoid resource leaks
        sc.close();
    }
}

Sample Output:

Input a number:  1427
6

Flowchart:

For more Practice: Solve these Related Problems:

• Write a Java program to compute the Hamming weight of an integer using bitwise operations without iterating over all 32 bits.
• Write a Java program to calculate the Hamming weight of an integer recursively.
• Write a Java program to compare two different methods for computing the Hamming weight of a 32-bit integer and output their results.
• Write a Java program to count the number of set bits in a binary representation of an integer using a well-known bit manipulation trick.

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