Convert integer to binary and print the sum of the digits

Question

The code below is supposed to take an integer, convert it to binary, and then print the sum of the digits:

Example:

Input = 15 

( 15 in Binary is "1111" and 1+1+1+1 = 4 )

Output = 4

This is the code I have so far. However, it is not working as expected. If you input 15 , it returns 0 instead of 4 as explained above:

  Scanner in = new Scanner(System.in);
  int X = in.nextInt();

  String binary = Integer.toBinaryString(X);   

  int amount = 0;  
  String input = Integer.toString(X);

  for(int b = 0; b < binary.length(); b++){
     if (binary.charAt(b) == 1){
      amount++;
     }
  }

  System.out.println(amount);

Answer 1

Characters are always single and written in single quotes.

1 is the character which is extracted from String and it should be written in single quotes.

Here is the correct code:

if (binary.charAt(b) == '1'){
  amount++;
}

Answer 2

Shorter approach:

Scanner in = new Scanner(System.in);
int X = in.nextInt();
String binary = Integer.toBinaryString(X);   
String input = Integer.toString(X)
System.out.println(StringUtils.countMatches(input, "1"));

Answer 3

Binary operations are enougth

    int p = 0b10000000000000000000000000010101;
    int t = 0;
    for(int i=0; i<32; i++) {
        t += p & 1;
        p = p >> 1;
    }
    System.out.println(t);