All the perfect numbers between 1 and 500 program algorithm says it works but it doesn't

Question

I am writing a program to tell me all the perfect numbers between 1 and 500 and I made this program, but it doesn't work although the algorithm makes sense.

import java.util.Scanner;

class allPerfect {
    public static void main(String args[]) {

        int sum = 0;
        System.out.println("All perfect numbers between 1 and 500 are:");
        for (int j = 0; j != 501; j++) {

            for (int i = 1; i < j; i++) {
                if (j % i == 0) {

                    sum = sum + i;
                    if (sum == j) {
                        System.out.println(j);
                        sum = 0;
                    } else {
                        sum = 0;
                    }
                } 
            }
        }       
    }
}

What's wrong here?

Answer 1

You are resetting sum before you finish finding all of a numbers divisors.

 for(int i = 1; i < j; i++ ){
        if(j % i == 0){

            sum = sum + i;
            if(sum == j){
                System.out.println(j);
                sum = 0;
            }else{

                sum = 0;
            }
        } 
    }

You should do the if(sum == j) only after you exit the inner for loop.

Answer 2

Please find the comment in-line.

class allPerfect {
  public static void main(String args[]){
    int sum;
    System.out.println("All perfect numbers between 1 and 500 are:");
    for(int j = 1; j!=501; j++){
      sum = 0;     //**You should reset the sum at the start of the inner loop.**
      for(int i = 1; i < j; i++ ){
        if(j % i == 0) {
          sum = sum + i;
        }
      }      
      if(sum == j) {              //------ statement-1
        System.out.println(j);
      }
    }
  }
}

statement-1 you check the sum of all the divisor is equal to the number, after summing up for all the numbers less than j .

You can ignore all the odd number (j) in outer loop, because odd perfect number does not exist. To be precise: it is not known that odd perfect number exists.