Why does my primality test stop after 1 and not seem to be able to go on?

Question

I apologize if this is a dumb question, I'm pretty new to Java, but I can't figure out why my mod operator based primality test for numbers 1-100 stops at 1. I've tried following my code and I don't understand why it fails to continue to the last else statement when possiblePrime = 2.

It should just take possiblePrime = 2 through the if and else if statements all the way to the last else statement and print "2 is a prime." and then continue on to possiblePrime = 3, but instead it goes through the loops as it should when possiblePrime = 1, as it is when it is initialized at the start, and then stops entirely once possiblePrime is incremented at the end of the last else statement, solely printing "1 is a prime."

Thank you for any help you might be able to offer, it is much appreciated! I am definitely racking my brain trying to figure this out, and I'm almost 100% sure it is some stupid and obvious mistake I'm just not seeing.

public class PrimeFind {
    public static void main(String[] args){
        int possiblePrime = 1;
        for(int i = 1 ; i <= 100 ; i++){
            int possibleDivisor = 1;
            if(possiblePrime%possibleDivisor != 0){
                possibleDivisor++;
            }
            else if(possiblePrime != possibleDivisor){
                possiblePrime++;
            }
            else{
                System.out.println(possiblePrime + " is a prime.");
                possiblePrime++;
            }
        }
    }
}

Answer 1

You are setting possibleDivisor = 1 inside the for loop. hence it will always equal 1. This in turn will make the modulo operation equal to 0 in every case. The possiblePrime will always be different from the possibleDivisor (1) except for 1. Hence you only get 1.

public class PrimeFind {
    public static void main(String[] args){
        int possiblePrime = 1;
        for(int i = 1 ; i <= 100 ; i++){
            int possibleDivisor = 1; // Always 1
            if(possiblePrime%possibleDivisor != 0){ // Always False because something%1 == 0
                possibleDivisor++;
            }
            else if(possiblePrime != possibleDivisor){ // Always True except for possiblePrime=1
                possiblePrime++;
            }
            else{
                System.out.println(possiblePrime + " is a prime.");
                possiblePrime++;
            }
        }
    }
}

Answer 2

You had put the wrong conditions that is why this is happening, your first if statement if(possiblePrime%possibleDivisor != 0) is always going to evaluated as false because you initialized possibleDivisor with value 1 and none of the numbers gives value other than 0 from division by 1 so in your code the statement possibleDivisor++; will never going to execute.

And your else if statement else if(possiblePrime != possibleDivisor) every time evaluates as true except i=0. That is why it won't go to else block and print your statement.

Your for loop doesn't get stop after i=0 but just because of your else block isn't executes, it seems to you as like loop stopped after the first iteration.