How to print out the index of a boolean array?

I'm trying to get all of the indexes of a Boolean array to be printed out where its element is true. The end goal is to be able to find a prime number of the indexes (where I change each index number that isn't prime to false in the array) then print out only what is left of the prime numbers of the indexes of the array.

The very first step I'm just trying to do is at least to get some integer index to print out, but nothing seems to be working and I don't know what is wrong.

public class PriNum{
    private boolean[] array;

    public PriNum(int max){
        if (max > 2){ //I don't have any problems with this if statement
            throw new IllegalArgumentException();
        }
        else{
            array = new boolean[max];

            for(int i = 0; i < max; i++){
                if(i == 0 || i == 1){ //Automatically makes 0 and 1 false 
                                      //because they are not prime
                    array[i] = false;
                }
                else{
                    array[i] = true;
                }
            }
            toString(); //I know for sure the code gets to here 
                        //because it prints out a string I have
                        // there, but not the index
        }
    }

    public String toString(){
        String s = "test"; //this only prints test so I can see if 
                           //the code gets here, otherwise it would just be ""

        for (int i = 0; i < array.length; i++){
            if(array[i] == true){
                s = s + i; //Initially I tried to have the indexes returned
                         //to be printed and separated by a comma,
                         //but nothing comes out at all, save for "test"
             }
         }

        return s;
    }
}

EDIT: Included is the driver class that's requesting the print of the class PriNum

class Driver{
    public static void main(String [] args){
        PriNum theprime = null;

        try{
            theprime = new PriNum(50);
        }
        catch (IllegalArgumentException oops){
            System.out.println("Max must be at least 2.");
        }

        System.out.println(theprime);
    }
}