what is difference between Array and ArrayList to get Prime numbers?

Question

i was solving problem about the prime numbers, i found solution using Arrays, but i wanted to use the array list for some issues, one of them is to understand the Arraylist well. but i found that the results are not the same when i use the arraylist, here are the 2 Codes:

//using Arrays

import java.util.Arrays;
public class Prime {
    public static void main(String[] args) {
        primeNumber(100);
    }
    public static void primeNumber(int end) {
        boolean primeNumbers[] = new boolean[end]; 
        for (int i = 0; i < end; i++) {
            primeNumbers[i] = true;
        }
        for (int i = 2; i < primeNumbers.length; i++) {
            if (primeNumbers[i] ) {
                for (int j = i+i; j < primeNumbers.length; j += i) {
                    primeNumbers[j]= false;
                }
            }
        }
        for (int j = 2; j < primeNumbers.length; j++) {
            if (primeNumbers[j]) {
                System.out.println(j);

            }
        }   
    }
}

//Using ArrayList

import java.util.ArrayList;
public class Prime {
    public static void main(String[] args) {
        primeNumber(100);
    }

    public static void primeNumber(int end) {
        ArrayList<Boolean> primeNumbers = new ArrayList<Boolean>(); 
        for (int i = 0; i < end; i++) {
            primeNumbers.add(i,true);
        }
        for (int i = 2; i < primeNumbers.size(); i++) {
            if (primeNumbers.get(i) ) {
                for (int j = i+i; j < primeNumbers.size(); j += i) {
                    primeNumbers.add(j, false);
                }
            }
        }
        for (int j = 2; j < primeNumbers.size(); j++) {
            if (primeNumbers.get(j)) {
                System.out.println(j);

            }
        }

    }
}

Can i know what is the wrong i did with using the Arraylist, and if i wanted to use the arraylist for the same result what should i do.

Great thanks for the help.

Answer 1

Instead of

primeNumbers.add(j, false);

you should use

primeNumbers.set(j, false);

since add() adds a new element at the specified position (meaning the ArrayList grows by one element), but set() only sets the value of the element at specified position.

Answer 2

primeNumbers.add(j, false);

adds the element to the array list, and shifting all elements with index >= j to the right [including the previous element j], while:

primeNumbers[j]= false

overrides the existing element j, and does not shift elements.

You are probably looking for ArrayList.set() instead of ArrayList.add()