ArrayList: IndexOutOfBounds exception issue

Question

I'm making a program to find prime numbers. I'm storing the prime numbers and all positive integers (right now till 100) in two ArrayList<Integer>. Here's the code:

import java.util.ArrayList;
public class PrimeNumbers { 
static ArrayList<Integer> num = new ArrayList<Integer>();
static ArrayList<Integer> prime = new ArrayList<Integer>();
public static void main(String[] args) {
    prime.add(2);
    prime.add(3);
    prime.add(5);
    for (int z = 1; z<=100; z++){
        num.add(z);
    }
    outer: for (int a = 1; a <=num.size(); a++){
        inner: for (int b = 1; b <=prime.size(); b++){
            if (num.get(a)%prime.get(b) != 0){//line 14
                if (prime.indexOf(b)+1 == prime.size()){
                    prime.add(a);
                    continue outer;
                }
                else 
                    continue inner;
            }
            else
                continue outer;
        }
    }
    System.out.println(prime);

    }
}

But when I run the program, these errors show up:

Exception in thread "main" java.lang.IndexOutOfBoundsException: Index: 3, Size: 3 at java.util.ArrayList.rangeCheck(ArrayList.java:653) at java.util.ArrayList.get(ArrayList.java:429) at PrimeNumbers.main(PrimeNumbers.java:14)

What is going wrong?

Answer 1

List indices run from 0 to size()-1, but you are looping from 1 to size().

Answer 2

The first element of a Collection has index 0, not 1. Thus for example, accessing the third element should be done as collection.get(2).

Your loops should iterate like follows:

outer: for (int a = 0; a < num.size(); a++){
    inner: for (int b = 0; b < prime.size(); b++){
        ...
    }
}

Note the strict (<) index comparison. Moreover, there is the following simpler way:

outer: for (Integer numItem : num){
    inner: for (Integer primeItem : prime){
        ...
    }
}

This for loop form iterates on the whole collection.

Answer 3

Try using Double instead of Integer. (Note how Double is an Object with a capital "D").