Declaring int[] Array without defining size?

No; an array's size must be declared ahead of time. Try taking a look at the different implementations of List or Vector.

If in the end you absolutely need an array, you may create an array from your List.

Lists are best for situations, when you don't know exact size of array. For example, you can use ArrayList.

You have to use List instead of an Array.

Compiler needs to know the size of an Array. When you define an Array like this. int[] primes = new int[0];

It creates space for only 1 integer. So, when you write to primes[1], it creates segmentation fault signal and your program crashes.

As mentioned by others, you can use an ArrayList, and if you still need the output as an array, you can convert the ArrayList to array at the end (once you know the size of the array).

Arrays are meant for the situation when we know about the size and number of elements we are going to put inside.If we are not sure about the size then in that case Collection API of Java can help you out. for example

List Interface implemented by ArrayList and Vector

ArrayList can extends its size 50% at runtime so this will solve your problem.Or else you can use Vector if your application dealing with thread safety.but avoid using Vector because It increases its size to double at runtime.

This code may help you.

public static void main(String[] args) {

    boolean isPrime;
    List<Integer> list = new ArrayList<Integer>();

    for (int i = 2; i <= 100; i++)
    {
        isPrime = true;
        for (int j = 2; j < i/2; j++)
        {
            int prime = i % j;
            if (prime == 0)
            {
                isPrime = false;
                break;
            }
        }

        if (isPrime == true){
            list.add(i);
        }
    }

    // all the prime numbers 
    for(Iterator<Integer> iterator = list.iterator(); iterator.hasNext();){
        System.out.print(iterator.next()+" ");
    }

}