Once an array is created, its size cannot be changed. If you want to change the size, you must create a new array and populates it using the values of the old array.
public class MainClass { public static void main(String[] args) { int[] numbers = { 1, 2, 3 }; int[] temp = new int[4]; int length = numbers.length; for (int j = 0; j < length; j++) { temp[j] = numbers[j]; } numbers = temp; } }