Class to flatten nested array of Integers

FlatArray.java

package com.antoniomartel;

import java.util.ArrayList;

public class FlatArray {

	protected ArrayList nestedArray = null;
	protected ArrayList flattenArray = null;

	// New exception created to be thrown when nested array contains something different from Integers
	class NotAnArrayOfIntegersException extends Exception {

		private static final long serialVersionUID = 1L;

		public NotAnArrayOfIntegersException (String msg){

			super(msg);
		}
	}

	// Class constructor
	public FlatArray(ArrayList arrayToBeFlatten) {

		nestedArray = arrayToBeFlatten;
	}

	// Private method to recursively loop thru arrays while adding found Integers to the flatten array that will come out as a result  
	private void loopArray(ArrayList array) throws NotAnArrayOfIntegersException {

		if (array != null) {
			for (int i = 0; i < array.size(); i++) {
				if (array.get(i).getClass().getName() == "java.lang.Integer") {
					flattenArray.add(array.get(i));
				}
				else 
					if (array.get(i).getClass().getName() == "java.util.ArrayList") {
						ArrayList subArray = (ArrayList) array.get(i);
						loopArray(subArray);
					}
					else {
						throw new NotAnArrayOfIntegersException("Nested array contains a class different from array of Integers: " + array.get(i).getClass().getName());
					}
			}
		}
	}

	// Public method that flattens arrays
	public ArrayList toArray() throws NotAnArrayOfIntegersException {

		flattenArray = new ArrayList<Integer>();
		loopArray(nestedArray);
		return flattenArray;
	}

	// Main method to create test env, execute method, print and check for result
	public static void main(String[] args) throws NotAnArrayOfIntegersException {

		ArrayList<Integer> array1 = new ArrayList<Integer>();
		array1.add(3);

		ArrayList array2 = new ArrayList();
		array2.add(1);
		array2.add(2);
		array2.add(array1);

		ArrayList array3 = new ArrayList();
		array3.add(array2);
		array3.add(4);

		FlatArray nestedArray = new FlatArray(array3);
		String result = nestedArray.toArray().toString();

		System.out.printf("Array to be flatten is : %s\n", array3.toString());
		System.out.printf("Flatten array is : %s\n", result);
		if (result.equals("[1, 2, 3, 4]") == false) 
			System.out.println("There was an error when nested array was tried to flatten");
		else
			System.out.println("Got expected result");
	}
}

Class FlatArray that transforms a nested arrays of Integers to a flatten array of Integers (no subarrays inside)