Use an array of any size to add any two numbers.

Counter.java

public class Counter {

    private int[] seq = null;

    public Counter(int size) {
        this.seq = new int[size];
    }

    public Counter() {
        this.seq = new int[10];
    }

    public int[] numToArray(int n) {
        // Convert to string
        String number = Integer.toString(n);
        // Init a new array at length
        int[] arr = new int[number.length()];
        // Set the values
        for(int i = 0; i < number.length(); i++) {
            arr[i] = number.toCharArray()[i]-48;
        }

        return arr;

    }

    private void carryOvers() {
        int carryOver = 0;

        for(int i = (seq.length -1); i > 0; i--) {
            seq[i] += carryOver;

            if(seq[i] > 9) {
                carryOver = ((seq[i] - (seq[i] % 10)) / 10);
                seq[i] = seq[i] - 10;
            } else {
                carryOver = 0;
            }
        }
    }

    public void add(int n) {
        int[] number = numToArray(n);

        int offset = seq.length - number.length;

        for(int i = (seq.length - 1); i > 0; i--) {

            seq[i] += number[i-offset];

            if(0 == (i - offset)) break;
        }

        carryOvers();

    }

    public String toString() {
        StringBuilder sb = new StringBuilder();
        for(int n : seq) {
            sb.append(n);
        }
        return sb.toString();
    }

    public int toValue() {
        return Integer.parseInt(this.toString());
    }


    public static void main (String[] args) {

        //
        // Example
        //

        Counter counter = new Counter();
        counter.add(236);
        counter.add(66);
        counter.add(10000);
        counter.add(10000);
        counter.add(10000);
        counter.add(10000);
        System.out.println(counter.toString());
    }
}

CounterTest.java

import org.junit.Before;
import org.junit.Test;

import static org.junit.Assert.*;

public class CounterTest {

    private Counter counter;

    @Before
    public void setUp() throws Exception {
        counter = new Counter(10);
    }

    @Test
    public void testNumToArray() throws Exception {
        int[] arr = new int[] {1, 2, 3};
        assertArrayEquals(arr, counter.numToArray(123));

        arr = new int[] {1};
        assertArrayEquals(arr, counter.numToArray(1));

        arr = new int[] {1,2,3,4,5,6,7,8,9};
        assertArrayEquals(arr, counter.numToArray(123456789));
    }

    @Test
    public void testAdd() throws Exception {
        counter.add(10);
        assertEquals(counter.toValue(), 10);

        counter.add(230);
        assertEquals(counter.toValue(), 240);

        counter.add(9);
        assertEquals(counter.toValue(), 249);

        counter.add(9);
        assertEquals(counter.toValue(), 258);

        counter.add(10000);
        assertEquals(counter.toValue(), 10258);
    }

    @Test
    public void testToString() throws Exception {
        counter.add(10);
        assertEquals(counter.toString(), "0000000010");

        counter.add(230);
        assertEquals(counter.toString(), "0000000240");

        counter.add(9);
        assertEquals(counter.toString(), "0000000249");

        counter.add(9);
        assertEquals(counter.toString(), "0000000258");

        counter.add(10000);
        assertEquals(counter.toString(), "0000010258");
    }
}

It actually sucks, but I had an hour to burn, so why not?