DynamicArray.java

import java.util.Arrays;

class DynamicArray<E> {
    private transient Object[] elementData;
    private int size;
    private int modCount = 0;
    private int a = 2;

    public DynamicArray(int initialCapacity) {
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal Capacity: " +
                                               initialCapacity);
        this.elementData = new Object[initialCapacity];
    }

    public DynamicArray() {
        this(10);
    }

    private void ensureCapacity(int minCapacity) {
        modCount++;

        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }

    private void grow(int minCapacity) {
        int oldCapacity = elementData.length;
        int newCapacity = oldCapacity * a;
        elementData = Arrays.copyOf(elementData, newCapacity);
    }

    public int size() {
        return size;
    }

    public boolean isEmpty() {
        return size == 0;
    }

    @SuppressWarnings("unchecked")
    E elementData(int index) {
        return (E) elementData[index];
    }

    public E get(int index) {
        rangeCheck(index);

        return elementData(index);
    }

    public E set(int index, E element) {
        rangeCheck(index);

        E oldValue = elementData(index);
        elementData[index] = element;
        return oldValue;
    }

    public boolean add(E element) {
        ensureCapacity(size + 1);
        elementData[size++] = element;
        return true;
    }

    private void rangeCheck(int index) {
        if (index >= size)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

    private String outOfBoundsMsg(int index) {
        return "Index: "+index+", Size: "+size;
    }
}

class DynamicArrayTest {
    public static void main(String[] args) {
        DynamicArray<String> array = new DynamicArray<String>();
        array.add("foo");
        array.add("bar");
        array.add("baz");

        assert array.size() == 3;
        assert array.get(0) == "foo";
        assert array.get(1) == "bar";
        assert array.get(2) == "baz";

        String[] foo = {"one",
                        "two",
                        "three",
                        "four",
                        "five",
                        "six",
                        "seven",
                        "eight"};

        // add another eight elements to the array, extending the size of the
        // array
        for (String item : foo) {
            array.add(item);
        }

        assert array.size() == 11;
    }
}