自己實現一個泛型ArrayList

自己實現一個ArrayList，保證最基本的 add(), remove() , get() , set() , size() ，增強for循環功能。

import java.util.Iterator;
import java.util.NoSuchElementException;

public class MyArrayList<E> implements Iterable<E> {
    private static final int DEFAULT_CAPACITY = 10;
    private int size;
    private Object[] elementData;

    public MyArrayList() {
        clear();
    }

    public void clear() {
        size = 0;
        ensureCapacity(DEFAULT_CAPACITY);
    }

    public int size() {
        return size;
    }

    public void trimToSize() {
        ensureCapacity(size);
    }

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

    public E get(int index) {
        if (index < 0 || index >= size)
            throw new ArrayIndexOutOfBoundsException();
        return elementData(index);
    }

    public E set(int index, E newValue) {
        if (index < 0 || index >= size)
            throw new ArrayIndexOutOfBoundsException();
        E oldValue = elementData(index);
        elementData[index] = newValue;
        return oldValue;
    }

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

    public boolean add(E e) {
        add(size, e);
        return true;
    }

    public void add(int index, E e) {
        if (elementData.length == size) {
            ensureCapacity(size << 1 + 1);
        }
        for (int i = size; i > index; i--) {
            elementData[i] = elementData[i - 1];
        }
        elementData[index] = e;
        size++;
    }

    public E remove(int index) {
        if (index < 0 || index >= size)
            return null;
        E old = elementData(index);
        for (int i = index; i < size - 1; i++) {
            elementData[i] = elementData[i + 1];
        }
        size--;
        return old;
    }

    private void ensureCapacity(int newCapacity) {
        if (newCapacity < size)
            return;
        Object[] old = elementData;
        elementData = new Object[newCapacity];
        for (int i = 0; i < size; i++) {
            elementData[i] = old[i];
        }
    }

    @Override
    public String toString() {
        StringBuilder builder = new StringBuilder();
        builder.append("[").append(elementData(0) == null ? null : elementData(0));
        for (int i = 1; i < size; i++) {
            builder.append(", ").append(elementData(i));
        }
        builder.append("]");
        return builder.toString();
    }

    @Override
    public Iterator<E> iterator() {
        return new Itr();
    }

    private class Itr implements Iterator<E> {
        private int current = 0;

        @Override
        public boolean hasNext() {
            return current < size;
        }

        @Override
        public E next() {
            if (!hasNext())
                throw new NoSuchElementException();
            return elementData(current++);
        }

        public void remove() {
            MyArrayList.this.remove(--current);
        }
    }
}