自己实现一个泛型ArrayList

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);
}
}
}