手写ArrayList

import javax.validation.constraints.NotNull;
import java.util.*;
import java.util.function.Consumer;

/**
 * @author Dongguabai
 * @date 2018/8/29 15:54
 */
public class MyArrayList<E> implements Iterable<E> {

    private static final Object[] EMPTY_ELEMENTDATA = {};
    private static final int DEFAULT_CAPACITY = 10;
    private transient int modCount = 0;
    private Object[] elementData;
    private int size;

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

    public MyArrayList(Collection<? extends E> c) {
        elementData = c.toArray();
        if ((size = elementData.length) != 0) {
            if (elementData.getClass() != Object[].class) {
                elementData = Arrays.copyOf(elementData, size, Object[].class);
            }
        } else {
            this.elementData = EMPTY_ELEMENTDATA;
        }
    }

    public MyArrayList() {
        this(DEFAULT_CAPACITY);
    }

    // 获取数组的容量
    public int getCapacity() {
        return elementData.length;
    }

    // 获取元素个数
    public int getSize() {
        return size;
    }

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

    public void add(int index, E e) {
        rangeCheckForAdd(index);
        if (size == elementData.length) {
            resize(2 * elementData.length);
        }
        for (int i = size - 1; i >= index; i--) {
            elementData[i + 1] = elementData[i];
        }
        elementData[index] = e;
        size++;
    }

    public void addLast(E e) {
        add(size, e);
    }

    public void addFirst(E e) {
        add(0, e);
    }

    public E get(int index) {
        rangeCheck(index);
        return elementData(elementData[index]);
    }

    public E getLast(){
        return get(size-1);
    } 

     public E getFirst(){
        return get(0);
    }

     private E elementData(Object elementDatum) {
        return (E) elementDatum;
    }

    public void set(int index, E e) {
        rangeCheck(index);
        elementData[index] = e;
    }

    public boolean contains(E e) {
        for (int i = 0; i < size; i++) {
            if (elementData[i].equals(e)) {
                return true;
            }
        }
        return false;
    }

    // 查找元素
    public int find(E e) {
        for (int i = 0; i < size; i++) {
            if (elementData[i].equals(e)) {
                return i;
            }
        }
        return -1;
    }

    // 删除元素
    public E remove(int index) {
        rangeCheck(index);
        E ret = elementData(elementData[index]);
        for (int i = index + 1; i < size; i++) {
            elementData[i - 1] = elementData[i];
        }
        size--;
        elementData[size] = null;

        if (size == elementData.length / 2) {
            resize(elementData.length / 2);
        }
        return ret;
    }

    // 删除第一个元素, 返回删除的元素
    public E removeFirst() {
        return remove(0);
    }

    // 删除最后一个元素, 返回删除的元素
    public E removeLast() {
        return remove(size - 1);
    }

    public void removeElement(E e) {
        int index = find(e);
        if (index != -1) {
            remove(index);
        }
    }

    @Override
    public String toString() {
        Iterator<E> it = iterator();
        if (!it.hasNext()) {
            return "[]";
        }
        StringBuilder sb = new StringBuilder();
        sb.append('[');
        for (; ; ) {
            E e = it.next();
            sb.append(e == this ? "(this Collection)" : e);
            if (!it.hasNext()) {
                return sb.append(']').toString();
            }
            sb.append(',').append(' ');
        }
    }

    private void resize(int newCapacity) {
        E[] newData = (E[]) new Object[newCapacity];
        for (int i = 0; i < size; i++) {
            newData[i] = elementData(elementData[i]);
        }
        elementData = newData;
    }

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

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

    private void rangeCheckForAdd(int index) {
        if (index > size || index < 0) {
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
        }
    }

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

    private class Itr implements Iterator<E> {
        int cursor;
        int lastRet = -1;
        int expectedModCount = modCount;

        @Override
        public boolean hasNext() {
            return cursor != size;
        }

        @SuppressWarnings("unchecked")
        @Override
        public E next() {
            checkForComodification();
            int i = cursor;
            if (i >= size) {
                throw new NoSuchElementException();
            }
            Object[] elementData = MyArrayList.this.elementData;
            if (i >= elementData.length) {
                throw new ConcurrentModificationException();
            }
            cursor = i + 1;
            return (E) elementData[lastRet = i];
        }

        @Override
        public void remove() {
            if (lastRet < 0) {
                throw new IllegalStateException();
            }
            checkForComodification();

            try {
                MyArrayList.this.remove(lastRet);
                cursor = lastRet;
                lastRet = -1;
                expectedModCount = modCount;
            } catch (IndexOutOfBoundsException ex) {
                throw new ConcurrentModificationException();
            }
        }

        @Override
        @SuppressWarnings("unchecked")
        public void forEachRemaining(Consumer<? super E> consumer) {
            Objects.requireNonNull(consumer);
            final int size = MyArrayList.this.size;
            int i = cursor;
            if (i >= size) {
                return;
            }
            final Object[] elementData = MyArrayList.this.elementData;
            if (i >= elementData.length) {
                throw new ConcurrentModificationException();
            }
            while (i != size && modCount == expectedModCount) {
                consumer.accept((E) elementData[i++]);
            }
            cursor = i;
            lastRet = i - 1;
            checkForComodification();
        }

        final void checkForComodification() {
            if (modCount != expectedModCount) {
                throw new ConcurrentModificationException();
            }
        }
    }

}