Java8 ArrayList的实现

ArrayList底层是使用数组的方式实现的，在java8中初始化则是分配一个内存为0的对象数组（常量类型），在添加时在不断分配新内存

• 初始化

public class ArrayList<E> extends AbstractList<E>
        implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{
   //默认的数组长度
    private static final int DEFAULT_CAPACITY = 10;

    //java8以后的ArrayList初始化之后并不会马上分配内存空间
    private static final Object[] EMPTY_ELEMENTDATA = {};

     //transient修饰非序列化
    transient Object[] elementData; // non-private to simplify nested class access


    private int size;


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


    public ArrayList() {
        super();
        this.elementData = EMPTY_ELEMENTDATA;
    }
}

• 添加节点

 public boolean add(E e) {
        //判断是否需要扩容（传入的参数是目前所需要的最小数组长度）
        ensureCapacityInternal(size + 1); 
        elementData[size++] = e;
        return true;
 }

private void ensureCapacityInternal(int minCapacity) {
        if (elementData == EMPTY_ELEMENTDATA) {
            //判断默认的长度是否符合要求
            minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
        }

        ensureExplicitCapacity(minCapacity);
    }

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

        // overflow-conscious code
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
}
//进行扩容
private void grow(int minCapacity) {
        // overflow-conscious code
        int oldCapacity = elementData.length;
        int newCapacity = oldCapacity + (oldCapacity >> 1);
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity;
        if (newCapacity - MAX_ARRAY_SIZE > 0)
            newCapacity = hugeCapacity(minCapacity);
        // minCapacity is usually close to size, so this is a win:
        //Arrays.copyOf(oldArray，newArrayLength)
        elementData = Arrays.copyOf(elementData, newCapacity);
    }

注意：Arrays.copyOf(elementData, newCapacity)在底层视线中非常常见，是将数组有一个数组复制到另一个数组中
区别：System.arraycopy(Object src, int srcPos, Object dest, int destPos, int length)表示将数组src从srcPost的位置开始复制，复制的长度为length，复制到dest数组的destPost位置上

• 删除节点（给出节点位置）

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

        modCount++;
        E oldValue = elementData(index);

        int numMoved = size - index - 1;
        if (numMoved > 0)
            System.arraycopy(elementData, index+1, elementData, index,
                             numMoved);
        elementData[--size] = null; // clear to let GC do its work

        return oldValue;
}
//判断节点位置是否存在
private void rangeCheck(int index) {
        if (index >= size)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}

• 删除节点（已知节点内容）

 public boolean remove(Object o) {
        if (o == null) {
            for (int index = 0; index < size; index++)
                if (elementData[index] == null) {
                    fastRemove(index);
                    return true;
                }
        } else {
            for (int index = 0; index < size; index++)
                if (o.equals(elementData[index])) {
                    fastRemove(index);
                    return true;
                }
        }
        return false;
    }
    //快速删除节点
private void fastRemove(int index) {
        modCount++;
        int numMoved = size - index - 1;
        if (numMoved > 0)
            System.arraycopy(elementData, index+1, elementData, index,
                             numMoved);
        elementData[--size] = null; // clear to let GC do its work
    }

同样的，在ArrayList中允许节点内容为空，也允许节点内容重复

• 获取某位置的节点

public E get(int index) {
            rangeCheck(index);
            checkForComodification();
            return ArrayList.this.elementData(offset + index);
}
//判断该节点是否已经被修改了（这个一般用于并发操作的控制，防止多个线程都写不一致）
private void checkForComodification() {
            if (ArrayList.this.modCount != this.modCount)
                throw new ConcurrentModificationException();
}

• 设置某位置的节点为指定内容

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

        E oldValue = elementData(index);
        elementData[index] = element;
        return oldValue;
}
//获取某位置的节点
 E elementData(int index) {
        return (E) elementData[index];
}

• 获取表长度

//同样这里增强了对并发操作的版本控制，查看目前的size是否被改变过
 public int size() {
            checkForComodification();
            return this.size;
}