ArrayList源码分析

    // 序列化ID
    private static final long serialVersionUID = 8683452581122892189L;
    // 默认初始化容量
    private static final int DEFAULT_CAPACITY = 10;
    // 空数组
    private static final Object[] EMPTY_ELEMENTDATA = {};
    // 空数组
    private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
    // 存储数据元素的数组
    transient Object[] elementData; // non-private to simplify nested class access
    // 当前arraylist集合的大小，也就是elementData数组中数据元素的个数
    private int size;

   /**
     * 一：无参构造方法
     */
    public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }
	
    /**
     * 二：携带一个int类型的参数，指定arraylist的初始容量
     */
    public ArrayList(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);
        }
    }
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    /**
     * 一：直接添加数据元素到arraylist的尾部
     */
    public boolean add(E e) {
	//是否扩容、记录modCount
        ensureCapacityInternal(size + 1);  // Increments modCount!!
	//把值添加到数组尾部
        elementData[size++] = e;
        return true;
    }
----------------------------------------------------------------------

private void ensureCapacityInternal(int minCapacity) {
	//minCapacity=size+1；
	//minCapacity表示如果添加成功后，数组的最小长度
	//如果为无参构造
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
	    //取默认长度和minCapacity的最大值，即10
            minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
        }
	//是否扩容
        ensureExplicitCapacity(minCapacity);
    }
----------------------------------------------------------------------

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

        // 如果添加后最小长度大于 数组长度
        if (minCapacity - elementData.length > 0)
	    //扩容
            grow(minCapacity);
    }
----------------------------------------------------------------------
 private void grow(int minCapacity) {
        //获取数组长度
        int oldCapacity = elementData.length;
	//1.5倍扩容
        int newCapacity = oldCapacity + (oldCapacity >> 1);
	//1.5倍扩容也不够用
        if (newCapacity - minCapacity < 0)
	    //扩容后长度=minCapacity
            newCapacity = minCapacity;
	//简直最大长度
        if (newCapacity - MAX_ARRAY_SIZE > 0)
            newCapacity = hugeCapacity(minCapacity);
        // 复制
        elementData = Arrays.copyOf(elementData, newCapacity);
    }

/*
    * 一：根据角标进行remove操作
    */
    public E remove(int index) {
        // 1. 对角标越界进行判断
        if (index >= size)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
        // 2.modCount自增1
        modCount++;
        // 3.获取到指定下角标位置的数据
        E oldValue = (E) elementData[index];
        // 4.计算需要移动的元素个数
        int numMoved = size - index - 1;
        if (numMoved > 0)
            // 5. 指定角标位置后的元素前移一位，效率低
            System.arraycopy(elementData, index+1, elementData, index,
                             numMoved);
        // 6.将size自减1，并将数组末尾置为null，便于垃圾回收
        elementData[--size] = null; // clear to let GC do its work
        // 7.最后将所要删除的数据元素return掉
        return oldValue;
    }

    /*
     * 二：根据数据元素进行remove操作
     */
    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;
    }
----------------------------------------------------------------------
private void fastRemove(int index) {
        // 1.modCount的值自增1
        modCount++;
        // 2.计算需要移动的元素个数
        int numMoved = size - index - 1;
        if (numMoved > 0)
             // 3. 指定角标位置后的元素前移一位
            System.arraycopy(elementData, index+1, elementData, index,
                             numMoved);
        // 4.将size自减1，并将数组末尾置为null，便于垃圾回收
        elementData[--size] = null; // clear to let GC do its work
    }

public void set(E e) {
            if (lastRet < 0)
                throw new IllegalStateException();
	    //检查modCount
            checkForComodification();

            try {
                ArrayList.this.set(lastRet, e);
            } catch (IndexOutOfBoundsException ex) {
                throw new ConcurrentModificationException();
            }
        }
----------------------------------------------------------------------
public E set(int index, E element) {
        rangeCheck(index);

        E oldValue = elementData(index);
        elementData[index] = element;
        return oldValue;
    }
----------------------------------------------------------------------
final void checkForComodification() {
	if (expectedModCount != ArrayList.this.modCount)
		throw new ConcurrentModificationException();
	}
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public E get(int index) {
            rangeCheck(index);
            checkForComodification();
            return ArrayList.this.elementData(offset + index);
        }
----------------------------------------------------------------------
E elementData(int index) {
        return (E) elementData[index];
    }

public void clear() {
        modCount++;

        // clear to let GC do its work
        for (int i = 0; i < size; i++)
            elementData[i] = null;

        size = 0;
    }
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public boolean contains(Object o) {
        return indexOf(o) >= 0;
    }
----------------------------------------------------------------------
public int indexOf(Object o) {
        if (o == null) {
            for (int i = 0; i < size; i++)
                if (elementData[i]==null)
                    return i;
        } else {
            for (int i = 0; i < size; i++)
                if (o.equals(elementData[i]))
                    return i;
        }
        return -1;
    }