//無參構(gòu)造函數(shù)，會(huì)構(gòu)造一個(gè)空數(shù)組。第一次擴(kuò)容時(shí)會(huì)創(chuàng)默認(rèn)大小為10的數(shù)組
    private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
    transient Object[] elementData;
    public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }
    private static final Object[] EMPTY_ELEMENTDATA = {};
    //如果給定集合大小，會(huì)創(chuàng)建一個(gè)對(duì)應(yīng)大小的數(shù)組
    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);
        }
    }

 //直接通過角標(biāo)從數(shù)組中取出元素
    public E get(int index) {
        if (index >= size)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
        return (E) elementData[index];
    }
    //set 方法，直接通過index 拿到元素，進(jìn)行修改就行。
    public E set(int index, E element) {
        if (index >= size)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
        E oldValue = (E) elementData[index];
        elementData[index] = element;
        return oldValue;
    }

//Add 方法 ，在末端插入不需要
    public boolean add(E e) {
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;
        return true;
    }

  public void add(int index, E element) {
        if (index > size || index < 0)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        //被插入的元素在index。則移動(dòng)的數(shù)組應(yīng)該從index開始，向后移動(dòng)一個(gè)，
		//移動(dòng)的數(shù)組長(zhǎng)度 size-index 從index開始的數(shù)組長(zhǎng)度
        System.arraycopy(elementData, index, elementData, index + 1,
                size - index);
        elementData[index] = element;
        size++;
    }

 private void ensureCapacityInternal(int minCapacity) {
        //通過無參構(gòu)造創(chuàng)建為設(shè)置初始大小時(shí)，判斷條件為true
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
            //創(chuàng)建默認(rèn)大小的數(shù)組，初始大小是10
            minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
        }
        ensureExplicitCapacity(minCapacity);
    }

 private void ensureExplicitCapacity(int minCapacity) {
        modCount++;
        // overflow-conscious code
        //新增后，數(shù)組容量已經(jīng)大于了數(shù)組長(zhǎng)度，則進(jìn)行擴(kuò)容
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }
    private void grow(int minCapacity) {
        // overflow-conscious code
        int oldCapacity = elementData.length;
		//新的數(shù)組大小是原來的1.5倍
        int newCapacity = oldCapacity + (oldCapacity >> 1);  
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity;
        //創(chuàng)建一個(gè)新長(zhǎng)度的數(shù)組。
        elementData = Arrays.copyOf(elementData, newCapacity);
    }

 //刪除
    public E remove(int index) {
        if (index >= size)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
        modCount++;
        //取出被刪除的元素
        E oldValue = (E) elementData[index];
        int numMoved = size - index - 1;
		//移動(dòng)的數(shù)組長(zhǎng)度，大于0才需要移動(dòng)，小于0說明被刪元素在最后一個(gè)
        if (numMoved > 0)
            //第一個(gè)參數(shù)是源數(shù)組，
            //第二個(gè)參數(shù)是移動(dòng)的起始位置：index+1，從被刪元素的后一個(gè)位置開始
            //第三個(gè)參數(shù)是目標(biāo)數(shù)組
            //第四個(gè)參數(shù)是移動(dòng)目標(biāo)位置：index，被刪除元素的位置
            //第五個(gè)參數(shù)是移動(dòng)的數(shù)組長(zhǎng)度 size - index - 1，從index+1位置開始之后的數(shù)組長(zhǎng)度
            //最終會(huì)調(diào)用到native方法進(jìn)行移動(dòng)。
            System.arraycopy(elementData, index+1, elementData, index,numMoved);
        elementData[--size] = null; // clear to let GC do its work
        return oldValue;
    }

public static native void arraycopy(java.lang.Object src, int srcPos, java.lang.Object dest, int destPos, int length);