集合之ArrayList

集合之ArrayList

/**
 * Default initial capacity.
 */
private static final int DEFAULT_CAPACITY = 10;

默认初始数组大小为10

    /**
     * Appends the specified element to the end of this list.
     *
     * @param e element to be appended to this list
     * @return <tt>true</tt> (as specified by {@link Collection#add})
     */
    public boolean add(E e) {
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        //放到数组最后边
        elementData[size++] = e;
        return true;
    }

//判断如果是初始值，如果是则先创建一个默认10的数组。
//如果不是。则判断是否超过现在的数组长度。如果超过，则扩容1.5倍
 private void ensureCapacityInternal(int minCapacity) {
    //如果是初始空数组值
    if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
        //创建一个不大于10的数组
        minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
    }

    ensureExplicitCapacity(minCapacity);
}

//判断是否扩容
 private void ensureExplicitCapacity(int minCapacity) {
    modCount++;

    // overflow-conscious code
    //如果比数组还大了。扩容
    if (minCapacity - elementData.length > 0)
        grow(minCapacity);
}

//扩容
private void grow(int minCapacity) {
    // overflow-conscious code
    int oldCapacity = elementData.length;
    //扩容原来的1.5倍
    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:
    elementData = Arrays.copyOf(elementData, newCapacity);
}


    //添加元素，
public void add(int index, E element) {
        if (index > size || index < 0)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
        //判断是否需要扩容
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        //在[index,leng]位置赋值到[index+1,leng+1],留出index位置
        //假设index = 2 , 原数组为12345 经过arraycopy后---> 120345 
        System.arraycopy(elementData, index, elementData, index + 1,
                         size - index);
        elementData[index] = element;
        size++;
}

public E get(int index) {
        if (index >= size)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
        //直接从数组里面获取
        return (E) elementData[index];
    }

//移除元素
public E remove(int index) {
        if (index >= size)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));

        modCount++;
        //拿到要移除的值
        E oldValue = (E) elementData[index];
        int numMoved = size - index - 1;
        if (numMoved > 0)
            //把[index+1,end] 赋值到[index,end]
            //假设index = 2 , 原数组为12345 经过arraycopy后---> 12455 
            System.arraycopy(elementData, index+1, elementData, index,
                             numMoved);
        //把最后一个置空
        elementData[--size] = null; // clear to let GC do its work

        return oldValue;
    }

小结：ArrayList集合就简单了。底层用数组实现。构造的时候不传大小默认大小为10，每次超过数组大小和扩容为原来的1.5倍。