ArrayList的具体实现

ArrayList是List接口的具体实现,是在实际生产环境中用的非常多的一种工具类

数据结构

1. 顾名思义,从ArrayList的名字中我们就可以看出,ArrayList是基于数组的一种实现.

private transient Object[] elementData; //对象存储
private int size; //实际含有对象的大小  size <= elementData.length

主要代码实现

构造函数

    // 构造函数比较简单, 初始化对象数组就可以了
    public ArrayList(int initialCapacity) {
        super();
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        this.elementData = new Object[initialCapacity];
    }

add

    //将对象添加到数组的末尾即可
    public boolean add(E e) {
        ensureCapacityInternal(size + 1);  //对List进行扩容
        elementData[size++] = e;
        return true;
    }
    //在index位置添加新的数据, >=index的则全部后移一位
    public void add(int index, E element) {
        rangeCheckForAdd(index);

        ensureCapacityInternal(size + 1);  // 扩容
        System.arraycopy(elementData, index, elementData, index + 1,
                         size - index); // 后移
        elementData[index] = element;
        size++;
    }

    //扩容实现
    private void grow(int minCapacity) {
        // overflow-conscious code
        int oldCapacity = elementData.length;
        int newCapacity = oldCapacity + (oldCapacity >> 1); // 计算扩容后的数组大小 JDK > 1.7的算法, 与1.7以前的算法不同
        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);
    }

set

    //替换index的原有数据
    public E set(int index, E element) {
        rangeCheck(index);

        E oldValue = elementData(index);
        elementData[index] = element;
        return oldValue;
    }

get

    //直接取值, 数组结构的最大优势
    E elementData(int index) {
        return (E) elementData[index];
    }

remove

    //根据index remove
    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;
    }
    //根据Object  remove
    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;
    }

其余函数基本与这几个类似

迭代器, TODO:暂时还没看太明白,改天继续分析