jdk源码阅读笔记-ArrayList

     /**
     * Default initial capacity.
     */
    //默认的初始化大小
    private static final int DEFAULT_CAPACITY = 10;

    /**
     * The array buffer into which the elements of the ArrayList are stored.
     * The capacity of the ArrayList is the length of this array buffer. Any
     * empty ArrayList with elementData ==         DEFAULTCAPACITY_EMPTY_ELEMENTDATA
     * will be expanded to DEFAULT_CAPACITY when the first element is added.
     */
    //这个是用来存放数据用的数组，add进来的数据都是放到这个数组里面的
    transient Object[] elementData; // non-private to simplify nested class access

    /**
     * The size of the ArrayList (the number of elements it contains).
     *
     * @serial
     */
    //数组的大小
    private int size;

**
     * Constructs an empty list with the specified initial capacity.
     *
     * @param  initialCapacity  the initial capacity of the list
     * @throws IllegalArgumentException if the specified initial capacity
     *         is negative
     */
    public ArrayList(int initialCapacity) {//创建一个 initialCapacity大小的空数组
        if (initialCapacity > 0) {
            this.elementData = new Object[initialCapacity];
        } else if (initialCapacity == 0) {
            this.elementData = EMPTY_ELEMENTDATA;
        } else {
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        }
    }

    /**
     * Constructs an empty list with an initial capacity of ten.
     */
    public ArrayList() {//创建空数组
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }

    /**
     * Constructs a list containing the elements of the specified
     * collection, in the order they are returned by the collection's
     * iterator.
     *
     * @param c the collection whose elements are to be placed into this list
     * @throws NullPointerException if the specified collection is null
     */
    public ArrayList(Collection<? extends E> c) {
        //将c转换成数组，toArray方法返回的数组类型有可能不是Object类型的
        elementData = c.toArray();
        if ((size = elementData.length) != 0) {
            // c.toArray might (incorrectly) not return Object[] (see 6260652)
            //数组类型不是Object类型，需要将类型转换成Object类，避免后面调用方法
            // 的时候出现类型转换异常
            if (elementData.getClass() != Object[].class)
                elementData = Arrays.copyOf(elementData, size, Object[].class);
        } else {
            // replace with empty array.
            this.elementData = EMPTY_ELEMENTDATA;
        }
    }

public static <T,U> T[] copyOf(U[] original, int newLength, Class<? extends T[]> newType) {
        @SuppressWarnings("unchecked")
        T[] copy = ((Object)newType == (Object)Object[].class)
            ? (T[]) new Object[newLength]
            : (T[]) Array.newInstance(newType.getComponentType(), newLength);
        System.arraycopy(original, 0, copy, 0,
                         Math.min(original.length, newLength));
        return copy;
    }

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

public static void main(String[] args) {
        int[] src = {1,2,3,4,5};
        int[] dest = new int[6];
        for (int i : dest) {
            System.out.print(i + " ");
        }
        System.out.println();
        System.arraycopy(src,0,dest,0,src.length);
        for (int i : dest) {
            System.out.print(i + " ");
        }
    }

    //运行结果：
    //0 0 0 0 0 0 
    //1 2 3 4 5 0 

/**
     * 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) {
        //确认当前数组大小不会发生越界异常，否者对数组进行扩容
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;
        return true;
    }

    /**
     * Inserts the specified element at the specified position in this
     * list. Shifts the element currently at that position (if any) and
     * any subsequent elements to the right (adds one to their indices).
     *
     * @param index index at which the specified element is to be inserted
     * @param element element to be inserted
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public void add(int index, E element) {
        //检查传入的下标是否在数组的范围之内
        rangeCheckForAdd(index);
        //检查数组是否需要扩容
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        //在index位置的元素全部往后移一位，为添加进来的元素腾出一个位置
        System.arraycopy(elementData, index, elementData, index + 1,
                         size - index);
        //将元素放入到index位置上
        elementData[index] = element;
        size++;
    }

private void ensureCapacityInternal(int minCapacity) {
    ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
}

private void ensureExplicitCapacity(int minCapacity) {
        //修改次数+1，主要实现快速失败机制的
        modCount++;

        // overflow-conscious code
        //如果最小所需容量比当前数组的长度大的话就进行扩容
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }


    /**
     * Increases the capacity to ensure that it can hold at least the
     * number of elements specified by the minimum capacity argument.
     *
     * @param minCapacity the desired minimum capacity
     */
    private void grow(int minCapacity) {
        // overflow-conscious code
        int oldCapacity = elementData.length;
        //每次扩容都是扩大原来数组大小的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:
        //创建一个新的数组，长度为 newCapacity，然后将原来数组的元素复制到新数组上并返回新数组，达到动态扩容数组的目的
        elementData = Arrays.copyOf(elementData, newCapacity);
    }

/**
     * Returns the element at the specified position in this list.
     *
     * @param  index index of the element to return
     * @return the element at the specified position in this list
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public E get(int index) {
        rangeCheck(index);

        return elementData(index);
    }

// Positional Access Operations

    @SuppressWarnings("unchecked")
    E elementData(int index) {
        return (E) elementData[index];
    }

/**
     * Removes the element at the specified position in this list.
     * Shifts any subsequent elements to the left (subtracts one from their
     * indices).
     *
     * @param index the index of the element to be removed
     * @return the element that was removed from the list
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public E remove(int index) {
        rangeCheck(index);

        modCount++;
        E oldValue = elementData(index);
        //移动区间
        int numMoved = size - index - 1;
        if (numMoved > 0)
            //在删除位置后面的所有元素都往前挪一位
            System.arraycopy(elementData, index+1, elementData, index,
                             numMoved);
        elementData[--size] = null; // clear to let GC do its work

        return oldValue;
    }


    /**
     * Removes the first occurrence of the specified element from this list,
     * if it is present.  If the list does not contain the element, it is
     * unchanged.  More formally, removes the element with the lowest index
     * <tt>i</tt> such that
     * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>
     * (if such an element exists).  Returns <tt>true</tt> if this list
     * contained the specified element (or equivalently, if this list
     * changed as a result of the call).
     *
     * @param o element to be removed from this list, if present
     * @return <tt>true</tt> if this list contained the specified element
     */
    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 remove method that skips bounds checking and does not
     * return the value removed.
     */
    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
    }

public static void main(String[] args) {
        List<Integer> list = new ArrayList<>();
        
        //添加操作
        list.add(1);
        list.add(2);
        list.add(3);
        
        //删除操作
        list.remove(0);
        
        //查询操作
        //1、随机访问：
        for (int i = 0; i < list.size(); i++) {
            System.out.println(list.get(i));
        }
        
        //增强for循环遍历
        for (Integer integer : list) {
            System.out.println(integer);
        }
        //使用迭代器遍历
        Iterator<Integer> iterator = list.iterator();
        while (iterator.hasNext()){
            Integer integer = iterator.next();
            System.out.println(integer);
        }
    }