ArrayList源码

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一些参数：

private static final long serialVersionUID = 8683452581122892189L; // 序列号

    /**     * 初始的容量大小，构造函数没有指定大小，用此     */    private static final int DEFAULT_CAPACITY = 10;

    /**     * Shared empty array instance used for empty instances.     * 空数组对象     */    private static final Object[] EMPTY_ELEMENTDATA = {};

    /**     * Shared empty array instance used for default sized empty instances. We     * distinguish this from EMPTY_ELEMENTDATA to know how much to inflate when     * first element is added.     * 默认的空数组对象，区别是当有第一元素被添加的时候，它就能知道增加了多大容量。就是指默认的10     */    private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

    /**     * 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.     * 数组缓存     */    transient Object[] elementData; // non-private to simplify nested class access

    /**     * The size of the ArrayList (the number of elements it contains).     *  ArrayList含有的元素个数     * @serial     */    private int size;

构造方法：

    /**     * Constructs an empty list with the specified initial capacity.     * 初始化容量大小     * @param  initialCapacity  the initial capacity of the list     * @throws IllegalArgumentException if the specified initial capacity     *         is negative     */    public ArrayList(int initialCapacity) {        if (initialCapacity > 0) {            this.elementData = new Object[initialCapacity];  // 存入array buffer        } else if (initialCapacity == 0) {            this.elementData = EMPTY_ELEMENTDATA;  // 空数组对象        } else {            throw new IllegalArgumentException("Illegal Capacity: "+                                               initialCapacity);        // 非法参数  抛出异常        }    }

    /**     * Constructs an empty list with an initial capacity of ten.     * 无参构造 ， 默认大小是 10     */    public ArrayList() {        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.     * 构造函数参数是集合 ，     * @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) {        elementData = c.toArray();        if ((size = elementData.length) != 0) {            // c.toArray might (incorrectly) not return Object[] (see 6260652)            // c.toArray方法可能不会一直返回Object[]结果 如果不是我们需要把它转成同样的            if (elementData.getClass() != Object[].class)                elementData = Arrays.copyOf(elementData, size, Object[].class);  // 复制数组到数组缓存里        } else {            // replace with empty array.            this.elementData = EMPTY_ELEMENTDATA; // 空对象数组        }    }

toArray:

  /** * Returns an array containing all of the elements in this list * in proper sequence (from first to last element). */ public Object[] toArray() {    return Arrays.copyOf(elementData, size);} public static <T> T[] copyOf(T[] original, int newLength) {    return (T[]) copyOf(original, newLength, original.getClass());} public static <T,U> T[] copyOf(U[] original, int newLength, Class<? extends T[]> newType) {    @SuppressWarnings("unchecked")    // 这句说明 返回的实际类型不一定是Object[]    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)); // 复制数组    return copy;}

扩容：

/** * 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;    int newCapacity = oldCapacity + (oldCapacity >> 1);  // 新容量大小是原来的1.5倍    if (newCapacity - minCapacity < 0)        newCapacity = minCapacity;  // 新容量小于ensureCapacity(int minCapacity)的参数  则用小的容量    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); // 新建数组}// 知道ArrayList的最大容量private static int hugeCapacity(int minCapacity) {    if (minCapacity < 0) // overflow  int 溢出成负数        throw new OutOfMemoryError();    return (minCapacity > MAX_ARRAY_SIZE) ?        Integer.MAX_VALUE :        MAX_ARRAY_SIZE;}

常用方法：

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

   /**     * Returns the element at the specified position in this list.     * GET方法     * @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);  // 判断是否过界，是抛出异常        return elementData(index);  // 数组值    }

    /**     * Replaces the element at the specified position in this list with     * the specified element.     *     * @param index index of the element to replace     * @param element element to be stored at the specified position     * @return the element previously at the specified position     * @throws IndexOutOfBoundsException {@inheritDoc}     */    public E set(int index, E element) {        rangeCheck(index);  // 判断是否过界，是抛出异常        E oldValue = elementData(index);        elementData[index] = element;  // 替换        return oldValue;  // 返回原来的值    }

    **末尾添加**    /**     * 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!! （多线程的时候，fast-fail），并且观察是否需要扩容        elementData[size++] = e;  // 添加        return true;  // 添加成功 true    }

    **指定位置添加**    /**     * 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) {        rangeCheckForAdd(index);        ensureCapacityInternal(size + 1);  // Increments modCount!!  fast-fail        System.arraycopy(elementData, index, elementData, index + 1,                         size - index);   // 同一个数组重新复制从index+1 开始长度为 size-index        elementData[index] = element;  // 指定位置添加        size++; // ArrayList长度增加1    }

移除

    /**     * 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++;  // fast - fail        E oldValue = elementData(index);        int numMoved = size - index - 1;        if (numMoved > 0)            System.arraycopy(elementData, index+1, elementData, index,                             numMoved); // 复制向前移动一位        elementData[--size] = null; // clear to let GC do its work        return oldValue; // 删除的值    }

    /**不同与上一个方法 他不需要检查越界问题     * 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; // 删除成功    }

    /*     * Private remove method that skips bounds checking and does not     * return the value removed.     * 区别不需要 进行边界校验     */    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    }

    /**     * Removes all of the elements from this list.  The list will     * be empty after this call returns.     * 清空ArrayList     */    public void clear() {        modCount++;        // clear to let GC do its work        for (int i = 0; i < size; i++)            elementData[i] = null;        size = 0;    }

批量删除

public boolean removeAll(Collection<?> c) {    Objects.requireNonNull(c);    return batchRemove(c, false);}

批量保留

public boolean retainAll(Collection<?> c) {    Objects.requireNonNull(c);    return batchRemove(c, true);}

    private boolean batchRemove(Collection<?> c, boolean complement) {        final Object[] elementData = this.elementData;  // final成内联效率高，安全等        int r = 0, w = 0;        boolean modified = false;          try {            for (; r < size; r++)                // complement = true 的时候保留， false 删除                  if (c.contains(elementData[r]) == complement)                    elementData[w++] = elementData[r];         } finally {            // Preserve behavioral compatibility with AbstractCollection,            // even if c.contains() throws.            if (r != size) {                // 出错的时候长度不相等  还原数组                System.arraycopy(elementData, r,                                 elementData, w,                                 size - r);                w += size - r;            }            if (w != size) {                // clear to let GC do its work                for (int i = w; i < size; i++)                    elementData[i] = null;                modCount += size - w;                size = w;                modified = true;            }        }        return modified; // 是否执行成功    }

遍历迭代器：

主要区别iterator可以向前遍历：listIterator  public E previous()

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