【java源码】ArrayList

题目：【java源码】ArrayList

ArrayList 常用功能：构造函数、增、批量增、删、批量删、批量保留
ArrayList 属性：
    // 默认数组长度（数组，而不是数据个数）
    private static final int DEFAULT_CAPACITY = 10;
    // 空数据
    private static final Object[] EMPTY_ELEMENTDATA = {};
    private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
    // 实际用于存放数据的地方
    transient Object[] elementData;
    // 数据个数
    private int size;
    
1、构造函数
    ①public ArrayList();
        只干了一件事：this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    ②public ArrayList(int initialCapacity);
        指定初始化elementData数组的初始大小。this.elementData = new Object[initialCapacity];
    ③public ArrayList(Collection<? extends E> c) {
        elementData = c.toArray();
        if ((size = elementData.length) != 0) {
            // c.toArray might (incorrectly) not return Object[] (see 6260652)
            if (elementData.getClass() != Object[].class)
                elementData = Arrays.copyOf(elementData, size, Object[].class);
        } else {
            // replace with empty array.
            this.elementData = EMPTY_ELEMENTDATA;
        }
    }
    
2、增：就是数组中插一个元素操作思路
    ①public boolean add(E e);
        1、确保elementData数组能够装下
            首先判断原来数组长度是否为0，如果为零，那么新数组长度为Math.max(DEFAULT_CAPACITY, 加入后数组最小应该长度)；
            判断原来数组是否还装的下，如果需要扩容那么：
            private void grow(int minCapacity) { // minCapacity：加入后数组最小应该长度
                // overflow-conscious code
                int oldCapacity = elementData.length;
                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);
            }
        2、elementData[size++] = e;return true;
    ②public void add(int index, E element);
        1、rangeCheckForAdd(index); // 检测index是否越接 if (index > size || index < 0)
        2、确保elementData数组能够装下
        3、System.arraycopy(elementData, index, elementData, index + 1, size - index); // 数组挪位
        4、elementData[index] = element;
        5、size++;

3、批量增：就是数组中插多个元素操作思路
    ①public boolean addAll(Collection<? extends E> c);
        public boolean addAll(Collection<? extends E> c) {
            Object[] a = c.toArray();
            int numNew = a.length;
            ensureCapacityInternal(size + numNew);  // 同上：确保elementData数组能够装下
            System.arraycopy(a, 0, elementData, size, numNew);
            size += numNew;
            return numNew != 0;
        }
    ②public boolean addAll(int index, Collection<? extends E> c)
        public boolean addAll(int index, Collection<? extends E> c) {
            rangeCheckForAdd(index);

            Object[] a = c.toArray();
            int numNew = a.length;
            ensureCapacityInternal(size + numNew);  // // 同上：确保elementData数组能够装下

            int numMoved = size - index;
            if (numMoved > 0)
                System.arraycopy(elementData, index, elementData, index + numNew,
                                 numMoved);

            System.arraycopy(a, 0, elementData, index, numNew);
            size += numNew;
            return numNew != 0;
        }

4、删：三点注意①按内容删，只删除第一个；②删的是equals为真的；③注意看fastRemove(index)源码
    ①public boolean remove(Object o);
        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;
        }
    ②public E remove(int index); // 同理
    ③private void fastRemove(int index);
        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
        }

5、批量删、批量保留：一点注意：batchRemove方法。
    ①public boolean removeAll(Collection<?> c);
        public boolean removeAll(Collection<?> c) {
            Objects.requireNonNull(c);
            return batchRemove(c, false);
        }
    ②public boolean retainAll(Collection<?> c);
        public boolean retainAll(Collection<?> c) {
            Objects.requireNonNull(c);
            return batchRemove(c, true);
        }
    ③private boolean batchRemove(Collection<?> c, boolean complement);
        // 思想：维护两个指针。一个读指针(r)，一个写指针(w)。读指针从0遍历到数组尾，在遍历中，如果符合条件就elementData[w++] = elementData[r];
        private boolean batchRemove(Collection<?> c, boolean complement) {
            final Object[] elementData = this.elementData;
            int r = 0, w = 0;
            boolean modified = false;
            try {
                for (; r < size; r++)
                    if (c.contains(elementData[r]) == complement)
                        elementData[w++] = elementData[r];
            } finally {
                // Preserve behavioral compatibility with AbstractCollection,
                // even if c.contains() throws.
                if (r != size) {
                    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;
        }
        
总结：
1、ArrayList通过内置一个Object数组实现顺序表功能。通过grow函数实现动态增长，最大长度可在源码中找答案。
2、删除功能通过fastRemove函数实现对一个删除，通过batchRemove函数实现对多个删除。通过elementData[i] = null；或elementData[--size] = null; 的方式，让JVM垃圾回收，自动回收。
    private void grow(int minCapacity) { // minCapacity：加入后数组最小应该长度
        // overflow-conscious code
        int oldCapacity = elementData.length;
        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);
    }
        
    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
    }
        
    private boolean batchRemove(Collection<?> c, boolean complement) {
        final Object[] elementData = this.elementData;
        int r = 0, w = 0;
        boolean modified = false;
        try {
            for (; r < size; r++)
                if (c.contains(elementData[r]) == complement)
                    elementData[w++] = elementData[r];
        } finally {
            // Preserve behavioral compatibility with AbstractCollection,
            // even if c.contains() throws.
            if (r != size) {
                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;
    }