【java源碼】ArrayList

題目：【java源碼】ArrayList

ArrayList 經常使用功能：構造函數、增、批量增、刪、批量刪、批量保留
ArrayList 屬性：
    // 默認數組長度（數組，而不是數據個數）
    private static final int DEFAULT_CAPACITY = 10;
    // 空數據
    private static final Object[] EMPTY_ELEMENTDATA = {};
    private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
    // 實際用於存放數據的地方
    transient Object[] elementData;
    // 數據個數
    private int size;
    
1、構造函數
    ①public ArrayList();
        只幹了一件事：this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    ②public ArrayList(int initialCapacity);
        指定初始化elementData數組的初始大小。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：就是數組中插一個元素操做思路
    ①public boolean add(E e);
        1、確保elementData數組可以裝下
            首先判斷原來數組長度是否爲0，若是爲零，那麼新數組長度爲Math.max(DEFAULT_CAPACITY, 加入後數組最小應該長度)；
            判斷原來數組是否還裝的下，若是須要擴容那麼：
            private void grow(int minCapacity) { // minCapacity：加入後數組最小應該長度
                // 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);
            }
        二、elementData[size++] = e;return true;
    ②public void add(int index, E element);
        一、rangeCheckForAdd(index); // 檢測index是否越接 if (index > size || index < 0)
        2、確保elementData數組可以裝下
        三、System.arraycopy(elementData, index, elementData, index + 1, size - index); // 數組挪位
        四、elementData[index] = element;
        五、size++;

3、批量增：就是數組中插多個元素操做思路
    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：確保elementData數組可以裝下
            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：確保elementData數組可以裝下

            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、刪：三點注意①按內容刪，只刪除第一個；②刪的是equals爲真的；③注意看fastRemove(index)源碼
    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、批量刪、批量保留：一點注意：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：維護兩個指針。一個讀指針(r)，一個寫指針(w)。讀指針從0遍歷到數組尾，在遍歷中，若是符合條件就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;
        }
        
總結：
1、ArrayList經過內置一個Object數組實現順序表功能。經過grow函數實現動態增加，最大長度可在源碼中找答案。
二、刪除功能經過fastRemove函數實現對一個刪除，經過batchRemove函數實現對多個刪除。經過elementData[i] = null；或elementData[--size] = null; 的方式，讓JVM垃圾回收，自動回收。
    private void grow(int minCapacity) { // minCapacity：加入後數組最小應該長度
        // 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;
    }