AbstractList源码分析

AbstractList

1 类图

2 字段

    // 默认容量
    private static final int DEFAULT_CAPACITY = 10;
    // 共享的空数组
    private static final Object[] EMPTY_ELEMENTDATA = {};
    // 共享个空数组，和EMPTY_ELEMENTDATA空数组相比是用于了解当添加第一个元素时数组应该扩容多少
    private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
    /**
     * ArrayList存储元素的buffer， ArrayList的容量就是这个数组buffer的长度
     * 任何拥有DEFAULTCAPACITY_EMPTY_ELEMENTDATA空数组的ArrayList在第一次
     * 添加元素的时候都会扩容到DEFAULT_CAPACITY = 10
     */
    transient Object[] elementData; // non-private to simplify nested class access
    // 持有元素的个数
    private int size;

3 构造器

    /**
     * 无参数的ArrayList的元素数组是DEFAULTCAPACITY_EMPTY_ELEMENTDATA
     */
    public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }

    /**
     * 初始容量为0的ArrayList的元素数组是EMPTY_ELEMENTDAT
     */
    public ArrayList(int initialCapacity) {
        if (initialCapacity > 0) {
            this.elementData = new Object[initialCapacity];
        } else if (initialCapacity == 0) {
            this.elementData = EMPTY_ELEMENTDATA;
        } else {
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        }
    }

    /**
     * 根据collection的迭代器返回的数据顺序构造一个ArrayList
     */
    public ArrayList(Collection<? extends E> c) {
        elementData = c.toArray();
        if ((size = elementData.length) != 0) {
            if (elementData.getClass() != Object[].class)
                elementData = Arrays.copyOf(elementData, size, Object[].class);
        } else {
            this.elementData = EMPTY_ELEMENTDATA;
        }
    }

4 增

    /**
     * 往列表末尾增加元素
     */
    public boolean add(E e) {
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;
        return true;
    }

1. 先内部确认容量
2. 数组buffer末尾添加元素
3. 返回true

    // 最小容量是当前size+1
    private void ensureCapacityInternal(int minCapacity) {
        // 如果当前的元素数组是DEFAULTCAPACITY_EMPTY_ELEMENTDATA
        // 那么最小容量取min(DEFAULT_CAPACITY, size+1)
        // 否则直接取size+1
        // 可以认为如果指定了容量，那么对容量的预期越低吗？
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
            minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
        }

        ensureExplicitCapacity(minCapacity);
    }

    // 确认容量的核心方法，然后扩容
    private void ensureExplicitCapacity(int minCapacity) {
        // 修改次数++
        modCount++;

        // 如果最小容量大于当前数组buffer的长度，那么需要扩容的
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }

    /**
     * 最大数组长度
     */
    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

    /**
     * 扩容，最起码可以容纳minCapacity个元素
     */
    private void grow(int minCapacity) {
        int oldCapacity = elementData.length;
        // 新容量是老容量1.5倍
        int newCapacity = oldCapacity + (oldCapacity >> 1);
        // 取minCapacity和newCapacity的更小值
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity;
        // 如果大于最大数组长度，再处理
        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);
    }

    // 溢出则抛出错误
    // 如果大于MAX_ARRAY_SIZE，取Integer最大值，否则取MAX_ARRAY_SIZE
    private static int hugeCapacity(int minCapacity) {
        if (minCapacity < 0) // overflow
            throw new OutOfMemoryError();
        return (minCapacity > MAX_ARRAY_SIZE) ?
            Integer.MAX_VALUE :
            MAX_ARRAY_SIZE;
    }

5 删除

    /**
     * 删除指定位置的元素，剩余右边元素都需要向左边移动一位
     */
    public E remove(int index) {
        rangeCheck(index);

        // 修改次数+1
        modCount++;
        E oldValue = elementData(index);

        int numMoved = size - index - 1;
        if (numMoved > 0)
            System.arraycopy(elementData, index+1, elementData, index, numMoved);
        elementData[--size] = null;

        return oldValue;
    }

    /**
     * 索引边界检查
     */
    private void rangeCheck(int index) {
        if (index >= size)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

7 更新

    /**
     * 替换指定位置上的元素
     */
    public E set(int index, E element) {
        rangeCheck(index);

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

8 查询

    /**
     * 返回指定位置元素
     */
    public E get(int index) {
        rangeCheck(index);

        return elementData(index);
    }

    /**
     * 返回指定元素第一次出现的索引，如果没有返回-1
     */
    public int indexOf(Object o) {
        if (o == null) {
            for (int i = 0; i < size; i++)
                if (elementData[i]==null)
                    return i;
        } else {
            for (int i = 0; i < size; i++)
                if (o.equals(elementData[i]))
                    return i;
        }
        return -1;
    }

9 遍历

    /**
     * 返回一个迭代器
     */
    public Iterator<E> iterator() {
        return new Itr();
    }

    private class Itr implements Iterator<E> {
        int cursor;       // 下一个返回的元素的索引
        int lastRet = -1; // 上一个返回的元素的索引，如果没有则返回-1
        int expectedModCount = modCount; // 期望的修改数目以及实际的修改数目
        public boolean hasNext() {
            return cursor != size;
        }

        // 如果实际修改次数和期望修改次数不相等则抛出ConcurrentModificationException
        final void checkForComodification() {
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
        }

        @SuppressWarnings("unchecked")
        public E next() {
            checkForComodification();
            int i = cursor;
            if (i >= size)
                throw new NoSuchElementException();
            Object[] elementData = ArrayList.this.elementData;
            if (i >= elementData.length)
                throw new ConcurrentModificationException();
            // 往后遍历，游标+1
            cursor = i + 1;
            // 上一个返回的元素的索引是i
            return (E) elementData[lastRet = i];
        }

        public void remove() {
            if (lastRet < 0)
                throw new IllegalStateException();
            checkForComodification();

            try {
                // 调用ArrayList.remove，modCount++
                ArrayList.this.remove(lastRet);
                // 下一个游标还是当前元素，比如说上一个索引0，数字2
                // 下一个索引1，数字3，当前游标是1
                // 删除2后，游标变成0，还会返回3
                // 那么上一个返回的元素就没有了，索引置-1
                cursor = lastRet;
                lastRet = -1;
                // 这里把expectedModCount设置为删除后的modCount
                expectedModCount = modCount;
            } catch (IndexOutOfBoundsException ex) {
                throw new ConcurrentModificationException();
            }
        }

        @Override
        @SuppressWarnings("unchecked")
        public void forEachRemaining(Consumer<? super E> consumer) {
            Objects.requireNonNull(consumer);
            final int size = ArrayList.this.size;
            int i = cursor;
            if (i >= size) {
                return;
            }
            final Object[] elementData = ArrayList.this.elementData;
            if (i >= elementData.length) {
                throw new ConcurrentModificationException();
            }
            while (i != size && modCount == expectedModCount) {
                consumer.accept((E) elementData[i++]);
            }
            // update once at end of iteration to reduce heap write traffic
            cursor = i;
            lastRet = i - 1;
            checkForComodification();
        }

    }

    /**
     * An optimized version of AbstractList.ListItr
     */
    private class ListItr extends Itr implements ListIterator<E> {
        ListItr(int index) {
            super();
            cursor = index;
        }

        public boolean hasPrevious() {
            return cursor != 0;
        }

        public int nextIndex() {
            return cursor;
        }

        public int previousIndex() {
            return cursor - 1;
        }

        // 可以向前遍历
        @SuppressWarnings("unchecked")
        public E previous() {
            checkForComodification();
            int i = cursor - 1;
            if (i < 0)
                throw new NoSuchElementException();
            Object[] elementData = ArrayList.this.elementData;
            if (i >= elementData.length)
                throw new ConcurrentModificationException();
            cursor = i;
            return (E) elementData[lastRet = i];
        }

        public void set(E e) {
            if (lastRet < 0)
                throw new IllegalStateException();
            checkForComodification();

            try {
                ArrayList.this.set(lastRet, e);
            } catch (IndexOutOfBoundsException ex) {
                throw new ConcurrentModificationException();
            }
        }

        public void add(E e) {
            checkForComodification();

            try {
                int i = cursor;
                ArrayList.this.add(i, e);
                cursor = i + 1;
                lastRet = -1;
                expectedModCount = modCount;
            } catch (IndexOutOfBoundsException ex) {
                throw new ConcurrentModificationException();
            }
        }
    }