JDK1.8源码分析：ArrayList

偏执的太偏执、 2022-03-20 05:26 317阅读 0赞

### 数据结构 ###

*  ArrayList相对于数组Array只是提供了动态拓容的功能，在内部也是使用一个数组来存储数据的。
 *  ArrayList也不是线程安全的，如果需要线程安全，则需要使用Collections.synchronziedList来进行包装，如：List list = Collections.synchronizedList(new ArrayList(…));
    
        /** * Default initial capacity. */
        private static final int DEFAULT_CAPACITY = 10;
        
        /** * Shared empty array instance used for empty instances. */
        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. */
        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. */
        transient Object[] elementData; // non-private to simplify nested class access
 *  元素个数size维护：size记录了当前list的内部数组存放了多少个元素，所以可以通过size方法来通过O(1)时间复杂度返回size，而不需要每次都对数组进行一次遍历计算。主要是在添加和删除元素时，执行size++或size–来同步更新：
    
        /** * 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!!
            
            // 递增size
            elementData[size++] = e;
            return true;
        }
        
        /** * 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);
        
            int numMoved = size - index - 1;
            if (numMoved > 0)
                System.arraycopy(elementData, index+1, elementData, index,
                                 numMoved);
            
            // 递减size
            elementData[--size] = null; // clear to let GC do its work
        
            return oldValue;
        }

### 拓容规则 ###

*  内部数组拓容规则：每次在调用add添加元素时，如果内部数组大小满了，则会进行拓容。拓容规则为：初始大小为10，之后每次拓容为当前数组容量的1.5倍，实现源码如下：主要在grow方法实现。
    
        /** * The maximum size of array to allocate. * Some VMs reserve some header words in an array. * Attempts to allocate larger arrays may result in * OutOfMemoryError: Requested array size exceeds VM limit */
        private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
        
        /** * 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;
            
            // 默认拓容为原来的1.5倍
            int newCapacity = oldCapacity + (oldCapacity >> 1);
            
            // 如果原来的容量加一比原来的1.5倍大，则使用该加一的版本
            if (newCapacity - minCapacity < 0)
                newCapacity = minCapacity;
                
            // 最大不超过Integer.MAX_VALUE - 8
            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 static int hugeCapacity(int minCapacity) { 
            if (minCapacity < 0) // overflow
                throw new OutOfMemoryError();
            return (minCapacity > MAX_ARRAY_SIZE) ?
                Integer.MAX_VALUE :
                MAX_ARRAY_SIZE;
        }
 *  拓容在确定好新的容量之后，会通过Arrays.copyOf方法来新建一个该新容量的数组，然后在Arrays.copyOf内部通过System.arraycopy方法将旧数组的内容拷贝到新数组，然后返回该新数组作为内部数组。其中System.arraycopy为本地native方法，性能较高。
    
        private void grow(int minCapacity) { 
                // 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:
                
                // 数组拓容
                elementData = Arrays.copyOf(elementData, newCapacity);
            }
            
            
            // Arrays.copyOf的实现：
            
            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")
                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;
            }

### 子集SubList ###

*  子集合SubList：SubList为List列表的一个子集，在底层与List列表公用一个内部数组来存储数据，所以对suList返回的子列表进行操作也会对主列表List产生影响。如下：set方法其实是对主列表的内部数组elementData进行操作的。
    
        private class SubList extends AbstractList<E> implements RandomAccess { 
            private final AbstractList<E> parent;
            private final int parentOffset;
            private final int offset;
            int size;
        
            SubList(AbstractList<E> parent,
                    int offset, int fromIndex, int toIndex) { 
                this.parent = parent;
                this.parentOffset = fromIndex;
                this.offset = offset + fromIndex;
                this.size = toIndex - fromIndex;
                this.modCount = ArrayList.this.modCount;
            }
        
            public E set(int index, E e) { 
                rangeCheck(index);
                checkForComodification();
                E oldValue = ArrayList.this.elementData(offset + index);
                ArrayList.this.elementData[offset + index] = e;
                return oldValue;
            }
            
            ...
            
        }

### 迭代器 ###

*  迭代器iterator：迭代器也是fail-fast的，注意判断fail-fast的条件是在当遍历到的数组下标i大于当前内部数组的size或者数组容量capacity时，才抛出异常。所以在使用迭代器的时候，不能依赖数组的位置关系，否则可能会出错，如在遍历第5个位置之前，其他线程在1到5之间插入了一个元素，则第5个其实是产生该迭代器的时候的第4个。
 *  如果需要依赖位置关系，即线程安全方面的考虑，则需要使用Collections.synchronizedList来包装成线程安全的List。
    
        public Iterator<E> iterator() { 
            return new Itr();
        }
        
        /** * An optimized version of AbstractList.Itr */
        private class Itr implements Iterator<E> { 
            int cursor;       // index of next element to return
            int lastRet = -1; // index of last element returned; -1 if no such
            int expectedModCount = modCount;
        
            public boolean hasNext() { 
                return cursor != size;
            }
        
            @SuppressWarnings("unchecked")
            public E next() { 
                checkForComodification();
                int i = cursor;
                
                // i 大于数组大小
                if (i >= size)
                    throw new NoSuchElementException();
                Object[] elementData = ArrayList.this.elementData;
                
                // i 大于数组容量
                if (i >= elementData.length)
                    throw new ConcurrentModificationException();
                cursor = i + 1;
                return (E) elementData[lastRet = i];
            }
            
            ...
            
        }