Java集合-ArrayList源码

	//默认初始容量
	private static final int DEFAULT_CAPACITY = 10;
	//空数组(用于空实例)
    private static final Object[] EMPTY_ELEMENTDATA = {};
	//用于默认大小空实例的共享空数组
    private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
	//保存数据的数组
    transient Object[] elementData; // non-private to simplify nested class access
	//元素个数
    private int size;

	//带有参数的构造器

	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);
        }
    }
	
	//这就是与上面不一样的地方
    public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }

    public ArrayList(Collection<? extends E> c) {
        Object[] a = c.toArray();
        if ((size = a.length) != 0) {
            if (c.getClass() == ArrayList.class) {
                elementData = a;
            } else {
                elementData = Arrays.copyOf(a, size, Object[].class);
            }
        } else {
            // replace with empty array.
            elementData = EMPTY_ELEMENTDATA;
        }
    }

	public void ensureCapacity(int minCapacity) {
		
		//这里就开始判处出前面两个空数组的区别了
		//如果这个list是无参构造的化没那么minExpand就为10，否则为0
        int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA)
            ? 
            : DEFAULT_CAPACITY;
		//如果最小容量大于已有的最大容量
        if (minCapacity > minExpand) {
            ensureExplicitCapacity(minCapacity);
        }
    }

   private void ensureExplicitCapacity(int minCapacity) {
        modCount++;

        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }

    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:
        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;
    }

 	private void ensureCapacityInternal(int minCapacity) {
        ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
    }

    private static int calculateCapacity(Object[] elementData, int minCapacity) {
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
            return Math.max(DEFAULT_CAPACITY, minCapacity);
        }
        return minCapacity;
    }

private class Itr implements Iterator<E> {
		//当前游标位置
        int cursor; 
        //上一个游标位置
        int lastRet = -1; 
        //修改次数
        int expectedModCount = modCount;

        Itr() {}
		
		//是否还存在下一个
        public boolean hasNext() {
            return cursor != size;
        }
        
        @SuppressWarnings("unchecked")
        public E next() {
        
        	//检查期待修改次数与实际的修改次数是否发生变化
        	//如果有变化就抛出异常
            checkForComodification();
            
            int i = cursor;
            
            if (i >= size) //判断是否越界
                throw new NoSuchElementException();
            Object[] elementData = ArrayList.this.elementData;
            
            if (i >= elementData.length) //判断是否越界
                throw new ConcurrentModificationException();
            
            cursor = i + 1; //游标变为下一个
            return (E) elementData[lastRet = i];
        }

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

            try {
                ArrayList.this.remove(lastRet);
                cursor = lastRet;
                lastRet = -1;
                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();
        }

        final void checkForComodification() {
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
        }
    }